xref: /linux/net/ipv4/tcp_output.c (revision b0249c0d41b306ddd79de58ca7fea543ab5e7a2e)

// SPDX-License-Identifier: GPL-2.0-only
/*
 * INET		An implementation of the TCP/IP protocol suite for the LINUX
 *		operating system.  INET is implemented using the  BSD Socket
 *		interface as the means of communication with the user level.
 *
 *		Implementation of the Transmission Control Protocol(TCP).
 *
 * Authors:	Ross Biro
 *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *		Mark Evans, <evansmp@uhura.aston.ac.uk>
 *		Corey Minyard <wf-rch!minyard@relay.EU.net>
 *		Florian La Roche, <flla@stud.uni-sb.de>
 *		Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 *		Linus Torvalds, <torvalds@cs.helsinki.fi>
 *		Alan Cox, <gw4pts@gw4pts.ampr.org>
 *		Matthew Dillon, <dillon@apollo.west.oic.com>
 *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *		Jorge Cwik, <jorge@laser.satlink.net>
 */

/*
 * Changes:	Pedro Roque	:	Retransmit queue handled by TCP.
 *				:	Fragmentation on mtu decrease
 *				:	Segment collapse on retransmit
 *				:	AF independence
 *
 *		Linus Torvalds	:	send_delayed_ack
 *		David S. Miller	:	Charge memory using the right skb
 *					during syn/ack processing.
 *		David S. Miller :	Output engine completely rewritten.
 *		Andrea Arcangeli:	SYNACK carry ts_recent in tsecr.
 *		Cacophonix Gaul :	draft-minshall-nagle-01
 *		J Hadi Salim	:	ECN support
 *
 */

#define pr_fmt(fmt) "TCP: " fmt

#include <net/tcp.h>
#include <net/tcp_ecn.h>
#include <net/mptcp.h>
#include <net/smc.h>
#include <net/proto_memory.h>
#include <net/psp.h>

#include <linux/compiler.h>
#include <linux/gfp.h>
#include <linux/module.h>
#include <linux/static_key.h>
#include <linux/skbuff_ref.h>

#include <trace/events/tcp.h>

/* Refresh clocks of a TCP socket,
 * ensuring monotically increasing values.
 */
void tcp_mstamp_refresh(struct tcp_sock *tp)
{
	u64 val = tcp_clock_ns();

	tp->tcp_clock_cache = val;
	tp->tcp_mstamp = div_u64(val, NSEC_PER_USEC);
}

static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
			   int push_one, gfp_t gfp);

/* Insert skb into rb tree, ordered by TCP_SKB_CB(skb)->seq */
void tcp_rbtree_insert(struct rb_root *root, struct sk_buff *skb)
{
	struct rb_node **p = &root->rb_node;
	struct rb_node *parent = NULL;
	struct sk_buff *skb1;

	while (*p) {
		parent = *p;
		skb1 = rb_to_skb(parent);
		if (before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb1)->seq))
			p = &parent->rb_left;
		else
			p = &parent->rb_right;
	}
	rb_link_node(&skb->rbnode, parent, p);
	rb_insert_color(&skb->rbnode, root);
}

/* Account for new data that has been sent to the network. */
static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned int prior_packets = tp->packets_out;

	WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(skb)->end_seq);

	__skb_unlink(skb, &sk->sk_write_queue);
	tcp_rbtree_insert(&sk->tcp_rtx_queue, skb);

	if (tp->highest_sack == NULL)
		tp->highest_sack = skb;

	tp->packets_out += tcp_skb_pcount(skb);
	if (!prior_packets || icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
		tcp_rearm_rto(sk);

	NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT,
		      tcp_skb_pcount(skb));
	tcp_check_space(sk);
}

/* SND.NXT, if window was not shrunk or the amount of shrunk was less than one
 * window scaling factor due to loss of precision.
 * If window has been shrunk, what should we make? It is not clear at all.
 * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
 * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
 * invalid. OK, let's make this for now:
 */
static inline __u32 tcp_acceptable_seq(const struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);

	if (!before(tcp_wnd_end(tp), tp->snd_nxt) ||
	    (tp->rx_opt.wscale_ok &&
	     ((tp->snd_nxt - tcp_wnd_end(tp)) < (1 << tp->rx_opt.rcv_wscale))))
		return tp->snd_nxt;
	else
		return tcp_wnd_end(tp);
}

/* Calculate mss to advertise in SYN segment.
 * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
 *
 * 1. It is independent of path mtu.
 * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
 * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
 *    attached devices, because some buggy hosts are confused by
 *    large MSS.
 * 4. We do not make 3, we advertise MSS, calculated from first
 *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
 *    This may be overridden via information stored in routing table.
 * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
 *    probably even Jumbo".
 */
static __u16 tcp_advertise_mss(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	const struct dst_entry *dst = __sk_dst_get(sk);
	int mss = tp->advmss;

	if (dst) {
		unsigned int metric = dst_metric_advmss(dst);

		if (metric < mss) {
			mss = metric;
			tp->advmss = mss;
		}
	}

	return (__u16)mss;
}

/* RFC2861. Reset CWND after idle period longer RTO to "restart window".
 * This is the first part of cwnd validation mechanism.
 */
void tcp_cwnd_restart(struct sock *sk, s32 delta)
{
	struct tcp_sock *tp = tcp_sk(sk);
	u32 restart_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
	u32 cwnd = tcp_snd_cwnd(tp);

	tcp_ca_event(sk, CA_EVENT_CWND_RESTART);

	tp->snd_ssthresh = tcp_current_ssthresh(sk);
	restart_cwnd = min(restart_cwnd, cwnd);

	while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
		cwnd >>= 1;
	tcp_snd_cwnd_set(tp, max(cwnd, restart_cwnd));
	tp->snd_cwnd_stamp = tcp_jiffies32;
	tp->snd_cwnd_used = 0;
}

/* Congestion state accounting after a packet has been sent. */
static void tcp_event_data_sent(struct tcp_sock *tp,
				struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	const u32 now = tcp_jiffies32;

	if (tcp_packets_in_flight(tp) == 0)
		tcp_ca_event(sk, CA_EVENT_TX_START);

	tp->lsndtime = now;

	/* If it is a reply for ato after last received
	 * packet, increase pingpong count.
	 */
	if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
		inet_csk_inc_pingpong_cnt(sk);
}

/* Account for an ACK we sent. */
static inline void tcp_event_ack_sent(struct sock *sk, u32 rcv_nxt)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (unlikely(tp->compressed_ack)) {
		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
			      tp->compressed_ack);
		tp->compressed_ack = 0;
		if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
			__sock_put(sk);
	}

	if (unlikely(rcv_nxt != tp->rcv_nxt))
		return;  /* Special ACK sent by DCTCP to reflect ECN */
	tcp_dec_quickack_mode(sk);
	inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
}

/* Determine a window scaling and initial window to offer.
 * Based on the assumption that the given amount of space
 * will be offered. Store the results in the tp structure.
 * NOTE: for smooth operation initial space offering should
 * be a multiple of mss if possible. We assume here that mss >= 1.
 * This MUST be enforced by all callers.
 */
void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
			       __u32 *rcv_wnd, __u32 *__window_clamp,
			       int wscale_ok, __u8 *rcv_wscale,
			       __u32 init_rcv_wnd)
{
	unsigned int space = (__space < 0 ? 0 : __space);
	u32 window_clamp = READ_ONCE(*__window_clamp);

	/* If no clamp set the clamp to the max possible scaled window */
	if (window_clamp == 0)
		window_clamp = (U16_MAX << TCP_MAX_WSCALE);
	space = min(window_clamp, space);

	/* Quantize space offering to a multiple of mss if possible. */
	if (space > mss)
		space = rounddown(space, mss);

	/* NOTE: offering an initial window larger than 32767
	 * will break some buggy TCP stacks. If the admin tells us
	 * it is likely we could be speaking with such a buggy stack
	 * we will truncate our initial window offering to 32K-1
	 * unless the remote has sent us a window scaling option,
	 * which we interpret as a sign the remote TCP is not
	 * misinterpreting the window field as a signed quantity.
	 */
	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
		(*rcv_wnd) = min(space, MAX_TCP_WINDOW);
	else
		(*rcv_wnd) = space;

	if (init_rcv_wnd)
		*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);

	*rcv_wscale = 0;
	if (wscale_ok) {
		/* Set window scaling on max possible window */
		space = max_t(u32, space, READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]));
		space = max_t(u32, space, READ_ONCE(sysctl_rmem_max));
		space = min_t(u32, space, window_clamp);
		*rcv_wscale = clamp_t(int, ilog2(space) - 15,
				      0, TCP_MAX_WSCALE);
	}
	/* Set the clamp no higher than max representable value */
	WRITE_ONCE(*__window_clamp,
		   min_t(__u32, U16_MAX << (*rcv_wscale), window_clamp));
}
EXPORT_IPV6_MOD(tcp_select_initial_window);

/* Chose a new window to advertise, update state in tcp_sock for the
 * socket, and return result with RFC1323 scaling applied.  The return
 * value can be stuffed directly into th->window for an outgoing
 * frame.
 */
static u16 tcp_select_window(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct net *net = sock_net(sk);
	u32 old_win = tp->rcv_wnd;
	u32 cur_win, new_win;

	/* Make the window 0 if we failed to queue the data because we
	 * are out of memory.
	 */
	if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) {
		tp->pred_flags = 0;
		tp->rcv_wnd = 0;
		tp->rcv_wup = tp->rcv_nxt;
		return 0;
	}

	cur_win = tcp_receive_window(tp);
	new_win = __tcp_select_window(sk);
	if (new_win < cur_win) {
		/* Danger Will Robinson!
		 * Don't update rcv_wup/rcv_wnd here or else
		 * we will not be able to advertise a zero
		 * window in time.  --DaveM
		 *
		 * Relax Will Robinson.
		 */
		if (!READ_ONCE(net->ipv4.sysctl_tcp_shrink_window) || !tp->rx_opt.rcv_wscale) {
			/* Never shrink the offered window */
			if (new_win == 0)
				NET_INC_STATS(net, LINUX_MIB_TCPWANTZEROWINDOWADV);
			new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
		}
	}

	tp->rcv_wnd = new_win;
	tp->rcv_wup = tp->rcv_nxt;

	/* Make sure we do not exceed the maximum possible
	 * scaled window.
	 */
	if (!tp->rx_opt.rcv_wscale &&
	    READ_ONCE(net->ipv4.sysctl_tcp_workaround_signed_windows))
		new_win = min(new_win, MAX_TCP_WINDOW);
	else
		new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));

	/* RFC1323 scaling applied */
	new_win >>= tp->rx_opt.rcv_wscale;

	/* If we advertise zero window, disable fast path. */
	if (new_win == 0) {
		tp->pred_flags = 0;
		if (old_win)
			NET_INC_STATS(net, LINUX_MIB_TCPTOZEROWINDOWADV);
	} else if (old_win == 0) {
		NET_INC_STATS(net, LINUX_MIB_TCPFROMZEROWINDOWADV);
	}

	return new_win;
}

/* Set up ECN state for a packet on a ESTABLISHED socket that is about to
 * be sent.
 */
static void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
			 struct tcphdr *th, int tcp_header_len)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (!tcp_ecn_mode_any(tp))
		return;

	if (tcp_ecn_mode_accecn(tp)) {
		if (!tcp_accecn_ace_fail_recv(tp) &&
		    !tcp_accecn_ace_fail_send(tp))
			INET_ECN_xmit(sk);
		else
			INET_ECN_dontxmit(sk);
		tcp_accecn_set_ace(tp, skb, th);
		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ACCECN;
	} else {
		/* Not-retransmitted data segment: set ECT and inject CWR. */
		if (skb->len != tcp_header_len &&
		    !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
			INET_ECN_xmit(sk);
			if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
				tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
				th->cwr = 1;
				skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
			}
		} else if (!tcp_ca_needs_ecn(sk)) {
			/* ACK or retransmitted segment: clear ECT|CE */
			INET_ECN_dontxmit(sk);
		}
		if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
			th->ece = 1;
	}
}

/* Constructs common control bits of non-data skb. If SYN/FIN is present,
 * auto increment end seqno.
 */
static void tcp_init_nondata_skb(struct sk_buff *skb, struct sock *sk,
				 u32 seq, u16 flags)
{
	skb->ip_summed = CHECKSUM_PARTIAL;

	TCP_SKB_CB(skb)->tcp_flags = flags;

	tcp_skb_pcount_set(skb, 1);
	psp_enqueue_set_decrypted(sk, skb);

	TCP_SKB_CB(skb)->seq = seq;
	if (flags & (TCPHDR_SYN | TCPHDR_FIN))
		seq++;
	TCP_SKB_CB(skb)->end_seq = seq;
}

static inline bool tcp_urg_mode(const struct tcp_sock *tp)
{
	return tp->snd_una != tp->snd_up;
}

#define OPTION_SACK_ADVERTISE	BIT(0)
#define OPTION_TS		BIT(1)
#define OPTION_MD5		BIT(2)
#define OPTION_WSCALE		BIT(3)
#define OPTION_FAST_OPEN_COOKIE	BIT(8)
#define OPTION_SMC		BIT(9)
#define OPTION_MPTCP		BIT(10)
#define OPTION_AO		BIT(11)
#define OPTION_ACCECN		BIT(12)

static void smc_options_write(__be32 *ptr, u16 *options)
{
#if IS_ENABLED(CONFIG_SMC)
	if (static_branch_unlikely(&tcp_have_smc)) {
		if (unlikely(OPTION_SMC & *options)) {
			*ptr++ = htonl((TCPOPT_NOP  << 24) |
				       (TCPOPT_NOP  << 16) |
				       (TCPOPT_EXP <<  8) |
				       (TCPOLEN_EXP_SMC_BASE));
			*ptr++ = htonl(TCPOPT_SMC_MAGIC);
		}
	}
#endif
}

struct tcp_out_options {
	u16 options;		/* bit field of OPTION_* */
	u16 mss;		/* 0 to disable */
	u8 ws;			/* window scale, 0 to disable */
	u8 num_sack_blocks;	/* number of SACK blocks to include */
	u8 num_accecn_fields:7,	/* number of AccECN fields needed */
	   use_synack_ecn_bytes:1; /* Use synack_ecn_bytes or not */
	u8 hash_size;		/* bytes in hash_location */
	u8 bpf_opt_len;		/* length of BPF hdr option */
	__u8 *hash_location;	/* temporary pointer, overloaded */
	__u32 tsval, tsecr;	/* need to include OPTION_TS */
	struct tcp_fastopen_cookie *fastopen_cookie;	/* Fast open cookie */
	struct mptcp_out_options mptcp;
};

static void mptcp_options_write(struct tcphdr *th, __be32 *ptr,
				struct tcp_sock *tp,
				struct tcp_out_options *opts)
{
#if IS_ENABLED(CONFIG_MPTCP)
	if (unlikely(OPTION_MPTCP & opts->options))
		mptcp_write_options(th, ptr, tp, &opts->mptcp);
#endif
}

#ifdef CONFIG_CGROUP_BPF
static int bpf_skops_write_hdr_opt_arg0(struct sk_buff *skb,
					enum tcp_synack_type synack_type)
{
	if (unlikely(!skb))
		return BPF_WRITE_HDR_TCP_CURRENT_MSS;

	if (unlikely(synack_type == TCP_SYNACK_COOKIE))
		return BPF_WRITE_HDR_TCP_SYNACK_COOKIE;

	return 0;
}

/* req, syn_skb and synack_type are used when writing synack */
static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
				  struct request_sock *req,
				  struct sk_buff *syn_skb,
				  enum tcp_synack_type synack_type,
				  struct tcp_out_options *opts,
				  unsigned int *remaining)
{
	struct bpf_sock_ops_kern sock_ops;
	int err;

	if (likely(!BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
					   BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG)) ||
	    !*remaining)
		return;

	/* *remaining has already been aligned to 4 bytes, so *remaining >= 4 */

	/* init sock_ops */
	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));

	sock_ops.op = BPF_SOCK_OPS_HDR_OPT_LEN_CB;

	if (req) {
		/* The listen "sk" cannot be passed here because
		 * it is not locked.  It would not make too much
		 * sense to do bpf_setsockopt(listen_sk) based
		 * on individual connection request also.
		 *
		 * Thus, "req" is passed here and the cgroup-bpf-progs
		 * of the listen "sk" will be run.
		 *
		 * "req" is also used here for fastopen even the "sk" here is
		 * a fullsock "child" sk.  It is to keep the behavior
		 * consistent between fastopen and non-fastopen on
		 * the bpf programming side.
		 */
		sock_ops.sk = (struct sock *)req;
		sock_ops.syn_skb = syn_skb;
	} else {
		sock_owned_by_me(sk);

		sock_ops.is_fullsock = 1;
		sock_ops.is_locked_tcp_sock = 1;
		sock_ops.sk = sk;
	}

	sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
	sock_ops.remaining_opt_len = *remaining;
	/* tcp_current_mss() does not pass a skb */
	if (skb)
		bpf_skops_init_skb(&sock_ops, skb, 0);

	err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);

	if (err || sock_ops.remaining_opt_len == *remaining)
		return;

	opts->bpf_opt_len = *remaining - sock_ops.remaining_opt_len;
	/* round up to 4 bytes */
	opts->bpf_opt_len = (opts->bpf_opt_len + 3) & ~3;

	*remaining -= opts->bpf_opt_len;
}

static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
				    struct request_sock *req,
				    struct sk_buff *syn_skb,
				    enum tcp_synack_type synack_type,
				    struct tcp_out_options *opts)
{
	u8 first_opt_off, nr_written, max_opt_len = opts->bpf_opt_len;
	struct bpf_sock_ops_kern sock_ops;
	int err;

	if (likely(!max_opt_len))
		return;

	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));

	sock_ops.op = BPF_SOCK_OPS_WRITE_HDR_OPT_CB;

	if (req) {
		sock_ops.sk = (struct sock *)req;
		sock_ops.syn_skb = syn_skb;
	} else {
		sock_owned_by_me(sk);

		sock_ops.is_fullsock = 1;
		sock_ops.is_locked_tcp_sock = 1;
		sock_ops.sk = sk;
	}

	sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
	sock_ops.remaining_opt_len = max_opt_len;
	first_opt_off = tcp_hdrlen(skb) - max_opt_len;
	bpf_skops_init_skb(&sock_ops, skb, first_opt_off);

	err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);

	if (err)
		nr_written = 0;
	else
		nr_written = max_opt_len - sock_ops.remaining_opt_len;

	if (nr_written < max_opt_len)
		memset(skb->data + first_opt_off + nr_written, TCPOPT_NOP,
		       max_opt_len - nr_written);
}
#else
static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
				  struct request_sock *req,
				  struct sk_buff *syn_skb,
				  enum tcp_synack_type synack_type,
				  struct tcp_out_options *opts,
				  unsigned int *remaining)
{
}

static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
				    struct request_sock *req,
				    struct sk_buff *syn_skb,
				    enum tcp_synack_type synack_type,
				    struct tcp_out_options *opts)
{
}
#endif

static __be32 *process_tcp_ao_options(struct tcp_sock *tp,
				      const struct tcp_request_sock *tcprsk,
				      struct tcp_out_options *opts,
				      struct tcp_key *key, __be32 *ptr)
{
#ifdef CONFIG_TCP_AO
	u8 maclen = tcp_ao_maclen(key->ao_key);

	if (tcprsk) {
		u8 aolen = maclen + sizeof(struct tcp_ao_hdr);

		*ptr++ = htonl((TCPOPT_AO << 24) | (aolen << 16) |
			       (tcprsk->ao_keyid << 8) |
			       (tcprsk->ao_rcv_next));
	} else {
		struct tcp_ao_key *rnext_key;
		struct tcp_ao_info *ao_info;

		ao_info = rcu_dereference_check(tp->ao_info,
			lockdep_sock_is_held(&tp->inet_conn.icsk_inet.sk));
		rnext_key = READ_ONCE(ao_info->rnext_key);
		if (WARN_ON_ONCE(!rnext_key))
			return ptr;
		*ptr++ = htonl((TCPOPT_AO << 24) |
			       (tcp_ao_len(key->ao_key) << 16) |
			       (key->ao_key->sndid << 8) |
			       (rnext_key->rcvid));
	}
	opts->hash_location = (__u8 *)ptr;
	ptr += maclen / sizeof(*ptr);
	if (unlikely(maclen % sizeof(*ptr))) {
		memset(ptr, TCPOPT_NOP, sizeof(*ptr));
		ptr++;
	}
#endif
	return ptr;
}

/* Initial values for AccECN option, ordered is based on ECN field bits
 * similar to received_ecn_bytes. Used for SYN/ACK AccECN option.
 */
static const u32 synack_ecn_bytes[3] = { 0, 0, 0 };

/* Write previously computed TCP options to the packet.
 *
 * Beware: Something in the Internet is very sensitive to the ordering of
 * TCP options, we learned this through the hard way, so be careful here.
 * Luckily we can at least blame others for their non-compliance but from
 * inter-operability perspective it seems that we're somewhat stuck with
 * the ordering which we have been using if we want to keep working with
 * those broken things (not that it currently hurts anybody as there isn't
 * particular reason why the ordering would need to be changed).
 *
 * At least SACK_PERM as the first option is known to lead to a disaster
 * (but it may well be that other scenarios fail similarly).
 */
static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
			      const struct tcp_request_sock *tcprsk,
			      struct tcp_out_options *opts,
			      struct tcp_key *key)
{
	u8 leftover_highbyte = TCPOPT_NOP; /* replace 1st NOP if avail */
	u8 leftover_lowbyte = TCPOPT_NOP;  /* replace 2nd NOP in succession */
	__be32 *ptr = (__be32 *)(th + 1);
	u16 options = opts->options;	/* mungable copy */

	if (tcp_key_is_md5(key)) {
		*ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
			       (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
		/* overload cookie hash location */
		opts->hash_location = (__u8 *)ptr;
		ptr += 4;
	} else if (tcp_key_is_ao(key)) {
		ptr = process_tcp_ao_options(tp, tcprsk, opts, key, ptr);
	}
	if (unlikely(opts->mss)) {
		*ptr++ = htonl((TCPOPT_MSS << 24) |
			       (TCPOLEN_MSS << 16) |
			       opts->mss);
	}

	if (likely(OPTION_TS & options)) {
		if (unlikely(OPTION_SACK_ADVERTISE & options)) {
			*ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
				       (TCPOLEN_SACK_PERM << 16) |
				       (TCPOPT_TIMESTAMP << 8) |
				       TCPOLEN_TIMESTAMP);
			options &= ~OPTION_SACK_ADVERTISE;
		} else {
			*ptr++ = htonl((TCPOPT_NOP << 24) |
				       (TCPOPT_NOP << 16) |
				       (TCPOPT_TIMESTAMP << 8) |
				       TCPOLEN_TIMESTAMP);
		}
		*ptr++ = htonl(opts->tsval);
		*ptr++ = htonl(opts->tsecr);
	}

	if (OPTION_ACCECN & options) {
		const u32 *ecn_bytes = opts->use_synack_ecn_bytes ?
				       synack_ecn_bytes :
				       tp->received_ecn_bytes;
		const u8 ect0_idx = INET_ECN_ECT_0 - 1;
		const u8 ect1_idx = INET_ECN_ECT_1 - 1;
		const u8 ce_idx = INET_ECN_CE - 1;
		u32 e0b;
		u32 e1b;
		u32 ceb;
		u8 len;

		e0b = ecn_bytes[ect0_idx] + TCP_ACCECN_E0B_INIT_OFFSET;
		e1b = ecn_bytes[ect1_idx] + TCP_ACCECN_E1B_INIT_OFFSET;
		ceb = ecn_bytes[ce_idx] + TCP_ACCECN_CEB_INIT_OFFSET;
		len = TCPOLEN_ACCECN_BASE +
		      opts->num_accecn_fields * TCPOLEN_ACCECN_PERFIELD;

		if (opts->num_accecn_fields == 2) {
			*ptr++ = htonl((TCPOPT_ACCECN1 << 24) | (len << 16) |
				       ((e1b >> 8) & 0xffff));
			*ptr++ = htonl(((e1b & 0xff) << 24) |
				       (ceb & 0xffffff));
		} else if (opts->num_accecn_fields == 1) {
			*ptr++ = htonl((TCPOPT_ACCECN1 << 24) | (len << 16) |
				       ((e1b >> 8) & 0xffff));
			leftover_highbyte = e1b & 0xff;
			leftover_lowbyte = TCPOPT_NOP;
		} else if (opts->num_accecn_fields == 0) {
			leftover_highbyte = TCPOPT_ACCECN1;
			leftover_lowbyte = len;
		} else if (opts->num_accecn_fields == 3) {
			*ptr++ = htonl((TCPOPT_ACCECN1 << 24) | (len << 16) |
				       ((e1b >> 8) & 0xffff));
			*ptr++ = htonl(((e1b & 0xff) << 24) |
				       (ceb & 0xffffff));
			*ptr++ = htonl(((e0b & 0xffffff) << 8) |
				       TCPOPT_NOP);
		}
		if (tp) {
			tp->accecn_minlen = 0;
			tp->accecn_opt_tstamp = tp->tcp_mstamp;
			tp->accecn_opt_sent_w_dsack = tp->rx_opt.dsack;
			if (tp->accecn_opt_demand)
				tp->accecn_opt_demand--;
		}
	} else if (tp) {
		tp->accecn_opt_sent_w_dsack = 0;
	}

	if (unlikely(OPTION_SACK_ADVERTISE & options)) {
		*ptr++ = htonl((leftover_highbyte << 24) |
			       (leftover_lowbyte << 16) |
			       (TCPOPT_SACK_PERM << 8) |
			       TCPOLEN_SACK_PERM);
		leftover_highbyte = TCPOPT_NOP;
		leftover_lowbyte = TCPOPT_NOP;
	}

	if (unlikely(OPTION_WSCALE & options)) {
		u8 highbyte = TCPOPT_NOP;

		/* Do not split the leftover 2-byte to fit into a single
		 * NOP, i.e., replace this NOP only when 1 byte is leftover
		 * within leftover_highbyte.
		 */
		if (unlikely(leftover_highbyte != TCPOPT_NOP &&
			     leftover_lowbyte == TCPOPT_NOP)) {
			highbyte = leftover_highbyte;
			leftover_highbyte = TCPOPT_NOP;
		}
		*ptr++ = htonl((highbyte << 24) |
			       (TCPOPT_WINDOW << 16) |
			       (TCPOLEN_WINDOW << 8) |
			       opts->ws);
	}

	if (unlikely(opts->num_sack_blocks)) {
		struct tcp_sack_block *sp = tp->rx_opt.dsack ?
			tp->duplicate_sack : tp->selective_acks;
		int this_sack;

		*ptr++ = htonl((leftover_highbyte << 24) |
			       (leftover_lowbyte << 16) |
			       (TCPOPT_SACK <<  8) |
			       (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
						     TCPOLEN_SACK_PERBLOCK)));
		leftover_highbyte = TCPOPT_NOP;
		leftover_lowbyte = TCPOPT_NOP;

		for (this_sack = 0; this_sack < opts->num_sack_blocks;
		     ++this_sack) {
			*ptr++ = htonl(sp[this_sack].start_seq);
			*ptr++ = htonl(sp[this_sack].end_seq);
		}

		tp->rx_opt.dsack = 0;
	} else if (unlikely(leftover_highbyte != TCPOPT_NOP ||
			    leftover_lowbyte != TCPOPT_NOP)) {
		*ptr++ = htonl((leftover_highbyte << 24) |
			       (leftover_lowbyte << 16) |
			       (TCPOPT_NOP << 8) |
			       TCPOPT_NOP);
		leftover_highbyte = TCPOPT_NOP;
		leftover_lowbyte = TCPOPT_NOP;
	}

	if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
		struct tcp_fastopen_cookie *foc = opts->fastopen_cookie;
		u8 *p = (u8 *)ptr;
		u32 len; /* Fast Open option length */

		if (foc->exp) {
			len = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
			*ptr = htonl((TCPOPT_EXP << 24) | (len << 16) |
				     TCPOPT_FASTOPEN_MAGIC);
			p += TCPOLEN_EXP_FASTOPEN_BASE;
		} else {
			len = TCPOLEN_FASTOPEN_BASE + foc->len;
			*p++ = TCPOPT_FASTOPEN;
			*p++ = len;
		}

		memcpy(p, foc->val, foc->len);
		if ((len & 3) == 2) {
			p[foc->len] = TCPOPT_NOP;
			p[foc->len + 1] = TCPOPT_NOP;
		}
		ptr += (len + 3) >> 2;
	}

	smc_options_write(ptr, &options);

	mptcp_options_write(th, ptr, tp, opts);
}

static void smc_set_option(struct tcp_sock *tp,
			   struct tcp_out_options *opts,
			   unsigned int *remaining)
{
#if IS_ENABLED(CONFIG_SMC)
	if (static_branch_unlikely(&tcp_have_smc) && tp->syn_smc) {
		tp->syn_smc = !!smc_call_hsbpf(1, tp, syn_option);
		/* re-check syn_smc */
		if (tp->syn_smc &&
		    *remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
			opts->options |= OPTION_SMC;
			*remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
		}
	}
#endif
}

static void smc_set_option_cond(const struct tcp_sock *tp,
				struct inet_request_sock *ireq,
				struct tcp_out_options *opts,
				unsigned int *remaining)
{
#if IS_ENABLED(CONFIG_SMC)
	if (static_branch_unlikely(&tcp_have_smc) && tp->syn_smc && ireq->smc_ok) {
		ireq->smc_ok = !!smc_call_hsbpf(1, tp, synack_option, ireq);
		/* re-check smc_ok */
		if (ireq->smc_ok &&
		    *remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
			opts->options |= OPTION_SMC;
			*remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
		}
	}
#endif
}

static void mptcp_set_option_cond(const struct request_sock *req,
				  struct tcp_out_options *opts,
				  unsigned int *remaining)
{
	if (rsk_is_mptcp(req)) {
		unsigned int size;

		if (mptcp_synack_options(req, &size, &opts->mptcp)) {
			if (*remaining >= size) {
				opts->options |= OPTION_MPTCP;
				*remaining -= size;
			}
		}
	}
}

static u32 tcp_synack_options_combine_saving(struct tcp_out_options *opts)
{
	/* How much there's room for combining with the alignment padding? */
	if ((opts->options & (OPTION_SACK_ADVERTISE | OPTION_TS)) ==
	    OPTION_SACK_ADVERTISE)
		return 2;
	else if (opts->options & OPTION_WSCALE)
		return 1;
	return 0;
}

/* Calculates how long AccECN option will fit to @remaining option space.
 *
 * AccECN option can sometimes replace NOPs used for alignment of other
 * TCP options (up to @max_combine_saving available).
 *
 * Only solutions with at least @required AccECN fields are accepted.
 *
 * Returns: The size of the AccECN option excluding space repurposed from
 * the alignment of the other options.
 */
static int tcp_options_fit_accecn(struct tcp_out_options *opts, int required,
				  int remaining)
{
	int size = TCP_ACCECN_MAXSIZE;
	int sack_blocks_reduce = 0;
	int max_combine_saving;
	int rem = remaining;
	int align_size;

	if (opts->use_synack_ecn_bytes)
		max_combine_saving = tcp_synack_options_combine_saving(opts);
	else
		max_combine_saving = opts->num_sack_blocks > 0 ? 2 : 0;
	opts->num_accecn_fields = TCP_ACCECN_NUMFIELDS;
	while (opts->num_accecn_fields >= required) {
		/* Pad to dword if cannot combine */
		if ((size & 0x3) > max_combine_saving)
			align_size = ALIGN(size, 4);
		else
			align_size = ALIGN_DOWN(size, 4);

		if (rem >= align_size) {
			size = align_size;
			break;
		} else if (opts->num_accecn_fields == required &&
			   opts->num_sack_blocks > 2 &&
			   required > 0) {
			/* Try to fit the option by removing one SACK block */
			opts->num_sack_blocks--;
			sack_blocks_reduce++;
			rem = rem + TCPOLEN_SACK_PERBLOCK;

			opts->num_accecn_fields = TCP_ACCECN_NUMFIELDS;
			size = TCP_ACCECN_MAXSIZE;
			continue;
		}

		opts->num_accecn_fields--;
		size -= TCPOLEN_ACCECN_PERFIELD;
	}
	if (sack_blocks_reduce > 0) {
		if (opts->num_accecn_fields >= required)
			size -= sack_blocks_reduce * TCPOLEN_SACK_PERBLOCK;
		else
			opts->num_sack_blocks += sack_blocks_reduce;
	}
	if (opts->num_accecn_fields < required)
		return 0;

	opts->options |= OPTION_ACCECN;
	return size;
}

/* Compute TCP options for SYN packets. This is not the final
 * network wire format yet.
 */
static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
				struct tcp_out_options *opts,
				struct tcp_key *key)
{
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned int remaining = MAX_TCP_OPTION_SPACE;
	struct tcp_fastopen_request *fastopen = tp->fastopen_req;
	bool timestamps;

	/* Better than switch (key.type) as it has static branches */
	if (tcp_key_is_md5(key)) {
		timestamps = false;
		opts->options |= OPTION_MD5;
		remaining -= TCPOLEN_MD5SIG_ALIGNED;
	} else {
		timestamps = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps);
		if (tcp_key_is_ao(key)) {
			opts->options |= OPTION_AO;
			remaining -= tcp_ao_len_aligned(key->ao_key);
		}
	}

	/* We always get an MSS option.  The option bytes which will be seen in
	 * normal data packets should timestamps be used, must be in the MSS
	 * advertised.  But we subtract them from tp->mss_cache so that
	 * calculations in tcp_sendmsg are simpler etc.  So account for this
	 * fact here if necessary.  If we don't do this correctly, as a
	 * receiver we won't recognize data packets as being full sized when we
	 * should, and thus we won't abide by the delayed ACK rules correctly.
	 * SACKs don't matter, we never delay an ACK when we have any of those
	 * going out.  */
	opts->mss = tcp_advertise_mss(sk);
	remaining -= TCPOLEN_MSS_ALIGNED;

	if (likely(timestamps)) {
		opts->options |= OPTION_TS;
		opts->tsval = tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb) + tp->tsoffset;
		opts->tsecr = tp->rx_opt.ts_recent;
		remaining -= TCPOLEN_TSTAMP_ALIGNED;
	}
	if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling))) {
		opts->ws = tp->rx_opt.rcv_wscale;
		opts->options |= OPTION_WSCALE;
		remaining -= TCPOLEN_WSCALE_ALIGNED;
	}
	if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_sack))) {
		opts->options |= OPTION_SACK_ADVERTISE;
		if (unlikely(!(OPTION_TS & opts->options)))
			remaining -= TCPOLEN_SACKPERM_ALIGNED;
	}

	if (fastopen && fastopen->cookie.len >= 0) {
		u32 need = fastopen->cookie.len;

		need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
					       TCPOLEN_FASTOPEN_BASE;
		need = (need + 3) & ~3U;  /* Align to 32 bits */
		if (remaining >= need) {
			opts->options |= OPTION_FAST_OPEN_COOKIE;
			opts->fastopen_cookie = &fastopen->cookie;
			remaining -= need;
			tp->syn_fastopen = 1;
			tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
		}
	}

	smc_set_option(tp, opts, &remaining);

	if (sk_is_mptcp(sk)) {
		unsigned int size;

		if (mptcp_syn_options(sk, skb, &size, &opts->mptcp)) {
			if (remaining >= size) {
				opts->options |= OPTION_MPTCP;
				remaining -= size;
			}
		}
	}

	/* Simultaneous open SYN/ACK needs AccECN option but not SYN.
	 * It is attempted to negotiate the use of AccECN also on the first
	 * retransmitted SYN, as mentioned in "3.1.4.1. Retransmitted SYNs"
	 * of AccECN draft.
	 */
	if (unlikely((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK) &&
		     tcp_ecn_mode_accecn(tp) &&
		     inet_csk(sk)->icsk_retransmits < 2 &&
		     READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option) &&
		     remaining >= TCPOLEN_ACCECN_BASE)) {
		opts->use_synack_ecn_bytes = 1;
		remaining -= tcp_options_fit_accecn(opts, 0, remaining);
	}

	bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);

	return MAX_TCP_OPTION_SPACE - remaining;
}

/* Set up TCP options for SYN-ACKs. */
static unsigned int tcp_synack_options(const struct sock *sk,
				       struct request_sock *req,
				       unsigned int mss, struct sk_buff *skb,
				       struct tcp_out_options *opts,
				       const struct tcp_key *key,
				       struct tcp_fastopen_cookie *foc,
				       enum tcp_synack_type synack_type,
				       struct sk_buff *syn_skb)
{
	struct inet_request_sock *ireq = inet_rsk(req);
	unsigned int remaining = MAX_TCP_OPTION_SPACE;
	struct tcp_request_sock *treq = tcp_rsk(req);

	if (tcp_key_is_md5(key)) {
		opts->options |= OPTION_MD5;
		remaining -= TCPOLEN_MD5SIG_ALIGNED;

		/* We can't fit any SACK blocks in a packet with MD5 + TS
		 * options. There was discussion about disabling SACK
		 * rather than TS in order to fit in better with old,
		 * buggy kernels, but that was deemed to be unnecessary.
		 */
		if (synack_type != TCP_SYNACK_COOKIE)
			ireq->tstamp_ok &= !ireq->sack_ok;
	} else if (tcp_key_is_ao(key)) {
		opts->options |= OPTION_AO;
		remaining -= tcp_ao_len_aligned(key->ao_key);
		ireq->tstamp_ok &= !ireq->sack_ok;
	}

	/* We always send an MSS option. */
	opts->mss = mss;
	remaining -= TCPOLEN_MSS_ALIGNED;

	if (likely(ireq->wscale_ok)) {
		opts->ws = ireq->rcv_wscale;
		opts->options |= OPTION_WSCALE;
		remaining -= TCPOLEN_WSCALE_ALIGNED;
	}
	if (likely(ireq->tstamp_ok)) {
		opts->options |= OPTION_TS;
		opts->tsval = tcp_skb_timestamp_ts(tcp_rsk(req)->req_usec_ts, skb) +
			      tcp_rsk(req)->ts_off;
		if (!tcp_rsk(req)->snt_tsval_first) {
			if (!opts->tsval)
				opts->tsval = ~0U;
			tcp_rsk(req)->snt_tsval_first = opts->tsval;
		}
		WRITE_ONCE(tcp_rsk(req)->snt_tsval_last, opts->tsval);
		opts->tsecr = req->ts_recent;
		remaining -= TCPOLEN_TSTAMP_ALIGNED;
	}
	if (likely(ireq->sack_ok)) {
		opts->options |= OPTION_SACK_ADVERTISE;
		if (unlikely(!ireq->tstamp_ok))
			remaining -= TCPOLEN_SACKPERM_ALIGNED;
	}
	if (foc != NULL && foc->len >= 0) {
		u32 need = foc->len;

		need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
				   TCPOLEN_FASTOPEN_BASE;
		need = (need + 3) & ~3U;  /* Align to 32 bits */
		if (remaining >= need) {
			opts->options |= OPTION_FAST_OPEN_COOKIE;
			opts->fastopen_cookie = foc;
			remaining -= need;
		}
	}

	mptcp_set_option_cond(req, opts, &remaining);

	smc_set_option_cond(tcp_sk(sk), ireq, opts, &remaining);

	if (treq->accecn_ok &&
	    READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option) &&
	    synack_type != TCP_SYNACK_RETRANS && remaining >= TCPOLEN_ACCECN_BASE) {
		opts->use_synack_ecn_bytes = 1;
		remaining -= tcp_options_fit_accecn(opts, 0, remaining);
	}

	bpf_skops_hdr_opt_len((struct sock *)sk, skb, req, syn_skb,
			      synack_type, opts, &remaining);

	return MAX_TCP_OPTION_SPACE - remaining;
}

/* Compute TCP options for ESTABLISHED sockets. This is not the
 * final wire format yet.
 */
static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb,
					struct tcp_out_options *opts,
					struct tcp_key *key)
{
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned int size = 0;
	unsigned int eff_sacks;

	opts->options = 0;

	/* Better than switch (key.type) as it has static branches */
	if (tcp_key_is_md5(key)) {
		opts->options |= OPTION_MD5;
		size += TCPOLEN_MD5SIG_ALIGNED;
	} else if (tcp_key_is_ao(key)) {
		opts->options |= OPTION_AO;
		size += tcp_ao_len_aligned(key->ao_key);
	}

	if (likely(tp->rx_opt.tstamp_ok)) {
		opts->options |= OPTION_TS;
		opts->tsval = skb ? tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb) +
				tp->tsoffset : 0;
		opts->tsecr = tp->rx_opt.ts_recent;
		size += TCPOLEN_TSTAMP_ALIGNED;
	}

	/* MPTCP options have precedence over SACK for the limited TCP
	 * option space because a MPTCP connection would be forced to
	 * fall back to regular TCP if a required multipath option is
	 * missing. SACK still gets a chance to use whatever space is
	 * left.
	 */
	if (sk_is_mptcp(sk)) {
		unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
		unsigned int opt_size = 0;

		if (mptcp_established_options(sk, skb, &opt_size, remaining,
					      &opts->mptcp)) {
			opts->options |= OPTION_MPTCP;
			size += opt_size;
		}
	}

	eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
	if (unlikely(eff_sacks)) {
		const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
		if (likely(remaining >= TCPOLEN_SACK_BASE_ALIGNED +
					TCPOLEN_SACK_PERBLOCK)) {
			opts->num_sack_blocks =
				min_t(unsigned int, eff_sacks,
				      (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
				      TCPOLEN_SACK_PERBLOCK);

			size += TCPOLEN_SACK_BASE_ALIGNED +
				opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
		} else {
			opts->num_sack_blocks = 0;
		}
	} else {
		opts->num_sack_blocks = 0;
	}

	if (tcp_ecn_mode_accecn(tp)) {
		int ecn_opt = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option);

		if (ecn_opt && tp->saw_accecn_opt &&
		    (ecn_opt >= TCP_ACCECN_OPTION_PERSIST ||
		     !tcp_accecn_opt_fail_send(tp)) &&
		    (ecn_opt >= TCP_ACCECN_OPTION_FULL || tp->accecn_opt_demand ||
		     tcp_accecn_option_beacon_check(sk))) {
			opts->use_synack_ecn_bytes = 0;
			size += tcp_options_fit_accecn(opts, tp->accecn_minlen,
						       MAX_TCP_OPTION_SPACE - size);
		}
	}

	if (unlikely(BPF_SOCK_OPS_TEST_FLAG(tp,
					    BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG))) {
		unsigned int remaining = MAX_TCP_OPTION_SPACE - size;

		bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);

		size = MAX_TCP_OPTION_SPACE - remaining;
	}

	return size;
}


/* TCP SMALL QUEUES (TSQ)
 *
 * TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
 * to reduce RTT and bufferbloat.
 * We do this using a special skb destructor (tcp_wfree).
 *
 * Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
 * needs to be reallocated in a driver.
 * The invariant being skb->truesize subtracted from sk->sk_wmem_alloc
 *
 * Since transmit from skb destructor is forbidden, we use a BH work item
 * to process all sockets that eventually need to send more skbs.
 * We use one work item per cpu, with its own queue of sockets.
 */
struct tsq_work {
	struct work_struct	work;
	struct list_head	head; /* queue of tcp sockets */
};
static DEFINE_PER_CPU(struct tsq_work, tsq_work);

static void tcp_tsq_write(struct sock *sk)
{
	if ((1 << sk->sk_state) &
	    (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |
	     TCPF_CLOSE_WAIT  | TCPF_LAST_ACK)) {
		struct tcp_sock *tp = tcp_sk(sk);

		if (tp->lost_out > tp->retrans_out &&
		    tcp_snd_cwnd(tp) > tcp_packets_in_flight(tp)) {
			tcp_mstamp_refresh(tp);
			tcp_xmit_retransmit_queue(sk);
		}

		tcp_write_xmit(sk, tcp_current_mss(sk), tp->nonagle,
			       0, GFP_ATOMIC);
	}
}

static void tcp_tsq_handler(struct sock *sk)
{
	bh_lock_sock(sk);
	if (!sock_owned_by_user(sk))
		tcp_tsq_write(sk);
	else if (!test_and_set_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags))
		sock_hold(sk);
	bh_unlock_sock(sk);
}
/*
 * One work item per cpu tries to send more skbs.
 * We run in BH context but need to disable irqs when
 * transferring tsq->head because tcp_wfree() might
 * interrupt us (non NAPI drivers)
 */
static void tcp_tsq_workfn(struct work_struct *work)
{
	struct tsq_work *tsq = container_of(work, struct tsq_work, work);
	LIST_HEAD(list);
	unsigned long flags;
	struct list_head *q, *n;
	struct tcp_sock *tp;
	struct sock *sk;

	local_irq_save(flags);
	list_splice_init(&tsq->head, &list);
	local_irq_restore(flags);

	list_for_each_safe(q, n, &list) {
		tp = list_entry(q, struct tcp_sock, tsq_node);
		list_del(&tp->tsq_node);

		sk = (struct sock *)tp;
		smp_mb__before_atomic();
		clear_bit(TSQ_QUEUED, &sk->sk_tsq_flags);

		tcp_tsq_handler(sk);
		sk_free(sk);
	}
}

#define TCP_DEFERRED_ALL (TCPF_TSQ_DEFERRED |		\
			  TCPF_WRITE_TIMER_DEFERRED |	\
			  TCPF_DELACK_TIMER_DEFERRED |	\
			  TCPF_MTU_REDUCED_DEFERRED |	\
			  TCPF_ACK_DEFERRED)
/**
 * tcp_release_cb - tcp release_sock() callback
 * @sk: socket
 *
 * called from release_sock() to perform protocol dependent
 * actions before socket release.
 */
void tcp_release_cb(struct sock *sk)
{
	unsigned long flags = smp_load_acquire(&sk->sk_tsq_flags);
	unsigned long nflags;

	/* perform an atomic operation only if at least one flag is set */
	do {
		if (!(flags & TCP_DEFERRED_ALL))
			return;
		nflags = flags & ~TCP_DEFERRED_ALL;
	} while (!try_cmpxchg(&sk->sk_tsq_flags, &flags, nflags));

	if (flags & TCPF_TSQ_DEFERRED) {
		tcp_tsq_write(sk);
		__sock_put(sk);
	}

	if (flags & TCPF_WRITE_TIMER_DEFERRED) {
		tcp_write_timer_handler(sk);
		__sock_put(sk);
	}
	if (flags & TCPF_DELACK_TIMER_DEFERRED) {
		tcp_delack_timer_handler(sk);
		__sock_put(sk);
	}
	if (flags & TCPF_MTU_REDUCED_DEFERRED) {
		inet_csk(sk)->icsk_af_ops->mtu_reduced(sk);
		__sock_put(sk);
	}
	if ((flags & TCPF_ACK_DEFERRED) && inet_csk_ack_scheduled(sk))
		tcp_send_ack(sk);
}
EXPORT_IPV6_MOD(tcp_release_cb);

void __init tcp_tsq_work_init(void)
{
	int i;

	for_each_possible_cpu(i) {
		struct tsq_work *tsq = &per_cpu(tsq_work, i);

		INIT_LIST_HEAD(&tsq->head);
		INIT_WORK(&tsq->work, tcp_tsq_workfn);
	}
}

/*
 * Write buffer destructor automatically called from kfree_skb.
 * We can't xmit new skbs from this context, as we might already
 * hold qdisc lock.
 */
void tcp_wfree(struct sk_buff *skb)
{
	struct sock *sk = skb->sk;
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned long flags, nval, oval;
	struct tsq_work *tsq;
	bool empty;

	/* Keep one reference on sk_wmem_alloc.
	 * Will be released by sk_free() from here or tcp_tsq_workfn()
	 */
	WARN_ON(refcount_sub_and_test(skb->truesize - 1, &sk->sk_wmem_alloc));

	/* If this softirq is serviced by ksoftirqd, we are likely under stress.
	 * Wait until our queues (qdisc + devices) are drained.
	 * This gives :
	 * - less callbacks to tcp_write_xmit(), reducing stress (batches)
	 * - chance for incoming ACK (processed by another cpu maybe)
	 *   to migrate this flow (skb->ooo_okay will be eventually set)
	 */
	if (refcount_read(&sk->sk_wmem_alloc) >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
		goto out;

	oval = smp_load_acquire(&sk->sk_tsq_flags);
	do {
		if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
			goto out;

		nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED;
	} while (!try_cmpxchg(&sk->sk_tsq_flags, &oval, nval));

	/* queue this socket to BH workqueue */
	local_irq_save(flags);
	tsq = this_cpu_ptr(&tsq_work);
	empty = list_empty(&tsq->head);
	list_add(&tp->tsq_node, &tsq->head);
	if (empty)
		queue_work(system_bh_wq, &tsq->work);
	local_irq_restore(flags);
	return;
out:
	sk_free(sk);
}

/* Note: Called under soft irq.
 * We can call TCP stack right away, unless socket is owned by user.
 */
enum hrtimer_restart tcp_pace_kick(struct hrtimer *timer)
{
	struct tcp_sock *tp = container_of(timer, struct tcp_sock, pacing_timer);
	struct sock *sk = (struct sock *)tp;

	tcp_tsq_handler(sk);
	sock_put(sk);

	return HRTIMER_NORESTART;
}

static void tcp_update_skb_after_send(struct sock *sk, struct sk_buff *skb,
				      u64 prior_wstamp)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (sk->sk_pacing_status != SK_PACING_NONE) {
		unsigned long rate = READ_ONCE(sk->sk_pacing_rate);

		/* Original sch_fq does not pace first 10 MSS
		 * Note that tp->data_segs_out overflows after 2^32 packets,
		 * this is a minor annoyance.
		 */
		if (rate != ~0UL && rate && tp->data_segs_out >= 10) {
			u64 len_ns = div64_ul((u64)skb->len * NSEC_PER_SEC, rate);
			u64 credit = tp->tcp_wstamp_ns - prior_wstamp;

			/* take into account OS jitter */
			len_ns -= min_t(u64, len_ns / 2, credit);
			tp->tcp_wstamp_ns += len_ns;
		}
	}
	list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
}

/* Snapshot the current delivery information in the skb, to generate
 * a rate sample later when the skb is (s)acked in tcp_rate_skb_delivered().
 */
static void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);

	 /* In general we need to start delivery rate samples from the
	  * time we received the most recent ACK, to ensure we include
	  * the full time the network needs to deliver all in-flight
	  * packets. If there are no packets in flight yet, then we
	  * know that any ACKs after now indicate that the network was
	  * able to deliver those packets completely in the sampling
	  * interval between now and the next ACK.
	  *
	  * Note that we use packets_out instead of tcp_packets_in_flight(tp)
	  * because the latter is a guess based on RTO and loss-marking
	  * heuristics. We don't want spurious RTOs or loss markings to cause
	  * a spuriously small time interval, causing a spuriously high
	  * bandwidth estimate.
	  */
	if (!tp->packets_out) {
		u64 tstamp_us = tcp_skb_timestamp_us(skb);

		tp->first_tx_mstamp  = tstamp_us;
		tp->delivered_mstamp = tstamp_us;
	}

	TCP_SKB_CB(skb)->tx.first_tx_mstamp	= tp->first_tx_mstamp;
	TCP_SKB_CB(skb)->tx.delivered_mstamp	= tp->delivered_mstamp;
	TCP_SKB_CB(skb)->tx.delivered		= tp->delivered;
	TCP_SKB_CB(skb)->tx.delivered_ce	= tp->delivered_ce;
	TCP_SKB_CB(skb)->tx.is_app_limited	= tp->app_limited ? 1 : 0;
}

INDIRECT_CALLABLE_DECLARE(int ip_queue_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
INDIRECT_CALLABLE_DECLARE(int inet6_csk_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));

/* This routine computes an IPv4 TCP checksum. */
static void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb)
{
	const struct inet_sock *inet = inet_sk(sk);

	__tcp_v4_send_check(skb, inet->inet_saddr, inet->inet_daddr);
}

#if IS_ENABLED(CONFIG_IPV6)
#include <net/ip6_checksum.h>

static void tcp_v6_send_check(struct sock *sk, struct sk_buff *skb)
{
	__tcp_v6_send_check(skb, &sk->sk_v6_rcv_saddr, &sk->sk_v6_daddr);
}
#endif

/* This routine actually transmits TCP packets queued in by
 * tcp_do_sendmsg().  This is used by both the initial
 * transmission and possible later retransmissions.
 * All SKB's seen here are completely headerless.  It is our
 * job to build the TCP header, and pass the packet down to
 * IP so it can do the same plus pass the packet off to the
 * device.
 *
 * We are working here with either a clone of the original
 * SKB, or a fresh unique copy made by the retransmit engine.
 */
static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
			      int clone_it, gfp_t gfp_mask, u32 rcv_nxt)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);
	struct inet_sock *inet;
	struct tcp_sock *tp;
	struct tcp_skb_cb *tcb;
	struct tcp_out_options opts;
	unsigned int tcp_options_size, tcp_header_size;
	struct sk_buff *oskb = NULL;
	struct tcp_key key;
	struct tcphdr *th;
	u64 prior_wstamp;
	int err;

	BUG_ON(!skb || !tcp_skb_pcount(skb));
	tp = tcp_sk(sk);
	prior_wstamp = tp->tcp_wstamp_ns;
	tp->tcp_wstamp_ns = max(tp->tcp_wstamp_ns, tp->tcp_clock_cache);
	skb_set_delivery_time(skb, tp->tcp_wstamp_ns, SKB_CLOCK_MONOTONIC);
	if (clone_it) {
		oskb = skb;

		tcp_skb_tsorted_save(oskb) {
			if (unlikely(skb_cloned(oskb)))
				skb = pskb_copy(oskb, gfp_mask);
			else
				skb = skb_clone(oskb, gfp_mask);
		} tcp_skb_tsorted_restore(oskb);

		if (unlikely(!skb))
			return -ENOBUFS;
		/* retransmit skbs might have a non zero value in skb->dev
		 * because skb->dev is aliased with skb->rbnode.rb_left
		 */
		skb->dev = NULL;
	}

	inet = inet_sk(sk);
	tcb = TCP_SKB_CB(skb);
	memset(&opts, 0, sizeof(opts));

	tcp_get_current_key(sk, &key);
	if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
		tcp_options_size = tcp_syn_options(sk, skb, &opts, &key);
	} else {
		tcp_options_size = tcp_established_options(sk, skb, &opts, &key);
		/* Force a PSH flag on all (GSO) packets to expedite GRO flush
		 * at receiver : This slightly improve GRO performance.
		 * Note that we do not force the PSH flag for non GSO packets,
		 * because they might be sent under high congestion events,
		 * and in this case it is better to delay the delivery of 1-MSS
		 * packets and thus the corresponding ACK packet that would
		 * release the following packet.
		 */
		if (tcp_skb_pcount(skb) > 1)
			tcb->tcp_flags |= TCPHDR_PSH;
	}
	tcp_header_size = tcp_options_size + sizeof(struct tcphdr);

	/* We set skb->ooo_okay to one if this packet can select
	 * a different TX queue than prior packets of this flow,
	 * to avoid self inflicted reorders.
	 * The 'other' queue decision is based on current cpu number
	 * if XPS is enabled, or sk->sk_txhash otherwise.
	 * We can switch to another (and better) queue if:
	 * 1) No packet with payload is in qdisc/device queues.
	 *    Delays in TX completion can defeat the test
	 *    even if packets were already sent.
	 * 2) Or rtx queue is empty.
	 *    This mitigates above case if ACK packets for
	 *    all prior packets were already processed.
	 */
	skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) ||
			tcp_rtx_queue_empty(sk);

	/* If we had to use memory reserve to allocate this skb,
	 * this might cause drops if packet is looped back :
	 * Other socket might not have SOCK_MEMALLOC.
	 * Packets not looped back do not care about pfmemalloc.
	 */
	skb->pfmemalloc = 0;

	__skb_push(skb, tcp_header_size);
	skb_reset_transport_header(skb);

	skb_orphan(skb);
	skb->sk = sk;
	skb->destructor = skb_is_tcp_pure_ack(skb) ? __sock_wfree : tcp_wfree;
	refcount_add(skb->truesize, &sk->sk_wmem_alloc);

	skb_set_dst_pending_confirm(skb, READ_ONCE(sk->sk_dst_pending_confirm));

	/* Build TCP header and checksum it. */
	th = (struct tcphdr *)skb->data;
	th->source		= inet->inet_sport;
	th->dest		= inet->inet_dport;
	th->seq			= htonl(tcb->seq);
	th->ack_seq		= htonl(rcv_nxt);
	*(((__be16 *)th) + 6)	= htons(((tcp_header_size >> 2) << 12) |
					(tcb->tcp_flags & TCPHDR_FLAGS_MASK));

	th->check		= 0;
	th->urg_ptr		= 0;

	/* The urg_mode check is necessary during a below snd_una win probe */
	if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) {
		if (before(tp->snd_up, tcb->seq + 0x10000)) {
			th->urg_ptr = htons(tp->snd_up - tcb->seq);
			th->urg = 1;
		} else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
			th->urg_ptr = htons(0xFFFF);
			th->urg = 1;
		}
	}

	skb_shinfo(skb)->gso_type = sk->sk_gso_type;
	if (likely(!(tcb->tcp_flags & TCPHDR_SYN))) {
		th->window      = htons(tcp_select_window(sk));
		tcp_ecn_send(sk, skb, th, tcp_header_size);
	} else {
		/* RFC1323: The window in SYN & SYN/ACK segments
		 * is never scaled.
		 */
		th->window	= htons(min(tp->rcv_wnd, 65535U));
	}

	tcp_options_write(th, tp, NULL, &opts, &key);

	if (tcp_key_is_md5(&key)) {
#ifdef CONFIG_TCP_MD5SIG
		/* Calculate the MD5 hash, as we have all we need now */
		sk_gso_disable(sk);
		tp->af_specific->calc_md5_hash(opts.hash_location,
					       key.md5_key, sk, skb);
#endif
	} else if (tcp_key_is_ao(&key)) {
		int err;

		err = tcp_ao_transmit_skb(sk, skb, key.ao_key, th,
					  opts.hash_location);
		if (err) {
			sk_skb_reason_drop(sk, skb, SKB_DROP_REASON_NOT_SPECIFIED);
			return -ENOMEM;
		}
	}

	/* BPF prog is the last one writing header option */
	bpf_skops_write_hdr_opt(sk, skb, NULL, NULL, 0, &opts);

#if IS_ENABLED(CONFIG_IPV6)
	if (likely(icsk->icsk_af_ops->net_header_len == sizeof(struct ipv6hdr)))
		tcp_v6_send_check(sk, skb);
	else
#endif
		tcp_v4_send_check(sk, skb);

	if (likely(tcb->tcp_flags & TCPHDR_ACK))
		tcp_event_ack_sent(sk, rcv_nxt);

	if (skb->len != tcp_header_size) {
		tcp_event_data_sent(tp, sk);
		tp->data_segs_out += tcp_skb_pcount(skb);
		tp->bytes_sent += skb->len - tcp_header_size;
	}

	if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
		TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
			      tcp_skb_pcount(skb));

	tp->segs_out += tcp_skb_pcount(skb);
	skb_set_hash_from_sk(skb, sk);
	/* OK, its time to fill skb_shinfo(skb)->gso_{segs|size} */
	skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
	skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);

	/* Leave earliest departure time in skb->tstamp (skb->skb_mstamp_ns) */

	/* Cleanup our debris for IP stacks */
	memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
			       sizeof(struct inet6_skb_parm)));

	tcp_add_tx_delay(skb, tp);

	err = INDIRECT_CALL_INET(icsk->icsk_af_ops->queue_xmit,
				 inet6_csk_xmit, ip_queue_xmit,
				 sk, skb, &inet->cork.fl);

	if (unlikely(err > 0)) {
		tcp_enter_cwr(sk);
		err = net_xmit_eval(err);
	}
	if (!err && oskb) {
		tcp_update_skb_after_send(sk, oskb, prior_wstamp);
		tcp_rate_skb_sent(sk, oskb);
	}
	return err;
}

static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
			    gfp_t gfp_mask)
{
	return __tcp_transmit_skb(sk, skb, clone_it, gfp_mask,
				  tcp_sk(sk)->rcv_nxt);
}

/* This routine just queues the buffer for sending.
 *
 * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
 * otherwise socket can stall.
 */
static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);

	/* Advance write_seq and place onto the write_queue. */
	WRITE_ONCE(tp->write_seq, TCP_SKB_CB(skb)->end_seq);
	__skb_header_release(skb);
	psp_enqueue_set_decrypted(sk, skb);
	tcp_add_write_queue_tail(sk, skb);
	sk_wmem_queued_add(sk, skb->truesize);
	sk_mem_charge(sk, skb->truesize);
}

/* Initialize TSO segments for a packet. */
static int tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
{
	int tso_segs;

	if (skb->len <= mss_now) {
		/* Avoid the costly divide in the normal
		 * non-TSO case.
		 */
		TCP_SKB_CB(skb)->tcp_gso_size = 0;
		tcp_skb_pcount_set(skb, 1);
		return 1;
	}
	TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
	tso_segs = DIV_ROUND_UP(skb->len, mss_now);
	tcp_skb_pcount_set(skb, tso_segs);
	return tso_segs;
}

/* Pcount in the middle of the write queue got changed, we need to do various
 * tweaks to fix counters
 */
static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int decr)
{
	struct tcp_sock *tp = tcp_sk(sk);

	tp->packets_out -= decr;

	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
		tp->sacked_out -= decr;
	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
		tp->retrans_out -= decr;
	if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
		tp->lost_out -= decr;

	/* Reno case is special. Sigh... */
	if (tcp_is_reno(tp) && decr > 0)
		tp->sacked_out -= min_t(u32, tp->sacked_out, decr);

	tcp_verify_left_out(tp);
}

static bool tcp_has_tx_tstamp(const struct sk_buff *skb)
{
	return TCP_SKB_CB(skb)->txstamp_ack ||
		(skb_shinfo(skb)->tx_flags & SKBTX_ANY_TSTAMP);
}

static void tcp_fragment_tstamp(struct sk_buff *skb, struct sk_buff *skb2)
{
	struct skb_shared_info *shinfo = skb_shinfo(skb);

	if (unlikely(tcp_has_tx_tstamp(skb)) &&
	    !before(shinfo->tskey, TCP_SKB_CB(skb2)->seq)) {
		struct skb_shared_info *shinfo2 = skb_shinfo(skb2);
		u8 tsflags = shinfo->tx_flags & SKBTX_ANY_TSTAMP;

		shinfo->tx_flags &= ~tsflags;
		shinfo2->tx_flags |= tsflags;
		swap(shinfo->tskey, shinfo2->tskey);
		TCP_SKB_CB(skb2)->txstamp_ack = TCP_SKB_CB(skb)->txstamp_ack;
		TCP_SKB_CB(skb)->txstamp_ack = 0;
	}
}

static void tcp_skb_fragment_eor(struct sk_buff *skb, struct sk_buff *skb2)
{
	TCP_SKB_CB(skb2)->eor = TCP_SKB_CB(skb)->eor;
	TCP_SKB_CB(skb)->eor = 0;
}

/* Insert buff after skb on the write or rtx queue of sk.  */
static void tcp_insert_write_queue_after(struct sk_buff *skb,
					 struct sk_buff *buff,
					 struct sock *sk,
					 enum tcp_queue tcp_queue)
{
	if (tcp_queue == TCP_FRAG_IN_WRITE_QUEUE)
		__skb_queue_after(&sk->sk_write_queue, skb, buff);
	else
		tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);
}

/* Function to create two new TCP segments.  Shrinks the given segment
 * to the specified size and appends a new segment with the rest of the
 * packet to the list.  This won't be called frequently, I hope.
 * Remember, these are still headerless SKBs at this point.
 */
int tcp_fragment(struct sock *sk, enum tcp_queue tcp_queue,
		 struct sk_buff *skb, u32 len,
		 unsigned int mss_now, gfp_t gfp)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *buff;
	int old_factor;
	long limit;
	u16 flags;
	int nlen;

	if (WARN_ON(len > skb->len))
		return -EINVAL;

	DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));

	/* tcp_sendmsg() can overshoot sk_wmem_queued by one full size skb.
	 * We need some allowance to not penalize applications setting small
	 * SO_SNDBUF values.
	 * Also allow first and last skb in retransmit queue to be split.
	 */
	limit = sk->sk_sndbuf + 2 * SKB_TRUESIZE(GSO_LEGACY_MAX_SIZE);
	if (unlikely((sk->sk_wmem_queued >> 1) > limit &&
		     tcp_queue != TCP_FRAG_IN_WRITE_QUEUE &&
		     skb != tcp_rtx_queue_head(sk) &&
		     skb != tcp_rtx_queue_tail(sk))) {
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
		return -ENOMEM;
	}

	if (skb_unclone_keeptruesize(skb, gfp))
		return -ENOMEM;

	/* Get a new skb... force flag on. */
	buff = tcp_stream_alloc_skb(sk, gfp, true);
	if (!buff)
		return -ENOMEM; /* We'll just try again later. */
	skb_copy_decrypted(buff, skb);
	mptcp_skb_ext_copy(buff, skb);

	sk_wmem_queued_add(sk, buff->truesize);
	sk_mem_charge(sk, buff->truesize);
	nlen = skb->len - len;
	buff->truesize += nlen;
	skb->truesize -= nlen;

	/* Correct the sequence numbers. */
	TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
	TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

	/* PSH and FIN should only be set in the second packet. */
	flags = TCP_SKB_CB(skb)->tcp_flags;
	TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
	TCP_SKB_CB(buff)->tcp_flags = flags;
	TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;
	tcp_skb_fragment_eor(skb, buff);

	skb_split(skb, buff, len);

	skb_set_delivery_time(buff, skb->tstamp, SKB_CLOCK_MONOTONIC);
	tcp_fragment_tstamp(skb, buff);

	old_factor = tcp_skb_pcount(skb);

	/* Fix up tso_factor for both original and new SKB.  */
	tcp_set_skb_tso_segs(skb, mss_now);
	tcp_set_skb_tso_segs(buff, mss_now);

	/* Update delivered info for the new segment */
	TCP_SKB_CB(buff)->tx = TCP_SKB_CB(skb)->tx;

	/* If this packet has been sent out already, we must
	 * adjust the various packet counters.
	 */
	if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
		int diff = old_factor - tcp_skb_pcount(skb) -
			tcp_skb_pcount(buff);

		if (diff)
			tcp_adjust_pcount(sk, skb, diff);
	}

	/* Link BUFF into the send queue. */
	__skb_header_release(buff);
	tcp_insert_write_queue_after(skb, buff, sk, tcp_queue);
	if (tcp_queue == TCP_FRAG_IN_RTX_QUEUE)
		list_add(&buff->tcp_tsorted_anchor, &skb->tcp_tsorted_anchor);

	return 0;
}

/* This is similar to __pskb_pull_tail(). The difference is that pulled
 * data is not copied, but immediately discarded.
 */
static int __pskb_trim_head(struct sk_buff *skb, int len)
{
	struct skb_shared_info *shinfo;
	int i, k, eat;

	DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));
	eat = len;
	k = 0;
	shinfo = skb_shinfo(skb);
	for (i = 0; i < shinfo->nr_frags; i++) {
		int size = skb_frag_size(&shinfo->frags[i]);

		if (size <= eat) {
			skb_frag_unref(skb, i);
			eat -= size;
		} else {
			shinfo->frags[k] = shinfo->frags[i];
			if (eat) {
				skb_frag_off_add(&shinfo->frags[k], eat);
				skb_frag_size_sub(&shinfo->frags[k], eat);
				eat = 0;
			}
			k++;
		}
	}
	shinfo->nr_frags = k;

	skb->data_len -= len;
	skb->len = skb->data_len;
	return len;
}

/* Remove acked data from a packet in the transmit queue. */
int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
{
	u32 delta_truesize;

	if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
		return -ENOMEM;

	delta_truesize = __pskb_trim_head(skb, len);

	TCP_SKB_CB(skb)->seq += len;

	skb->truesize	   -= delta_truesize;
	sk_wmem_queued_add(sk, -delta_truesize);
	if (!skb_zcopy_pure(skb))
		sk_mem_uncharge(sk, delta_truesize);

	/* Any change of skb->len requires recalculation of tso factor. */
	if (tcp_skb_pcount(skb) > 1)
		tcp_set_skb_tso_segs(skb, tcp_skb_mss(skb));

	return 0;
}

/* Calculate MSS not accounting any TCP options.  */
static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	const struct inet_connection_sock *icsk = inet_csk(sk);
	int mss_now;

	/* Calculate base mss without TCP options:
	   It is MMS_S - sizeof(tcphdr) of rfc1122
	 */
	mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);

	/* Clamp it (mss_clamp does not include tcp options) */
	if (mss_now > tp->rx_opt.mss_clamp)
		mss_now = tp->rx_opt.mss_clamp;

	/* Now subtract optional transport overhead */
	mss_now -= icsk->icsk_ext_hdr_len;

	/* Then reserve room for full set of TCP options and 8 bytes of data */
	mss_now = max(mss_now,
		      READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_snd_mss));
	return mss_now;
}

/* Calculate MSS. Not accounting for SACKs here.  */
int tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
	/* Subtract TCP options size, not including SACKs */
	return __tcp_mtu_to_mss(sk, pmtu) -
	       (tcp_sk(sk)->tcp_header_len - sizeof(struct tcphdr));
}
EXPORT_IPV6_MOD(tcp_mtu_to_mss);

/* Inverse of above */
int tcp_mss_to_mtu(struct sock *sk, int mss)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	const struct inet_connection_sock *icsk = inet_csk(sk);

	return mss +
	      tp->tcp_header_len +
	      icsk->icsk_ext_hdr_len +
	      icsk->icsk_af_ops->net_header_len;
}
EXPORT_SYMBOL(tcp_mss_to_mtu);

/* MTU probing init per socket */
void tcp_mtup_init(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct net *net = sock_net(sk);

	icsk->icsk_mtup.enabled = READ_ONCE(net->ipv4.sysctl_tcp_mtu_probing) > 1;
	icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
			       icsk->icsk_af_ops->net_header_len;
	icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, READ_ONCE(net->ipv4.sysctl_tcp_base_mss));
	icsk->icsk_mtup.probe_size = 0;
	if (icsk->icsk_mtup.enabled)
		icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
}

/* This function synchronize snd mss to current pmtu/exthdr set.

   tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
   for TCP options, but includes only bare TCP header.

   tp->rx_opt.mss_clamp is mss negotiated at connection setup.
   It is minimum of user_mss and mss received with SYN.
   It also does not include TCP options.

   inet_csk(sk)->icsk_pmtu_cookie is last pmtu, seen by this function.

   tp->mss_cache is current effective sending mss, including
   all tcp options except for SACKs. It is evaluated,
   taking into account current pmtu, but never exceeds
   tp->rx_opt.mss_clamp.

   NOTE1. rfc1122 clearly states that advertised MSS
   DOES NOT include either tcp or ip options.

   NOTE2. inet_csk(sk)->icsk_pmtu_cookie and tp->mss_cache
   are READ ONLY outside this function.		--ANK (980731)
 */
unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct inet_connection_sock *icsk = inet_csk(sk);
	int mss_now;

	if (icsk->icsk_mtup.search_high > pmtu)
		icsk->icsk_mtup.search_high = pmtu;

	mss_now = tcp_mtu_to_mss(sk, pmtu);
	mss_now = tcp_bound_to_half_wnd(tp, mss_now);

	/* And store cached results */
	icsk->icsk_pmtu_cookie = pmtu;
	if (icsk->icsk_mtup.enabled)
		mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
	tp->mss_cache = mss_now;

	return mss_now;
}
EXPORT_IPV6_MOD(tcp_sync_mss);

/* Compute the current effective MSS, taking SACKs and IP options,
 * and even PMTU discovery events into account.
 */
unsigned int tcp_current_mss(struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	const struct dst_entry *dst = __sk_dst_get(sk);
	u32 mss_now;
	unsigned int header_len;
	struct tcp_out_options opts;
	struct tcp_key key;

	mss_now = tp->mss_cache;

	if (dst) {
		u32 mtu = dst_mtu(dst);
		if (mtu != inet_csk(sk)->icsk_pmtu_cookie)
			mss_now = tcp_sync_mss(sk, mtu);
	}
	tcp_get_current_key(sk, &key);
	header_len = tcp_established_options(sk, NULL, &opts, &key) +
		     sizeof(struct tcphdr);
	/* The mss_cache is sized based on tp->tcp_header_len, which assumes
	 * some common options. If this is an odd packet (because we have SACK
	 * blocks etc) then our calculated header_len will be different, and
	 * we have to adjust mss_now correspondingly */
	if (header_len != tp->tcp_header_len) {
		int delta = (int) header_len - tp->tcp_header_len;
		mss_now -= delta;
	}

	return mss_now;
}

/* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
 * As additional protections, we do not touch cwnd in retransmission phases,
 * and if application hit its sndbuf limit recently.
 */
static void tcp_cwnd_application_limited(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
	    sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
		/* Limited by application or receiver window. */
		u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
		u32 win_used = max(tp->snd_cwnd_used, init_win);
		if (win_used < tcp_snd_cwnd(tp)) {
			tp->snd_ssthresh = tcp_current_ssthresh(sk);
			tcp_snd_cwnd_set(tp, (tcp_snd_cwnd(tp) + win_used) >> 1);
		}
		tp->snd_cwnd_used = 0;
	}
	tp->snd_cwnd_stamp = tcp_jiffies32;
}

static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
{
	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
	struct tcp_sock *tp = tcp_sk(sk);

	/* Track the strongest available signal of the degree to which the cwnd
	 * is fully utilized. If cwnd-limited then remember that fact for the
	 * current window. If not cwnd-limited then track the maximum number of
	 * outstanding packets in the current window. (If cwnd-limited then we
	 * chose to not update tp->max_packets_out to avoid an extra else
	 * clause with no functional impact.)
	 */
	if (!before(tp->snd_una, tp->cwnd_usage_seq) ||
	    is_cwnd_limited ||
	    (!tp->is_cwnd_limited &&
	     tp->packets_out > tp->max_packets_out)) {
		tp->is_cwnd_limited = is_cwnd_limited;
		tp->max_packets_out = tp->packets_out;
		tp->cwnd_usage_seq = tp->snd_nxt;
	}

	if (tcp_is_cwnd_limited(sk)) {
		/* Network is feed fully. */
		tp->snd_cwnd_used = 0;
		tp->snd_cwnd_stamp = tcp_jiffies32;
	} else {
		/* Network starves. */
		if (tp->packets_out > tp->snd_cwnd_used)
			tp->snd_cwnd_used = tp->packets_out;

		if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_slow_start_after_idle) &&
		    (s32)(tcp_jiffies32 - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto &&
		    !ca_ops->cong_control)
			tcp_cwnd_application_limited(sk);

		/* The following conditions together indicate the starvation
		 * is caused by insufficient sender buffer:
		 * 1) just sent some data (see tcp_write_xmit)
		 * 2) not cwnd limited (this else condition)
		 * 3) no more data to send (tcp_write_queue_empty())
		 * 4) application is hitting buffer limit (SOCK_NOSPACE)
		 */
		if (tcp_write_queue_empty(sk) && sk->sk_socket &&
		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags) &&
		    (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT))
			tcp_chrono_start(sk, TCP_CHRONO_SNDBUF_LIMITED);
	}
}

/* Minshall's variant of the Nagle send check. */
static bool tcp_minshall_check(const struct tcp_sock *tp)
{
	return after(tp->snd_sml, tp->snd_una) &&
		!after(tp->snd_sml, tp->snd_nxt);
}

/* Update snd_sml if this skb is under mss
 * Note that a TSO packet might end with a sub-mss segment
 * The test is really :
 * if ((skb->len % mss) != 0)
 *        tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
 * But we can avoid doing the divide again given we already have
 *  skb_pcount = skb->len / mss_now
 */
static void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss_now,
				const struct sk_buff *skb)
{
	if (skb->len < tcp_skb_pcount(skb) * mss_now)
		tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
}

/* Return false, if packet can be sent now without violation Nagle's rules:
 * 1. It is full sized. (provided by caller in %partial bool)
 * 2. Or it contains FIN. (already checked by caller)
 * 3. Or TCP_CORK is not set, and TCP_NODELAY is set.
 * 4. Or TCP_CORK is not set, and all sent packets are ACKed.
 *    With Minshall's modification: all sent small packets are ACKed.
 */
static bool tcp_nagle_check(bool partial, const struct tcp_sock *tp,
			    int nonagle)
{
	return partial &&
		((nonagle & TCP_NAGLE_CORK) ||
		 (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
}

/* Return how many segs we'd like on a TSO packet,
 * depending on current pacing rate, and how close the peer is.
 *
 * Rationale is:
 * - For close peers, we rather send bigger packets to reduce
 *   cpu costs, because occasional losses will be repaired fast.
 * - For long distance/rtt flows, we would like to get ACK clocking
 *   with 1 ACK per ms.
 *
 * Use min_rtt to help adapt TSO burst size, with smaller min_rtt resulting
 * in bigger TSO bursts. We we cut the RTT-based allowance in half
 * for every 2^9 usec (aka 512 us) of RTT, so that the RTT-based allowance
 * is below 1500 bytes after 6 * ~500 usec = 3ms.
 */
static u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
			    int min_tso_segs)
{
	unsigned long bytes;
	u32 r;

	bytes = READ_ONCE(sk->sk_pacing_rate) >> READ_ONCE(sk->sk_pacing_shift);

	r = tcp_min_rtt(tcp_sk(sk)) >> READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_rtt_log);
	if (r < BITS_PER_TYPE(sk->sk_gso_max_size))
		bytes += sk->sk_gso_max_size >> r;

	bytes = min_t(unsigned long, bytes, sk->sk_gso_max_size);

	return max_t(u32, bytes / mss_now, min_tso_segs);
}

/* Return the number of segments we want in the skb we are transmitting.
 * See if congestion control module wants to decide; otherwise, autosize.
 */
static u32 tcp_tso_segs(struct sock *sk, unsigned int mss_now)
{
	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
	u32 min_tso, tso_segs;

	min_tso = ca_ops->min_tso_segs ?
			ca_ops->min_tso_segs(sk) :
			READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_tso_segs);

	tso_segs = tcp_tso_autosize(sk, mss_now, min_tso);
	return min_t(u32, tso_segs, sk->sk_gso_max_segs);
}

/* Returns the portion of skb which can be sent right away */
static unsigned int tcp_mss_split_point(const struct sock *sk,
					const struct sk_buff *skb,
					unsigned int mss_now,
					unsigned int max_segs,
					int nonagle)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	u32 partial, needed, window, max_len;

	window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
	max_len = mss_now * max_segs;

	if (likely(max_len <= window && skb != tcp_write_queue_tail(sk)))
		return max_len;

	needed = min(skb->len, window);

	if (max_len <= needed)
		return max_len;

	partial = needed % mss_now;
	/* If last segment is not a full MSS, check if Nagle rules allow us
	 * to include this last segment in this skb.
	 * Otherwise, we'll split the skb at last MSS boundary
	 */
	if (tcp_nagle_check(partial != 0, tp, nonagle))
		return needed - partial;

	return needed;
}

/* Can at least one segment of SKB be sent right now, according to the
 * congestion window rules?  If so, return how many segments are allowed.
 */
static u32 tcp_cwnd_test(const struct tcp_sock *tp)
{
	u32 in_flight, cwnd, halfcwnd;

	in_flight = tcp_packets_in_flight(tp);
	cwnd = tcp_snd_cwnd(tp);
	if (in_flight >= cwnd)
		return 0;

	/* For better scheduling, ensure we have at least
	 * 2 GSO packets in flight.
	 */
	halfcwnd = max(cwnd >> 1, 1U);
	return min(halfcwnd, cwnd - in_flight);
}

/* Initialize TSO state of a skb.
 * This must be invoked the first time we consider transmitting
 * SKB onto the wire.
 */
static int tcp_init_tso_segs(struct sk_buff *skb, unsigned int mss_now)
{
	int tso_segs = tcp_skb_pcount(skb);

	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now))
		return tcp_set_skb_tso_segs(skb, mss_now);

	return tso_segs;
}


/* Return true if the Nagle test allows this packet to be
 * sent now.
 */
static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buff *skb,
				  unsigned int cur_mss, int nonagle)
{
	/* Nagle rule does not apply to frames, which sit in the middle of the
	 * write_queue (they have no chances to get new data).
	 *
	 * This is implemented in the callers, where they modify the 'nonagle'
	 * argument based upon the location of SKB in the send queue.
	 */
	if (nonagle & TCP_NAGLE_PUSH)
		return true;

	/* Don't use the nagle rule for urgent data (or for the final FIN). */
	if (tcp_urg_mode(tp) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
		return true;

	if (!tcp_nagle_check(skb->len < cur_mss, tp, nonagle))
		return true;

	return false;
}

/* Does at least the first segment of SKB fit into the send window? */
static bool tcp_snd_wnd_test(const struct tcp_sock *tp,
			     const struct sk_buff *skb,
			     unsigned int cur_mss)
{
	u32 end_seq = TCP_SKB_CB(skb)->end_seq;

	if (skb->len > cur_mss)
		end_seq = TCP_SKB_CB(skb)->seq + cur_mss;

	return !after(end_seq, tcp_wnd_end(tp));
}

/* Trim TSO SKB to LEN bytes, put the remaining data into a new packet
 * which is put after SKB on the list.  It is very much like
 * tcp_fragment() except that it may make several kinds of assumptions
 * in order to speed up the splitting operation.  In particular, we
 * know that all the data is in scatter-gather pages, and that the
 * packet has never been sent out before (and thus is not cloned).
 */
static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
			unsigned int mss_now, gfp_t gfp)
{
	int nlen = skb->len - len;
	struct sk_buff *buff;
	u16 flags;

	/* All of a TSO frame must be composed of paged data.  */
	DEBUG_NET_WARN_ON_ONCE(skb->len != skb->data_len);

	buff = tcp_stream_alloc_skb(sk, gfp, true);
	if (unlikely(!buff))
		return -ENOMEM;
	skb_copy_decrypted(buff, skb);
	mptcp_skb_ext_copy(buff, skb);

	sk_wmem_queued_add(sk, buff->truesize);
	sk_mem_charge(sk, buff->truesize);
	buff->truesize += nlen;
	skb->truesize -= nlen;

	/* Correct the sequence numbers. */
	TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
	TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

	/* PSH and FIN should only be set in the second packet. */
	flags = TCP_SKB_CB(skb)->tcp_flags;
	TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
	TCP_SKB_CB(buff)->tcp_flags = flags;

	tcp_skb_fragment_eor(skb, buff);

	skb_split(skb, buff, len);
	tcp_fragment_tstamp(skb, buff);

	/* Fix up tso_factor for both original and new SKB.  */
	tcp_set_skb_tso_segs(skb, mss_now);
	tcp_set_skb_tso_segs(buff, mss_now);

	/* Link BUFF into the send queue. */
	__skb_header_release(buff);
	tcp_insert_write_queue_after(skb, buff, sk, TCP_FRAG_IN_WRITE_QUEUE);

	return 0;
}

/* Try to defer sending, if possible, in order to minimize the amount
 * of TSO splitting we do.  View it as a kind of TSO Nagle test.
 *
 * This algorithm is from John Heffner.
 */
static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb,
				 bool *is_cwnd_limited,
				 bool *is_rwnd_limited,
				 u32 max_segs)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);
	u32 send_win, cong_win, limit, in_flight, threshold;
	u64 srtt_in_ns, expected_ack, how_far_is_the_ack;
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *head;
	int win_divisor;
	s64 delta;

	if (icsk->icsk_ca_state >= TCP_CA_Recovery)
		goto send_now;

	/* Avoid bursty behavior by allowing defer
	 * only if the last write was recent (1 ms).
	 * Note that tp->tcp_wstamp_ns can be in the future if we have
	 * packets waiting in a qdisc or device for EDT delivery.
	 */
	delta = tp->tcp_clock_cache - tp->tcp_wstamp_ns - NSEC_PER_MSEC;
	if (delta > 0)
		goto send_now;

	in_flight = tcp_packets_in_flight(tp);

	BUG_ON(tcp_skb_pcount(skb) <= 1);
	BUG_ON(tcp_snd_cwnd(tp) <= in_flight);

	send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

	/* From in_flight test above, we know that cwnd > in_flight.  */
	cong_win = (tcp_snd_cwnd(tp) - in_flight) * tp->mss_cache;

	limit = min(send_win, cong_win);

	/* If a full-sized TSO skb can be sent, do it. */
	if (limit >= max_segs * tp->mss_cache)
		goto send_now;

	/* Middle in queue won't get any more data, full sendable already? */
	if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
		goto send_now;

	win_divisor = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_win_divisor);
	if (win_divisor) {
		u32 chunk = min(tp->snd_wnd, tcp_snd_cwnd(tp) * tp->mss_cache);

		/* If at least some fraction of a window is available,
		 * just use it.
		 */
		chunk /= win_divisor;
		if (limit >= chunk)
			goto send_now;
	} else {
		/* Different approach, try not to defer past a single
		 * ACK.  Receiver should ACK every other full sized
		 * frame, so if we have space for more than 3 frames
		 * then send now.
		 */
		if (limit > tcp_max_tso_deferred_mss(tp) * tp->mss_cache)
			goto send_now;
	}

	/* TODO : use tsorted_sent_queue ? */
	head = tcp_rtx_queue_head(sk);
	if (!head)
		goto send_now;

	srtt_in_ns = (u64)(NSEC_PER_USEC >> 3) * tp->srtt_us;
	/* When is the ACK expected ? */
	expected_ack = head->tstamp + srtt_in_ns;
	/* How far from now is the ACK expected ? */
	how_far_is_the_ack = expected_ack - tp->tcp_clock_cache;

	/* If next ACK is likely to come too late,
	 * ie in more than min(1ms, half srtt), do not defer.
	 */
	threshold = min(srtt_in_ns >> 1, NSEC_PER_MSEC);

	if ((s64)(how_far_is_the_ack - threshold) > 0)
		goto send_now;

	/* Ok, it looks like it is advisable to defer.
	 * Three cases are tracked :
	 * 1) We are cwnd-limited
	 * 2) We are rwnd-limited
	 * 3) We are application limited.
	 */
	if (cong_win < send_win) {
		if (cong_win <= skb->len) {
			*is_cwnd_limited = true;
			return true;
		}
	} else {
		if (send_win <= skb->len) {
			*is_rwnd_limited = true;
			return true;
		}
	}

	/* If this packet won't get more data, do not wait. */
	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) ||
	    TCP_SKB_CB(skb)->eor)
		goto send_now;

	return true;

send_now:
	return false;
}

static inline void tcp_mtu_check_reprobe(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	struct net *net = sock_net(sk);
	u32 interval;
	s32 delta;

	interval = READ_ONCE(net->ipv4.sysctl_tcp_probe_interval);
	delta = tcp_jiffies32 - icsk->icsk_mtup.probe_timestamp;
	if (unlikely(delta >= interval * HZ)) {
		int mss = tcp_current_mss(sk);

		/* Update current search range */
		icsk->icsk_mtup.probe_size = 0;
		icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp +
			sizeof(struct tcphdr) +
			icsk->icsk_af_ops->net_header_len;
		icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);

		/* Update probe time stamp */
		icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
	}
}

static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
{
	struct sk_buff *skb, *next;

	skb = tcp_send_head(sk);
	tcp_for_write_queue_from_safe(skb, next, sk) {
		if (len <= skb->len)
			break;

		if (tcp_has_tx_tstamp(skb) || !tcp_skb_can_collapse(skb, next))
			return false;

		len -= skb->len;
	}

	return true;
}

static int tcp_clone_payload(struct sock *sk, struct sk_buff *to,
			     int probe_size)
{
	skb_frag_t *lastfrag = NULL, *fragto = skb_shinfo(to)->frags;
	int i, todo, len = 0, nr_frags = 0;
	const struct sk_buff *skb;

	if (!sk_wmem_schedule(sk, to->truesize + probe_size))
		return -ENOMEM;

	skb_queue_walk(&sk->sk_write_queue, skb) {
		const skb_frag_t *fragfrom = skb_shinfo(skb)->frags;

		if (skb_headlen(skb))
			return -EINVAL;

		for (i = 0; i < skb_shinfo(skb)->nr_frags; i++, fragfrom++) {
			if (len >= probe_size)
				goto commit;
			todo = min_t(int, skb_frag_size(fragfrom),
				     probe_size - len);
			len += todo;
			if (lastfrag &&
			    skb_frag_page(fragfrom) == skb_frag_page(lastfrag) &&
			    skb_frag_off(fragfrom) == skb_frag_off(lastfrag) +
						      skb_frag_size(lastfrag)) {
				skb_frag_size_add(lastfrag, todo);
				continue;
			}
			if (unlikely(nr_frags == MAX_SKB_FRAGS))
				return -E2BIG;
			skb_frag_page_copy(fragto, fragfrom);
			skb_frag_off_copy(fragto, fragfrom);
			skb_frag_size_set(fragto, todo);
			nr_frags++;
			lastfrag = fragto++;
		}
	}
commit:
	WARN_ON_ONCE(len != probe_size);
	for (i = 0; i < nr_frags; i++)
		skb_frag_ref(to, i);

	skb_shinfo(to)->nr_frags = nr_frags;
	to->truesize += probe_size;
	to->len += probe_size;
	to->data_len += probe_size;
	__skb_header_release(to);
	return 0;
}

/* tcp_mtu_probe() and tcp_grow_skb() can both eat an skb (src) if
 * all its payload was moved to another one (dst).
 * Make sure to transfer tcp_flags, eor, and tstamp.
 */
static void tcp_eat_one_skb(struct sock *sk,
			    struct sk_buff *dst,
			    struct sk_buff *src)
{
	TCP_SKB_CB(dst)->tcp_flags |= TCP_SKB_CB(src)->tcp_flags;
	TCP_SKB_CB(dst)->eor = TCP_SKB_CB(src)->eor;
	tcp_skb_collapse_tstamp(dst, src);
	tcp_unlink_write_queue(src, sk);
	tcp_wmem_free_skb(sk, src);
}

/* Create a new MTU probe if we are ready.
 * MTU probe is regularly attempting to increase the path MTU by
 * deliberately sending larger packets.  This discovers routing
 * changes resulting in larger path MTUs.
 *
 * Returns 0 if we should wait to probe (no cwnd available),
 *         1 if a probe was sent,
 *         -1 otherwise
 */
static int tcp_mtu_probe(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb, *nskb, *next;
	struct net *net = sock_net(sk);
	int probe_size;
	int size_needed;
	int copy, len;
	int mss_now;
	int interval;

	/* Not currently probing/verifying,
	 * not in recovery,
	 * have enough cwnd, and
	 * not SACKing (the variable headers throw things off)
	 */
	if (likely(!icsk->icsk_mtup.enabled ||
		   icsk->icsk_mtup.probe_size ||
		   inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
		   tcp_snd_cwnd(tp) < 11 ||
		   tp->rx_opt.num_sacks || tp->rx_opt.dsack))
		return -1;

	/* Use binary search for probe_size between tcp_mss_base,
	 * and current mss_clamp. if (search_high - search_low)
	 * smaller than a threshold, backoff from probing.
	 */
	mss_now = tcp_current_mss(sk);
	probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +
				    icsk->icsk_mtup.search_low) >> 1);
	size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
	interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
	/* When misfortune happens, we are reprobing actively,
	 * and then reprobe timer has expired. We stick with current
	 * probing process by not resetting search range to its orignal.
	 */
	if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||
	    interval < READ_ONCE(net->ipv4.sysctl_tcp_probe_threshold)) {
		/* Check whether enough time has elaplased for
		 * another round of probing.
		 */
		tcp_mtu_check_reprobe(sk);
		return -1;
	}

	/* Have enough data in the send queue to probe? */
	if (tp->write_seq - tp->snd_nxt < size_needed)
		return -1;

	if (tp->snd_wnd < size_needed)
		return -1;
	if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
		return 0;

	/* Do we need to wait to drain cwnd? With none in flight, don't stall */
	if (tcp_packets_in_flight(tp) + 2 > tcp_snd_cwnd(tp)) {
		if (!tcp_packets_in_flight(tp))
			return -1;
		else
			return 0;
	}

	if (!tcp_can_coalesce_send_queue_head(sk, probe_size))
		return -1;

	/* We're allowed to probe.  Build it now. */
	nskb = tcp_stream_alloc_skb(sk, GFP_ATOMIC, false);
	if (!nskb)
		return -1;

	/* build the payload, and be prepared to abort if this fails. */
	if (tcp_clone_payload(sk, nskb, probe_size)) {
		tcp_skb_tsorted_anchor_cleanup(nskb);
		consume_skb(nskb);
		return -1;
	}
	sk_wmem_queued_add(sk, nskb->truesize);
	sk_mem_charge(sk, nskb->truesize);

	skb = tcp_send_head(sk);
	skb_copy_decrypted(nskb, skb);
	mptcp_skb_ext_copy(nskb, skb);

	TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
	TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
	TCP_SKB_CB(nskb)->tcp_flags = TCPHDR_ACK;

	tcp_insert_write_queue_before(nskb, skb, sk);
	tcp_highest_sack_replace(sk, skb, nskb);

	len = 0;
	tcp_for_write_queue_from_safe(skb, next, sk) {
		copy = min_t(int, skb->len, probe_size - len);

		if (skb->len <= copy) {
			tcp_eat_one_skb(sk, nskb, skb);
		} else {
			TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
						   ~(TCPHDR_FIN|TCPHDR_PSH);
			__pskb_trim_head(skb, copy);
			tcp_set_skb_tso_segs(skb, mss_now);
			TCP_SKB_CB(skb)->seq += copy;
		}

		len += copy;

		if (len >= probe_size)
			break;
	}
	tcp_init_tso_segs(nskb, nskb->len);

	/* We're ready to send.  If this fails, the probe will
	 * be resegmented into mss-sized pieces by tcp_write_xmit().
	 */
	if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
		/* Decrement cwnd here because we are sending
		 * effectively two packets. */
		tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - 1);
		tcp_event_new_data_sent(sk, nskb);

		icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
		tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
		tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;

		return 1;
	}

	return -1;
}

static bool tcp_pacing_check(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (!tcp_needs_internal_pacing(sk))
		return false;

	if (tp->tcp_wstamp_ns <= tp->tcp_clock_cache)
		return false;

	if (!hrtimer_is_queued(&tp->pacing_timer)) {
		hrtimer_start(&tp->pacing_timer,
			      ns_to_ktime(tp->tcp_wstamp_ns),
			      HRTIMER_MODE_ABS_PINNED_SOFT);
		sock_hold(sk);
	}
	return true;
}

static bool tcp_rtx_queue_empty_or_single_skb(const struct sock *sk)
{
	const struct rb_node *node = sk->tcp_rtx_queue.rb_node;

	/* No skb in the rtx queue. */
	if (!node)
		return true;

	/* Only one skb in rtx queue. */
	return !node->rb_left && !node->rb_right;
}

/* TCP Small Queues :
 * Control number of packets in qdisc/devices to two packets / or ~1 ms.
 * (These limits are doubled for retransmits)
 * This allows for :
 *  - better RTT estimation and ACK scheduling
 *  - faster recovery
 *  - high rates
 * Alas, some drivers / subsystems require a fair amount
 * of queued bytes to ensure line rate.
 * One example is wifi aggregation (802.11 AMPDU)
 */
static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
				  unsigned int factor)
{
	unsigned long limit;

	limit = max_t(unsigned long,
		      2 * skb->truesize,
		      READ_ONCE(sk->sk_pacing_rate) >> READ_ONCE(sk->sk_pacing_shift));
	limit = min_t(unsigned long, limit,
		      READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes));
	limit <<= factor;

	if (static_branch_unlikely(&tcp_tx_delay_enabled) &&
	    tcp_sk(sk)->tcp_tx_delay) {
		u64 extra_bytes = (u64)READ_ONCE(sk->sk_pacing_rate) *
				  tcp_sk(sk)->tcp_tx_delay;

		/* TSQ is based on skb truesize sum (sk_wmem_alloc), so we
		 * approximate our needs assuming an ~100% skb->truesize overhead.
		 * USEC_PER_SEC is approximated by 2^20.
		 * do_div(extra_bytes, USEC_PER_SEC/2) is replaced by a right shift.
		 */
		extra_bytes >>= (20 - 1);
		limit += extra_bytes;
	}
	if (refcount_read(&sk->sk_wmem_alloc) > limit) {
		/* Always send skb if rtx queue is empty or has one skb.
		 * No need to wait for TX completion to call us back,
		 * after softirq schedule.
		 * This helps when TX completions are delayed too much.
		 */
		if (tcp_rtx_queue_empty_or_single_skb(sk))
			return false;

		set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
		/* It is possible TX completion already happened
		 * before we set TSQ_THROTTLED, so we must
		 * test again the condition.
		 */
		smp_mb__after_atomic();
		if (refcount_read(&sk->sk_wmem_alloc) > limit)
			return true;
	}
	return false;
}

static void tcp_chrono_set(struct tcp_sock *tp, const enum tcp_chrono new)
{
	const u32 now = tcp_jiffies32;
	enum tcp_chrono old = tp->chrono_type;

	if (old > TCP_CHRONO_UNSPEC)
		tp->chrono_stat[old - 1] += now - tp->chrono_start;
	tp->chrono_start = now;
	tp->chrono_type = new;
}

void tcp_chrono_start(struct sock *sk, const enum tcp_chrono type)
{
	struct tcp_sock *tp = tcp_sk(sk);

	/* If there are multiple conditions worthy of tracking in a
	 * chronograph then the highest priority enum takes precedence
	 * over the other conditions. So that if something "more interesting"
	 * starts happening, stop the previous chrono and start a new one.
	 */
	if (type > tp->chrono_type)
		tcp_chrono_set(tp, type);
}

void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type)
{
	struct tcp_sock *tp = tcp_sk(sk);


	/* There are multiple conditions worthy of tracking in a
	 * chronograph, so that the highest priority enum takes
	 * precedence over the other conditions (see tcp_chrono_start).
	 * If a condition stops, we only stop chrono tracking if
	 * it's the "most interesting" or current chrono we are
	 * tracking and starts busy chrono if we have pending data.
	 */
	if (tcp_rtx_and_write_queues_empty(sk))
		tcp_chrono_set(tp, TCP_CHRONO_UNSPEC);
	else if (type == tp->chrono_type)
		tcp_chrono_set(tp, TCP_CHRONO_BUSY);
}

/* First skb in the write queue is smaller than ideal packet size.
 * Check if we can move payload from the second skb in the queue.
 */
static void tcp_grow_skb(struct sock *sk, struct sk_buff *skb, int amount)
{
	struct sk_buff *next_skb = skb->next;
	unsigned int nlen;

	if (tcp_skb_is_last(sk, skb))
		return;

	if (!tcp_skb_can_collapse(skb, next_skb))
		return;

	nlen = min_t(u32, amount, next_skb->len);
	if (!nlen || !skb_shift(skb, next_skb, nlen))
		return;

	TCP_SKB_CB(skb)->end_seq += nlen;
	TCP_SKB_CB(next_skb)->seq += nlen;

	if (!next_skb->len) {
		/* In case FIN is set, we need to update end_seq */
		TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;

		tcp_eat_one_skb(sk, skb, next_skb);
	}
}

/* This routine writes packets to the network.  It advances the
 * send_head.  This happens as incoming acks open up the remote
 * window for us.
 *
 * LARGESEND note: !tcp_urg_mode is overkill, only frames between
 * snd_up-64k-mss .. snd_up cannot be large. However, taking into
 * account rare use of URG, this is not a big flaw.
 *
 * Send at most one packet when push_one > 0. Temporarily ignore
 * cwnd limit to force at most one packet out when push_one == 2.

 * Returns true, if no segments are in flight and we have queued segments,
 * but cannot send anything now because of SWS or another problem.
 */
static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
			   int push_one, gfp_t gfp)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;
	unsigned int tso_segs, sent_pkts;
	u32 cwnd_quota, max_segs;
	int result;
	bool is_cwnd_limited = false, is_rwnd_limited = false;

	sent_pkts = 0;

	tcp_mstamp_refresh(tp);

	/* AccECN option beacon depends on mstamp, it may change mss */
	if (tcp_ecn_mode_accecn(tp) && tcp_accecn_option_beacon_check(sk))
		mss_now = tcp_current_mss(sk);

	if (!push_one) {
		/* Do MTU probing. */
		result = tcp_mtu_probe(sk);
		if (!result) {
			return false;
		} else if (result > 0) {
			sent_pkts = 1;
		}
	}

	max_segs = tcp_tso_segs(sk, mss_now);
	while ((skb = tcp_send_head(sk))) {
		unsigned int limit;
		int missing_bytes;

		if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
			/* "skb_mstamp_ns" is used as a start point for the retransmit timer */
			tp->tcp_wstamp_ns = tp->tcp_clock_cache;
			skb_set_delivery_time(skb, tp->tcp_wstamp_ns, SKB_CLOCK_MONOTONIC);
			list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
			tcp_init_tso_segs(skb, mss_now);
			goto repair; /* Skip network transmission */
		}

		if (tcp_pacing_check(sk))
			break;

		cwnd_quota = tcp_cwnd_test(tp);
		if (!cwnd_quota) {
			if (push_one == 2)
				/* Force out a loss probe pkt. */
				cwnd_quota = 1;
			else
				break;
		}
		cwnd_quota = min(cwnd_quota, max_segs);
		missing_bytes = cwnd_quota * mss_now - skb->len;
		if (missing_bytes > 0)
			tcp_grow_skb(sk, skb, missing_bytes);

		tso_segs = tcp_set_skb_tso_segs(skb, mss_now);

		if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
			is_rwnd_limited = true;
			break;
		}

		if (tso_segs == 1) {
			if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
						     (tcp_skb_is_last(sk, skb) ?
						      nonagle : TCP_NAGLE_PUSH))))
				break;
		} else {
			if (!push_one &&
			    tcp_tso_should_defer(sk, skb, &is_cwnd_limited,
						 &is_rwnd_limited, max_segs))
				break;
		}

		limit = mss_now;
		if (tso_segs > 1 && !tcp_urg_mode(tp))
			limit = tcp_mss_split_point(sk, skb, mss_now,
						    cwnd_quota,
						    nonagle);

		if (skb->len > limit &&
		    unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
			break;

		if (tcp_small_queue_check(sk, skb, 0))
			break;

		/* Argh, we hit an empty skb(), presumably a thread
		 * is sleeping in sendmsg()/sk_stream_wait_memory().
		 * We do not want to send a pure-ack packet and have
		 * a strange looking rtx queue with empty packet(s).
		 */
		if (TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq)
			break;

		if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
			break;

repair:
		/* Advance the send_head.  This one is sent out.
		 * This call will increment packets_out.
		 */
		tcp_event_new_data_sent(sk, skb);

		tcp_minshall_update(tp, mss_now, skb);
		sent_pkts += tcp_skb_pcount(skb);

		if (push_one)
			break;
	}

	if (is_rwnd_limited)
		tcp_chrono_start(sk, TCP_CHRONO_RWND_LIMITED);
	else
		tcp_chrono_stop(sk, TCP_CHRONO_RWND_LIMITED);

	is_cwnd_limited |= (tcp_packets_in_flight(tp) >= tcp_snd_cwnd(tp));
	if (likely(sent_pkts || is_cwnd_limited))
		tcp_cwnd_validate(sk, is_cwnd_limited);

	if (likely(sent_pkts)) {
		if (tcp_in_cwnd_reduction(sk))
			tp->prr_out += sent_pkts;

		/* Send one loss probe per tail loss episode. */
		if (push_one != 2)
			tcp_schedule_loss_probe(sk, false);
		return false;
	}
	return !tp->packets_out && !tcp_write_queue_empty(sk);
}

bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	u32 timeout, timeout_us, rto_delta_us;
	int early_retrans;

	/* Don't do any loss probe on a Fast Open connection before 3WHS
	 * finishes.
	 */
	if (rcu_access_pointer(tp->fastopen_rsk))
		return false;

	early_retrans = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_early_retrans);
	/* Schedule a loss probe in 2*RTT for SACK capable connections
	 * not in loss recovery, that are either limited by cwnd or application.
	 */
	if ((early_retrans != 3 && early_retrans != 4) ||
	    !tcp_is_sack(tp) ||
	    (icsk->icsk_ca_state != TCP_CA_Open &&
	     icsk->icsk_ca_state != TCP_CA_CWR))
		return false;

	/* Probe timeout is 2*rtt. Add minimum RTO to account
	 * for delayed ack when there's one outstanding packet. If no RTT
	 * sample is available then probe after TCP_TIMEOUT_INIT.
	 */
	if (tp->srtt_us) {
		timeout_us = tp->srtt_us >> 2;
		if (tp->packets_out == 1)
			timeout_us += tcp_rto_min_us(sk);
		else
			timeout_us += TCP_TIMEOUT_MIN_US;
		timeout = usecs_to_jiffies(timeout_us);
	} else {
		timeout = TCP_TIMEOUT_INIT;
	}

	/* If the RTO formula yields an earlier time, then use that time. */
	rto_delta_us = advancing_rto ?
			jiffies_to_usecs(inet_csk(sk)->icsk_rto) :
			tcp_rto_delta_us(sk);  /* How far in future is RTO? */
	if (rto_delta_us > 0)
		timeout = min_t(u32, timeout, usecs_to_jiffies(rto_delta_us));

	tcp_reset_xmit_timer(sk, ICSK_TIME_LOSS_PROBE, timeout, true);
	return true;
}

/* Thanks to skb fast clones, we can detect if a prior transmit of
 * a packet is still in a qdisc or driver queue.
 * In this case, there is very little point doing a retransmit !
 */
static bool skb_still_in_host_queue(struct sock *sk,
				    const struct sk_buff *skb)
{
	if (unlikely(skb_fclone_busy(sk, skb))) {
		set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
		smp_mb__after_atomic();
		if (skb_fclone_busy(sk, skb)) {
			NET_INC_STATS(sock_net(sk),
				      LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
			return true;
		}
	}
	return false;
}

/* When probe timeout (PTO) fires, try send a new segment if possible, else
 * retransmit the last segment.
 */
void tcp_send_loss_probe(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;
	int pcount;
	int mss = tcp_current_mss(sk);

	/* At most one outstanding TLP */
	if (tp->tlp_high_seq)
		goto rearm_timer;

	tp->tlp_retrans = 0;
	skb = tcp_send_head(sk);
	if (skb && tcp_snd_wnd_test(tp, skb, mss)) {
		pcount = tp->packets_out;
		tcp_write_xmit(sk, mss, TCP_NAGLE_OFF, 2, GFP_ATOMIC);
		if (tp->packets_out > pcount)
			goto probe_sent;
		goto rearm_timer;
	}
	skb = skb_rb_last(&sk->tcp_rtx_queue);
	if (unlikely(!skb)) {
		tcp_warn_once(sk, tp->packets_out, "invalid inflight: ");
		smp_store_release(&inet_csk(sk)->icsk_pending, 0);
		return;
	}

	if (skb_still_in_host_queue(sk, skb))
		goto rearm_timer;

	pcount = tcp_skb_pcount(skb);
	if (WARN_ON(!pcount))
		goto rearm_timer;

	if ((pcount > 1) && (skb->len > (pcount - 1) * mss)) {
		if (unlikely(tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
					  (pcount - 1) * mss, mss,
					  GFP_ATOMIC)))
			goto rearm_timer;
		skb = skb_rb_next(skb);
	}

	if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
		goto rearm_timer;

	if (__tcp_retransmit_skb(sk, skb, 1))
		goto rearm_timer;

	tp->tlp_retrans = 1;

probe_sent:
	/* Record snd_nxt for loss detection. */
	tp->tlp_high_seq = tp->snd_nxt;

	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSPROBES);
	/* Reset s.t. tcp_rearm_rto will restart timer from now */
	smp_store_release(&inet_csk(sk)->icsk_pending, 0);
rearm_timer:
	tcp_rearm_rto(sk);
}

/* Push out any pending frames which were held back due to
 * TCP_CORK or attempt at coalescing tiny packets.
 * The socket must be locked by the caller.
 */
void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
			       int nonagle)
{
	/* If we are closed, the bytes will have to remain here.
	 * In time closedown will finish, we empty the write queue and
	 * all will be happy.
	 */
	if (unlikely(sk->sk_state == TCP_CLOSE))
		return;

	if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
			   sk_gfp_mask(sk, GFP_ATOMIC)))
		tcp_check_probe_timer(sk);
}

/* Send _single_ skb sitting at the send head. This function requires
 * true push pending frames to setup probe timer etc.
 */
void tcp_push_one(struct sock *sk, unsigned int mss_now)
{
	struct sk_buff *skb = tcp_send_head(sk);

	BUG_ON(!skb || skb->len < mss_now);

	tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
}

/* This function returns the amount that we can raise the
 * usable window based on the following constraints
 *
 * 1. The window can never be shrunk once it is offered (RFC 793)
 * 2. We limit memory per socket
 *
 * RFC 1122:
 * "the suggested [SWS] avoidance algorithm for the receiver is to keep
 *  RECV.NEXT + RCV.WIN fixed until:
 *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
 *
 * i.e. don't raise the right edge of the window until you can raise
 * it at least MSS bytes.
 *
 * Unfortunately, the recommended algorithm breaks header prediction,
 * since header prediction assumes th->window stays fixed.
 *
 * Strictly speaking, keeping th->window fixed violates the receiver
 * side SWS prevention criteria. The problem is that under this rule
 * a stream of single byte packets will cause the right side of the
 * window to always advance by a single byte.
 *
 * Of course, if the sender implements sender side SWS prevention
 * then this will not be a problem.
 *
 * BSD seems to make the following compromise:
 *
 *	If the free space is less than the 1/4 of the maximum
 *	space available and the free space is less than 1/2 mss,
 *	then set the window to 0.
 *	[ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
 *	Otherwise, just prevent the window from shrinking
 *	and from being larger than the largest representable value.
 *
 * This prevents incremental opening of the window in the regime
 * where TCP is limited by the speed of the reader side taking
 * data out of the TCP receive queue. It does nothing about
 * those cases where the window is constrained on the sender side
 * because the pipeline is full.
 *
 * BSD also seems to "accidentally" limit itself to windows that are a
 * multiple of MSS, at least until the free space gets quite small.
 * This would appear to be a side effect of the mbuf implementation.
 * Combining these two algorithms results in the observed behavior
 * of having a fixed window size at almost all times.
 *
 * Below we obtain similar behavior by forcing the offered window to
 * a multiple of the mss when it is feasible to do so.
 *
 * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
 * Regular options like TIMESTAMP are taken into account.
 */
u32 __tcp_select_window(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	struct net *net = sock_net(sk);
	/* MSS for the peer's data.  Previous versions used mss_clamp
	 * here.  I don't know if the value based on our guesses
	 * of peer's MSS is better for the performance.  It's more correct
	 * but may be worse for the performance because of rcv_mss
	 * fluctuations.  --SAW  1998/11/1
	 */
	int mss = icsk->icsk_ack.rcv_mss;
	int free_space = tcp_space(sk);
	int allowed_space = tcp_full_space(sk);
	int full_space, window;

	if (sk_is_mptcp(sk))
		mptcp_space(sk, &free_space, &allowed_space);

	full_space = min_t(int, tp->window_clamp, allowed_space);

	if (unlikely(mss > full_space)) {
		mss = full_space;
		if (mss <= 0)
			return 0;
	}

	/* Only allow window shrink if the sysctl is enabled and we have
	 * a non-zero scaling factor in effect.
	 */
	if (READ_ONCE(net->ipv4.sysctl_tcp_shrink_window) && tp->rx_opt.rcv_wscale)
		goto shrink_window_allowed;

	/* do not allow window to shrink */

	if (free_space < (full_space >> 1)) {
		icsk->icsk_ack.quick = 0;

		if (tcp_under_memory_pressure(sk))
			tcp_adjust_rcv_ssthresh(sk);

		/* free_space might become our new window, make sure we don't
		 * increase it due to wscale.
		 */
		free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);

		/* if free space is less than mss estimate, or is below 1/16th
		 * of the maximum allowed, try to move to zero-window, else
		 * tcp_clamp_window() will grow rcv buf up to tcp_rmem[2], and
		 * new incoming data is dropped due to memory limits.
		 * With large window, mss test triggers way too late in order
		 * to announce zero window in time before rmem limit kicks in.
		 */
		if (free_space < (allowed_space >> 4) || free_space < mss)
			return 0;
	}

	if (free_space > tp->rcv_ssthresh)
		free_space = tp->rcv_ssthresh;

	/* Don't do rounding if we are using window scaling, since the
	 * scaled window will not line up with the MSS boundary anyway.
	 */
	if (tp->rx_opt.rcv_wscale) {
		window = free_space;

		/* Advertise enough space so that it won't get scaled away.
		 * Import case: prevent zero window announcement if
		 * 1<<rcv_wscale > mss.
		 */
		window = ALIGN(window, (1 << tp->rx_opt.rcv_wscale));
	} else {
		window = tp->rcv_wnd;
		/* Get the largest window that is a nice multiple of mss.
		 * Window clamp already applied above.
		 * If our current window offering is within 1 mss of the
		 * free space we just keep it. This prevents the divide
		 * and multiply from happening most of the time.
		 * We also don't do any window rounding when the free space
		 * is too small.
		 */
		if (window <= free_space - mss || window > free_space)
			window = rounddown(free_space, mss);
		else if (mss == full_space &&
			 free_space > window + (full_space >> 1))
			window = free_space;
	}

	return window;

shrink_window_allowed:
	/* new window should always be an exact multiple of scaling factor */
	free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);

	if (free_space < (full_space >> 1)) {
		icsk->icsk_ack.quick = 0;

		if (tcp_under_memory_pressure(sk))
			tcp_adjust_rcv_ssthresh(sk);

		/* if free space is too low, return a zero window */
		if (free_space < (allowed_space >> 4) || free_space < mss ||
			free_space < (1 << tp->rx_opt.rcv_wscale))
			return 0;
	}

	if (free_space > tp->rcv_ssthresh) {
		free_space = tp->rcv_ssthresh;
		/* new window should always be an exact multiple of scaling factor
		 *
		 * For this case, we ALIGN "up" (increase free_space) because
		 * we know free_space is not zero here, it has been reduced from
		 * the memory-based limit, and rcv_ssthresh is not a hard limit
		 * (unlike sk_rcvbuf).
		 */
		free_space = ALIGN(free_space, (1 << tp->rx_opt.rcv_wscale));
	}

	return free_space;
}

void tcp_skb_collapse_tstamp(struct sk_buff *skb,
			     const struct sk_buff *next_skb)
{
	if (unlikely(tcp_has_tx_tstamp(next_skb))) {
		const struct skb_shared_info *next_shinfo =
			skb_shinfo(next_skb);
		struct skb_shared_info *shinfo = skb_shinfo(skb);

		shinfo->tx_flags |= next_shinfo->tx_flags & SKBTX_ANY_TSTAMP;
		shinfo->tskey = next_shinfo->tskey;
		TCP_SKB_CB(skb)->txstamp_ack |=
			TCP_SKB_CB(next_skb)->txstamp_ack;
	}
}

/* Collapses two adjacent SKB's during retransmission. */
static bool tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *next_skb = skb_rb_next(skb);
	int next_skb_size;

	next_skb_size = next_skb->len;

	BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);

	if (next_skb_size && !tcp_skb_shift(skb, next_skb, 1, next_skb_size))
		return false;

	tcp_highest_sack_replace(sk, next_skb, skb);

	/* Update sequence range on original skb. */
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;

	/* Merge over control information. This moves PSH/FIN etc. over */
	TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;

	/* All done, get rid of second SKB and account for it so
	 * packet counting does not break.
	 */
	TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;
	TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;

	/* changed transmit queue under us so clear hints */
	if (next_skb == tp->retransmit_skb_hint)
		tp->retransmit_skb_hint = skb;

	tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));

	tcp_skb_collapse_tstamp(skb, next_skb);

	tcp_rtx_queue_unlink_and_free(next_skb, sk);
	return true;
}

/* Check if coalescing SKBs is legal. */
static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
{
	if (tcp_skb_pcount(skb) > 1)
		return false;
	if (skb_cloned(skb))
		return false;
	if (!skb_frags_readable(skb))
		return false;
	/* Some heuristics for collapsing over SACK'd could be invented */
	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
		return false;

	return true;
}

/* Collapse packets in the retransmit queue to make to create
 * less packets on the wire. This is only done on retransmission.
 */
static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
				     int space)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb = to, *tmp;
	bool first = true;

	if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_retrans_collapse))
		return;
	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
		return;

	skb_rbtree_walk_from_safe(skb, tmp) {
		if (!tcp_can_collapse(sk, skb))
			break;

		if (!tcp_skb_can_collapse(to, skb))
			break;

		space -= skb->len;

		if (first) {
			first = false;
			continue;
		}

		if (space < 0)
			break;

		if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
			break;

		if (!tcp_collapse_retrans(sk, to))
			break;
	}
}

/* This retransmits one SKB.  Policy decisions and retransmit queue
 * state updates are done by the caller.  Returns non-zero if an
 * error occurred which prevented the send.
 */
int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned int cur_mss;
	int diff, len, err;
	int avail_wnd;

	/* Inconclusive MTU probe */
	if (icsk->icsk_mtup.probe_size)
		icsk->icsk_mtup.probe_size = 0;

	if (skb_still_in_host_queue(sk, skb)) {
		err = -EBUSY;
		goto out;
	}

start:
	if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
		if (unlikely(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
			TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_SYN;
			TCP_SKB_CB(skb)->seq++;
			goto start;
		}
		if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
			WARN_ON_ONCE(1);
			err = -EINVAL;
			goto out;
		}
		if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq)) {
			err = -ENOMEM;
			goto out;
		}
	}

	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk)) {
		err = -EHOSTUNREACH; /* Routing failure or similar. */
		goto out;
	}

	cur_mss = tcp_current_mss(sk);
	avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

	/* If receiver has shrunk his window, and skb is out of
	 * new window, do not retransmit it. The exception is the
	 * case, when window is shrunk to zero. In this case
	 * our retransmit of one segment serves as a zero window probe.
	 */
	if (avail_wnd <= 0) {
		if (TCP_SKB_CB(skb)->seq != tp->snd_una) {
			err = -EAGAIN;
			goto out;
		}
		avail_wnd = cur_mss;
	}

	len = cur_mss * segs;
	if (len > avail_wnd) {
		len = rounddown(avail_wnd, cur_mss);
		if (!len)
			len = avail_wnd;
	}
	if (skb->len > len) {
		if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
				 cur_mss, GFP_ATOMIC)) {
			err = -ENOMEM;  /* We'll try again later. */
			goto out;
		}
	} else {
		if (skb_unclone_keeptruesize(skb, GFP_ATOMIC)) {
			err = -ENOMEM;
			goto out;
		}

		diff = tcp_skb_pcount(skb);
		tcp_set_skb_tso_segs(skb, cur_mss);
		diff -= tcp_skb_pcount(skb);
		if (diff)
			tcp_adjust_pcount(sk, skb, diff);
		avail_wnd = min_t(int, avail_wnd, cur_mss);
		if (skb->len < avail_wnd)
			tcp_retrans_try_collapse(sk, skb, avail_wnd);
	}

	if (!tcp_ecn_mode_pending(tp) || icsk->icsk_retransmits > 1) {
		/* RFC3168, section 6.1.1.1. ECN fallback
		 * As AccECN uses the same SYN flags (+ AE), this check
		 * covers both cases.
		 */
		if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) ==
		    TCPHDR_SYN_ECN)
			tcp_ecn_clear_syn(sk, skb);
	}

	/* Update global and local TCP statistics. */
	segs = tcp_skb_pcount(skb);
	TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
	tp->total_retrans += segs;
	tp->bytes_retrans += skb->len;

	/* make sure skb->data is aligned on arches that require it
	 * and check if ack-trimming & collapsing extended the headroom
	 * beyond what csum_start can cover.
	 */
	if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
		     skb_headroom(skb) >= 0xFFFF)) {
		struct sk_buff *nskb;

		tcp_skb_tsorted_save(skb) {
			nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
			if (nskb) {
				nskb->dev = NULL;
				err = tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC);
			} else {
				err = -ENOBUFS;
			}
		} tcp_skb_tsorted_restore(skb);

		if (!err) {
			tcp_update_skb_after_send(sk, skb, tp->tcp_wstamp_ns);
			tcp_rate_skb_sent(sk, skb);
		}
	} else {
		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
	}

	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
				  TCP_SKB_CB(skb)->seq, segs, err);

	if (unlikely(err) && err != -EBUSY)
		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);

	/* To avoid taking spuriously low RTT samples based on a timestamp
	 * for a transmit that never happened, always mark EVER_RETRANS
	 */
	TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;

out:
	trace_tcp_retransmit_skb(sk, skb, err);
	return err;
}

int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
{
	struct tcp_sock *tp = tcp_sk(sk);
	int err = __tcp_retransmit_skb(sk, skb, segs);

	if (err == 0) {
#if FASTRETRANS_DEBUG > 0
		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
			net_dbg_ratelimited("retrans_out leaked\n");
		}
#endif
		TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
		tp->retrans_out += tcp_skb_pcount(skb);
	}

	/* Save stamp of the first (attempted) retransmit. */
	if (!tp->retrans_stamp)
		tp->retrans_stamp = tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb);

	if (tp->undo_retrans < 0)
		tp->undo_retrans = 0;
	tp->undo_retrans += tcp_skb_pcount(skb);
	return err;
}

/* This gets called after a retransmit timeout, and the initially
 * retransmitted data is acknowledged.  It tries to continue
 * resending the rest of the retransmit queue, until either
 * we've sent it all or the congestion window limit is reached.
 */
void tcp_xmit_retransmit_queue(struct sock *sk)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);
	struct sk_buff *skb, *rtx_head, *hole = NULL;
	struct tcp_sock *tp = tcp_sk(sk);
	bool rearm_timer = false;
	u32 max_segs;
	int mib_idx;

	if (!tp->packets_out)
		return;

	rtx_head = tcp_rtx_queue_head(sk);
	skb = tp->retransmit_skb_hint ?: rtx_head;
	max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
	skb_rbtree_walk_from(skb) {
		__u8 sacked;
		int segs;

		if (tcp_pacing_check(sk))
			break;

		/* we could do better than to assign each time */
		if (!hole)
			tp->retransmit_skb_hint = skb;

		segs = tcp_snd_cwnd(tp) - tcp_packets_in_flight(tp);
		if (segs <= 0)
			break;
		sacked = TCP_SKB_CB(skb)->sacked;
		/* In case tcp_shift_skb_data() have aggregated large skbs,
		 * we need to make sure not sending too bigs TSO packets
		 */
		segs = min_t(int, segs, max_segs);

		if (tp->retrans_out >= tp->lost_out) {
			break;
		} else if (!(sacked & TCPCB_LOST)) {
			if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
				hole = skb;
			continue;

		} else {
			if (icsk->icsk_ca_state != TCP_CA_Loss)
				mib_idx = LINUX_MIB_TCPFASTRETRANS;
			else
				mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
		}

		if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
			continue;

		if (tcp_small_queue_check(sk, skb, 1))
			break;

		if (tcp_retransmit_skb(sk, skb, segs))
			break;

		NET_ADD_STATS(sock_net(sk), mib_idx, tcp_skb_pcount(skb));

		if (tcp_in_cwnd_reduction(sk))
			tp->prr_out += tcp_skb_pcount(skb);

		if (skb == rtx_head &&
		    icsk->icsk_pending != ICSK_TIME_REO_TIMEOUT)
			rearm_timer = true;

	}
	if (rearm_timer)
		tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
				     inet_csk(sk)->icsk_rto, true);
}

/* Send a FIN. The caller locks the socket for us.
 * We should try to send a FIN packet really hard, but eventually give up.
 */
void tcp_send_fin(struct sock *sk)
{
	struct sk_buff *skb, *tskb, *tail = tcp_write_queue_tail(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	/* Optimization, tack on the FIN if we have one skb in write queue and
	 * this skb was not yet sent, or we are under memory pressure.
	 * Note: in the latter case, FIN packet will be sent after a timeout,
	 * as TCP stack thinks it has already been transmitted.
	 */
	tskb = tail;
	if (!tskb && tcp_under_memory_pressure(sk))
		tskb = skb_rb_last(&sk->tcp_rtx_queue);

	if (tskb) {
		TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;
		TCP_SKB_CB(tskb)->end_seq++;
		tp->write_seq++;
		if (!tail) {
			/* This means tskb was already sent.
			 * Pretend we included the FIN on previous transmit.
			 * We need to set tp->snd_nxt to the value it would have
			 * if FIN had been sent. This is because retransmit path
			 * does not change tp->snd_nxt.
			 */
			WRITE_ONCE(tp->snd_nxt, tp->snd_nxt + 1);
			return;
		}
	} else {
		skb = alloc_skb_fclone(MAX_TCP_HEADER,
				       sk_gfp_mask(sk, GFP_ATOMIC |
						       __GFP_NOWARN));
		if (unlikely(!skb))
			return;

		INIT_LIST_HEAD(&skb->tcp_tsorted_anchor);
		skb_reserve(skb, MAX_TCP_HEADER);
		sk_forced_mem_schedule(sk, skb->truesize);
		/* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
		tcp_init_nondata_skb(skb, sk, tp->write_seq,
				     TCPHDR_ACK | TCPHDR_FIN);
		tcp_queue_skb(sk, skb);
	}
	__tcp_push_pending_frames(sk, tcp_current_mss(sk), TCP_NAGLE_OFF);
}

/* We get here when a process closes a file descriptor (either due to
 * an explicit close() or as a byproduct of exit()'ing) and there
 * was unread data in the receive queue.  This behavior is recommended
 * by RFC 2525, section 2.17.  -DaveM
 */
void tcp_send_active_reset(struct sock *sk, gfp_t priority,
			   enum sk_rst_reason reason)
{
	struct sk_buff *skb;

	TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);

	/* NOTE: No TCP options attached and we never retransmit this. */
	skb = alloc_skb(MAX_TCP_HEADER, priority);
	if (!skb) {
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
		return;
	}

	/* Reserve space for headers and prepare control bits. */
	skb_reserve(skb, MAX_TCP_HEADER);
	tcp_init_nondata_skb(skb, sk, tcp_acceptable_seq(sk),
			     TCPHDR_ACK | TCPHDR_RST);
	tcp_mstamp_refresh(tcp_sk(sk));
	/* Send it off. */
	if (tcp_transmit_skb(sk, skb, 0, priority))
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);

	/* skb of trace_tcp_send_reset() keeps the skb that caused RST,
	 * skb here is different to the troublesome skb, so use NULL
	 */
	trace_tcp_send_reset(sk, NULL, reason);
}

/* Send a crossed SYN-ACK during socket establishment.
 * WARNING: This routine must only be called when we have already sent
 * a SYN packet that crossed the incoming SYN that caused this routine
 * to get called. If this assumption fails then the initial rcv_wnd
 * and rcv_wscale values will not be correct.
 */
int tcp_send_synack(struct sock *sk)
{
	struct sk_buff *skb;

	skb = tcp_rtx_queue_head(sk);
	if (!skb || !(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
		pr_err("%s: wrong queue state\n", __func__);
		return -EFAULT;
	}
	if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
		if (skb_cloned(skb)) {
			struct sk_buff *nskb;

			tcp_skb_tsorted_save(skb) {
				nskb = skb_copy(skb, GFP_ATOMIC);
			} tcp_skb_tsorted_restore(skb);
			if (!nskb)
				return -ENOMEM;
			INIT_LIST_HEAD(&nskb->tcp_tsorted_anchor);
			tcp_highest_sack_replace(sk, skb, nskb);
			tcp_rtx_queue_unlink_and_free(skb, sk);
			__skb_header_release(nskb);
			tcp_rbtree_insert(&sk->tcp_rtx_queue, nskb);
			sk_wmem_queued_add(sk, nskb->truesize);
			sk_mem_charge(sk, nskb->truesize);
			skb = nskb;
		}

		TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ACK;
		tcp_ecn_send_synack(sk, skb);
	}
	return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
}

/**
 * tcp_make_synack - Allocate one skb and build a SYNACK packet.
 * @sk: listener socket
 * @dst: dst entry attached to the SYNACK. It is consumed and caller
 *       should not use it again.
 * @req: request_sock pointer
 * @foc: cookie for tcp fast open
 * @synack_type: Type of synack to prepare
 * @syn_skb: SYN packet just received.  It could be NULL for rtx case.
 */
struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
				struct request_sock *req,
				struct tcp_fastopen_cookie *foc,
				enum tcp_synack_type synack_type,
				struct sk_buff *syn_skb)
{
	struct inet_request_sock *ireq = inet_rsk(req);
	const struct tcp_sock *tp = tcp_sk(sk);
	struct tcp_out_options opts;
	struct tcp_key key = {};
	struct sk_buff *skb;
	int tcp_header_size;
	struct tcphdr *th;
	int mss;
	u64 now;

	skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
	if (unlikely(!skb)) {
		dst_release(dst);
		return NULL;
	}
	/* Reserve space for headers. */
	skb_reserve(skb, MAX_TCP_HEADER);

	switch (synack_type) {
	case TCP_SYNACK_NORMAL:
	case TCP_SYNACK_RETRANS:
		skb_set_owner_edemux(skb, req_to_sk(req));
		break;
	case TCP_SYNACK_COOKIE:
		/* Under synflood, we do not attach skb to a socket,
		 * to avoid false sharing.
		 */
		break;
	case TCP_SYNACK_FASTOPEN:
		/* sk is a const pointer, because we want to express multiple
		 * cpu might call us concurrently.
		 * sk->sk_wmem_alloc in an atomic, we can promote to rw.
		 */
		skb_set_owner_w(skb, (struct sock *)sk);
		break;
	}
	skb_dst_set(skb, dst);

	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));

	memset(&opts, 0, sizeof(opts));
	now = tcp_clock_ns();
#ifdef CONFIG_SYN_COOKIES
	if (unlikely(synack_type == TCP_SYNACK_COOKIE && ireq->tstamp_ok))
		skb_set_delivery_time(skb, cookie_init_timestamp(req, now),
				      SKB_CLOCK_MONOTONIC);
	else
#endif
	{
		skb_set_delivery_time(skb, now, SKB_CLOCK_MONOTONIC);
		if (!tcp_rsk(req)->snt_synack) /* Timestamp first SYNACK */
			tcp_rsk(req)->snt_synack = tcp_skb_timestamp_us(skb);
	}

#if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
	rcu_read_lock();
#endif
	if (tcp_rsk_used_ao(req)) {
#ifdef CONFIG_TCP_AO
		struct tcp_ao_key *ao_key = NULL;
		u8 keyid = tcp_rsk(req)->ao_keyid;
		u8 rnext = tcp_rsk(req)->ao_rcv_next;

		ao_key = tcp_sk(sk)->af_specific->ao_lookup(sk, req_to_sk(req),
							    keyid, -1);
		/* If there is no matching key - avoid sending anything,
		 * especially usigned segments. It could try harder and lookup
		 * for another peer-matching key, but the peer has requested
		 * ao_keyid (RFC5925 RNextKeyID), so let's keep it simple here.
		 */
		if (unlikely(!ao_key)) {
			trace_tcp_ao_synack_no_key(sk, keyid, rnext);
			rcu_read_unlock();
			kfree_skb(skb);
			net_warn_ratelimited("TCP-AO: the keyid %u from SYN packet is not present - not sending SYNACK\n",
					     keyid);
			return NULL;
		}
		key.ao_key = ao_key;
		key.type = TCP_KEY_AO;
#endif
	} else {
#ifdef CONFIG_TCP_MD5SIG
		key.md5_key = tcp_rsk(req)->af_specific->req_md5_lookup(sk,
					req_to_sk(req));
		if (key.md5_key)
			key.type = TCP_KEY_MD5;
#endif
	}
	skb_set_hash(skb, READ_ONCE(tcp_rsk(req)->txhash), PKT_HASH_TYPE_L4);
	/* bpf program will be interested in the tcp_flags */
	TCP_SKB_CB(skb)->tcp_flags = TCPHDR_SYN | TCPHDR_ACK;
	tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts,
					     &key, foc, synack_type, syn_skb)
					+ sizeof(*th);

	skb_push(skb, tcp_header_size);
	skb_reset_transport_header(skb);

	th = (struct tcphdr *)skb->data;
	memset(th, 0, sizeof(struct tcphdr));
	th->syn = 1;
	th->ack = 1;
	tcp_ecn_make_synack(req, th, synack_type);
	th->source = htons(ireq->ir_num);
	th->dest = ireq->ir_rmt_port;
	skb->mark = ireq->ir_mark;
	skb->ip_summed = CHECKSUM_PARTIAL;
	th->seq = htonl(tcp_rsk(req)->snt_isn);
	/* XXX data is queued and acked as is. No buffer/window check */
	th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);

	/* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
	th->window = htons(min(req->rsk_rcv_wnd, 65535U));
	tcp_options_write(th, NULL, tcp_rsk(req), &opts, &key);
	th->doff = (tcp_header_size >> 2);
	TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);

	/* Okay, we have all we need - do the md5 hash if needed */
	if (tcp_key_is_md5(&key)) {
#ifdef CONFIG_TCP_MD5SIG
		tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
					key.md5_key, req_to_sk(req), skb);
#endif
	} else if (tcp_key_is_ao(&key)) {
#ifdef CONFIG_TCP_AO
		tcp_rsk(req)->af_specific->ao_synack_hash(opts.hash_location,
					key.ao_key, req, skb,
					opts.hash_location - (u8 *)th, 0);
#endif
	}
#if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
	rcu_read_unlock();
#endif

	bpf_skops_write_hdr_opt((struct sock *)sk, skb, req, syn_skb,
				synack_type, &opts);

	skb_set_delivery_time(skb, now, SKB_CLOCK_MONOTONIC);
	tcp_add_tx_delay(skb, tp);

	return skb;
}
EXPORT_IPV6_MOD(tcp_make_synack);

static void tcp_ca_dst_init(struct sock *sk, const struct dst_entry *dst)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	const struct tcp_congestion_ops *ca;
	u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);

	if (ca_key == TCP_CA_UNSPEC)
		return;

	rcu_read_lock();
	ca = tcp_ca_find_key(ca_key);
	if (likely(ca && bpf_try_module_get(ca, ca->owner))) {
		bpf_module_put(icsk->icsk_ca_ops, icsk->icsk_ca_ops->owner);
		icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
		icsk->icsk_ca_ops = ca;
	}
	rcu_read_unlock();
}

/* Do all connect socket setups that can be done AF independent. */
static void tcp_connect_init(struct sock *sk)
{
	const struct dst_entry *dst = __sk_dst_get(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	__u8 rcv_wscale;
	u16 user_mss;
	u32 rcv_wnd;

	/* We'll fix this up when we get a response from the other end.
	 * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
	 */
	tp->tcp_header_len = sizeof(struct tcphdr);
	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps))
		tp->tcp_header_len += TCPOLEN_TSTAMP_ALIGNED;

	tcp_ao_connect_init(sk);

	/* If user gave his TCP_MAXSEG, record it to clamp */
	user_mss = READ_ONCE(tp->rx_opt.user_mss);
	if (user_mss)
		tp->rx_opt.mss_clamp = user_mss;
	tp->max_window = 0;
	tcp_mtup_init(sk);
	tcp_sync_mss(sk, dst_mtu(dst));

	tcp_ca_dst_init(sk, dst);

	if (!tp->window_clamp)
		WRITE_ONCE(tp->window_clamp, dst_metric(dst, RTAX_WINDOW));
	tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));

	tcp_initialize_rcv_mss(sk);

	/* limit the window selection if the user enforce a smaller rx buffer */
	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
	    (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
		WRITE_ONCE(tp->window_clamp, tcp_full_space(sk));

	rcv_wnd = tcp_rwnd_init_bpf(sk);
	if (rcv_wnd == 0)
		rcv_wnd = dst_metric(dst, RTAX_INITRWND);

	tcp_select_initial_window(sk, tcp_full_space(sk),
				  tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
				  &tp->rcv_wnd,
				  &tp->window_clamp,
				  READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling),
				  &rcv_wscale,
				  rcv_wnd);

	tp->rx_opt.rcv_wscale = rcv_wscale;
	tp->rcv_ssthresh = tp->rcv_wnd;

	WRITE_ONCE(sk->sk_err, 0);
	sock_reset_flag(sk, SOCK_DONE);
	tp->snd_wnd = 0;
	tcp_init_wl(tp, 0);
	tcp_write_queue_purge(sk);
	tp->snd_una = tp->write_seq;
	tp->snd_sml = tp->write_seq;
	tp->snd_up = tp->write_seq;
	WRITE_ONCE(tp->snd_nxt, tp->write_seq);

	if (likely(!tp->repair))
		tp->rcv_nxt = 0;
	else
		tp->rcv_tstamp = tcp_jiffies32;
	tp->rcv_wup = tp->rcv_nxt;
	WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);

	inet_csk(sk)->icsk_rto = tcp_timeout_init(sk);
	WRITE_ONCE(inet_csk(sk)->icsk_retransmits, 0);
	tcp_clear_retrans(tp);
}

static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);

	tcb->end_seq += skb->len;
	__skb_header_release(skb);
	sk_wmem_queued_add(sk, skb->truesize);
	sk_mem_charge(sk, skb->truesize);
	WRITE_ONCE(tp->write_seq, tcb->end_seq);
	tp->packets_out += tcp_skb_pcount(skb);
}

/* Build and send a SYN with data and (cached) Fast Open cookie. However,
 * queue a data-only packet after the regular SYN, such that regular SYNs
 * are retransmitted on timeouts. Also if the remote SYN-ACK acknowledges
 * only the SYN sequence, the data are retransmitted in the first ACK.
 * If cookie is not cached or other error occurs, falls back to send a
 * regular SYN with Fast Open cookie request option.
 */
static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	struct tcp_fastopen_request *fo = tp->fastopen_req;
	struct page_frag *pfrag = sk_page_frag(sk);
	struct sk_buff *syn_data;
	int space, err = 0;

	tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
	if (!tcp_fastopen_cookie_check(sk, &tp->rx_opt.mss_clamp, &fo->cookie))
		goto fallback;

	/* MSS for SYN-data is based on cached MSS and bounded by PMTU and
	 * user-MSS. Reserve maximum option space for middleboxes that add
	 * private TCP options. The cost is reduced data space in SYN :(
	 */
	tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp);
	/* Sync mss_cache after updating the mss_clamp */
	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);

	space = __tcp_mtu_to_mss(sk, icsk->icsk_pmtu_cookie) -
		MAX_TCP_OPTION_SPACE;

	space = min_t(size_t, space, fo->size);

	if (space &&
	    !skb_page_frag_refill(min_t(size_t, space, PAGE_SIZE),
				  pfrag, sk->sk_allocation))
		goto fallback;
	syn_data = tcp_stream_alloc_skb(sk, sk->sk_allocation, false);
	if (!syn_data)
		goto fallback;
	memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
	if (space) {
		space = min_t(size_t, space, pfrag->size - pfrag->offset);
		space = tcp_wmem_schedule(sk, space);
	}
	if (space) {
		space = copy_page_from_iter(pfrag->page, pfrag->offset,
					    space, &fo->data->msg_iter);
		if (unlikely(!space)) {
			tcp_skb_tsorted_anchor_cleanup(syn_data);
			kfree_skb(syn_data);
			goto fallback;
		}
		skb_fill_page_desc(syn_data, 0, pfrag->page,
				   pfrag->offset, space);
		page_ref_inc(pfrag->page);
		pfrag->offset += space;
		skb_len_add(syn_data, space);
		skb_zcopy_set(syn_data, fo->uarg, NULL);
	}
	/* No more data pending in inet_wait_for_connect() */
	if (space == fo->size)
		fo->data = NULL;
	fo->copied = space;

	tcp_connect_queue_skb(sk, syn_data);
	if (syn_data->len)
		tcp_chrono_start(sk, TCP_CHRONO_BUSY);

	err = tcp_transmit_skb(sk, syn_data, 1, sk->sk_allocation);

	skb_set_delivery_time(syn, syn_data->skb_mstamp_ns, SKB_CLOCK_MONOTONIC);

	/* Now full SYN+DATA was cloned and sent (or not),
	 * remove the SYN from the original skb (syn_data)
	 * we keep in write queue in case of a retransmit, as we
	 * also have the SYN packet (with no data) in the same queue.
	 */
	TCP_SKB_CB(syn_data)->seq++;
	TCP_SKB_CB(syn_data)->tcp_flags = TCPHDR_ACK | TCPHDR_PSH;
	if (!err) {
		tp->syn_data = (fo->copied > 0);
		tcp_rbtree_insert(&sk->tcp_rtx_queue, syn_data);
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT);
		goto done;
	}

	/* data was not sent, put it in write_queue */
	__skb_queue_tail(&sk->sk_write_queue, syn_data);
	tp->packets_out -= tcp_skb_pcount(syn_data);

fallback:
	/* Send a regular SYN with Fast Open cookie request option */
	if (fo->cookie.len > 0)
		fo->cookie.len = 0;
	err = tcp_transmit_skb(sk, syn, 1, sk->sk_allocation);
	if (err)
		tp->syn_fastopen = 0;
done:
	fo->cookie.len = -1;  /* Exclude Fast Open option for SYN retries */
	return err;
}

/* Build a SYN and send it off. */
int tcp_connect(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *buff;
	int err;

	tcp_call_bpf(sk, BPF_SOCK_OPS_TCP_CONNECT_CB, 0, NULL);

#if defined(CONFIG_TCP_MD5SIG) && defined(CONFIG_TCP_AO)
	/* Has to be checked late, after setting daddr/saddr/ops.
	 * Return error if the peer has both a md5 and a tcp-ao key
	 * configured as this is ambiguous.
	 */
	if (unlikely(rcu_dereference_protected(tp->md5sig_info,
					       lockdep_sock_is_held(sk)))) {
		bool needs_ao = !!tp->af_specific->ao_lookup(sk, sk, -1, -1);
		bool needs_md5 = !!tp->af_specific->md5_lookup(sk, sk);
		struct tcp_ao_info *ao_info;

		ao_info = rcu_dereference_check(tp->ao_info,
						lockdep_sock_is_held(sk));
		if (ao_info) {
			/* This is an extra check: tcp_ao_required() in
			 * tcp_v{4,6}_parse_md5_keys() should prevent adding
			 * md5 keys on ao_required socket.
			 */
			needs_ao |= ao_info->ao_required;
			WARN_ON_ONCE(ao_info->ao_required && needs_md5);
		}
		if (needs_md5 && needs_ao)
			return -EKEYREJECTED;

		/* If we have a matching md5 key and no matching tcp-ao key
		 * then free up ao_info if allocated.
		 */
		if (needs_md5) {
			tcp_ao_destroy_sock(sk, false);
		} else if (needs_ao) {
			tcp_clear_md5_list(sk);
			kfree(rcu_replace_pointer(tp->md5sig_info, NULL,
						  lockdep_sock_is_held(sk)));
		}
	}
#endif
#ifdef CONFIG_TCP_AO
	if (unlikely(rcu_dereference_protected(tp->ao_info,
					       lockdep_sock_is_held(sk)))) {
		/* Don't allow connecting if ao is configured but no
		 * matching key is found.
		 */
		if (!tp->af_specific->ao_lookup(sk, sk, -1, -1))
			return -EKEYREJECTED;
	}
#endif

	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
		return -EHOSTUNREACH; /* Routing failure or similar. */

	tcp_connect_init(sk);

	if (unlikely(tp->repair)) {
		tcp_finish_connect(sk, NULL);
		return 0;
	}

	buff = tcp_stream_alloc_skb(sk, sk->sk_allocation, true);
	if (unlikely(!buff))
		return -ENOBUFS;

	/* SYN eats a sequence byte, write_seq updated by
	 * tcp_connect_queue_skb().
	 */
	tcp_init_nondata_skb(buff, sk, tp->write_seq, TCPHDR_SYN);
	tcp_mstamp_refresh(tp);
	tp->retrans_stamp = tcp_time_stamp_ts(tp);
	tcp_connect_queue_skb(sk, buff);
	tcp_ecn_send_syn(sk, buff);
	tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);

	/* Send off SYN; include data in Fast Open. */
	err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
	      tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
	if (err == -ECONNREFUSED)
		return err;

	/* We change tp->snd_nxt after the tcp_transmit_skb() call
	 * in order to make this packet get counted in tcpOutSegs.
	 */
	WRITE_ONCE(tp->snd_nxt, tp->write_seq);
	tp->pushed_seq = tp->write_seq;
	buff = tcp_send_head(sk);
	if (unlikely(buff)) {
		WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(buff)->seq);
		tp->pushed_seq	= TCP_SKB_CB(buff)->seq;
	}
	TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);

	/* Timer for repeating the SYN until an answer. */
	tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
			     inet_csk(sk)->icsk_rto, false);
	return 0;
}
EXPORT_SYMBOL(tcp_connect);

u32 tcp_delack_max(const struct sock *sk)
{
	u32 delack_from_rto_min = max(tcp_rto_min(sk), 2) - 1;

	return min(READ_ONCE(inet_csk(sk)->icsk_delack_max), delack_from_rto_min);
}

/* Send out a delayed ack, the caller does the policy checking
 * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
 * for details.
 */
void tcp_send_delayed_ack(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	int ato = icsk->icsk_ack.ato;
	unsigned long timeout;

	if (ato > TCP_DELACK_MIN) {
		const struct tcp_sock *tp = tcp_sk(sk);
		int max_ato = HZ / 2;

		if (inet_csk_in_pingpong_mode(sk) ||
		    (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
			max_ato = TCP_DELACK_MAX;

		/* Slow path, intersegment interval is "high". */

		/* If some rtt estimate is known, use it to bound delayed ack.
		 * Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
		 * directly.
		 */
		if (tp->srtt_us) {
			int rtt = max_t(int, usecs_to_jiffies(tp->srtt_us >> 3),
					TCP_DELACK_MIN);

			if (rtt < max_ato)
				max_ato = rtt;
		}

		ato = min(ato, max_ato);
	}

	ato = min_t(u32, ato, tcp_delack_max(sk));

	/* Stay within the limit we were given */
	timeout = jiffies + ato;

	/* Use new timeout only if there wasn't a older one earlier. */
	if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
		/* If delack timer is about to expire, send ACK now. */
		if (time_before_eq(icsk_delack_timeout(icsk), jiffies + (ato >> 2))) {
			tcp_send_ack(sk);
			return;
		}

		if (!time_before(timeout, icsk_delack_timeout(icsk)))
			timeout = icsk_delack_timeout(icsk);
	}
	smp_store_release(&icsk->icsk_ack.pending,
			  icsk->icsk_ack.pending | ICSK_ACK_SCHED | ICSK_ACK_TIMER);
	sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
}

/* This routine sends an ack and also updates the window. */
void __tcp_send_ack(struct sock *sk, u32 rcv_nxt, u16 flags)
{
	struct sk_buff *buff;

	/* If we have been reset, we may not send again. */
	if (sk->sk_state == TCP_CLOSE)
		return;

	/* We are not putting this on the write queue, so
	 * tcp_transmit_skb() will set the ownership to this
	 * sock.
	 */
	buff = alloc_skb(MAX_TCP_HEADER,
			 sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
	if (unlikely(!buff)) {
		struct inet_connection_sock *icsk = inet_csk(sk);
		unsigned long delay;

		delay = TCP_DELACK_MAX << icsk->icsk_ack.retry;
		if (delay < tcp_rto_max(sk))
			icsk->icsk_ack.retry++;
		inet_csk_schedule_ack(sk);
		icsk->icsk_ack.ato = TCP_ATO_MIN;
		tcp_reset_xmit_timer(sk, ICSK_TIME_DACK, delay, false);
		return;
	}

	/* Reserve space for headers and prepare control bits. */
	skb_reserve(buff, MAX_TCP_HEADER);
	tcp_init_nondata_skb(buff, sk,
			     tcp_acceptable_seq(sk), TCPHDR_ACK | flags);

	/* We do not want pure acks influencing TCP Small Queues or fq/pacing
	 * too much.
	 * SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784
	 */
	skb_set_tcp_pure_ack(buff);

	/* Send it off, this clears delayed acks for us. */
	__tcp_transmit_skb(sk, buff, 0, (__force gfp_t)0, rcv_nxt);
}
EXPORT_SYMBOL_GPL(__tcp_send_ack);

void tcp_send_ack(struct sock *sk)
{
	__tcp_send_ack(sk, tcp_sk(sk)->rcv_nxt, 0);
}

/* This routine sends a packet with an out of date sequence
 * number. It assumes the other end will try to ack it.
 *
 * Question: what should we make while urgent mode?
 * 4.4BSD forces sending single byte of data. We cannot send
 * out of window data, because we have SND.NXT==SND.MAX...
 *
 * Current solution: to send TWO zero-length segments in urgent mode:
 * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
 * out-of-date with SND.UNA-1 to probe window.
 */
static int tcp_xmit_probe_skb(struct sock *sk, int urgent, int mib)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;

	/* We don't queue it, tcp_transmit_skb() sets ownership. */
	skb = alloc_skb(MAX_TCP_HEADER,
			sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
	if (!skb)
		return -1;

	/* Reserve space for headers and set control bits. */
	skb_reserve(skb, MAX_TCP_HEADER);
	/* Use a previous sequence.  This should cause the other
	 * end to send an ack.  Don't queue or clone SKB, just
	 * send it.
	 */
	tcp_init_nondata_skb(skb, sk, tp->snd_una - !urgent, TCPHDR_ACK);
	NET_INC_STATS(sock_net(sk), mib);
	return tcp_transmit_skb(sk, skb, 0, (__force gfp_t)0);
}

/* Called from setsockopt( ... TCP_REPAIR ) */
void tcp_send_window_probe(struct sock *sk)
{
	if (sk->sk_state == TCP_ESTABLISHED) {
		tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1;
		tcp_mstamp_refresh(tcp_sk(sk));
		tcp_xmit_probe_skb(sk, 0, LINUX_MIB_TCPWINPROBE);
	}
}

/* Initiate keepalive or window probe from timer. */
int tcp_write_wakeup(struct sock *sk, int mib)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;

	if (sk->sk_state == TCP_CLOSE)
		return -1;

	skb = tcp_send_head(sk);
	if (skb && before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
		int err;
		unsigned int mss = tcp_current_mss(sk);
		unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

		if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
			tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;

		/* We are probing the opening of a window
		 * but the window size is != 0
		 * must have been a result SWS avoidance ( sender )
		 */
		if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
		    skb->len > mss) {
			seg_size = min(seg_size, mss);
			TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
			if (tcp_fragment(sk, TCP_FRAG_IN_WRITE_QUEUE,
					 skb, seg_size, mss, GFP_ATOMIC))
				return -1;
		} else if (!tcp_skb_pcount(skb))
			tcp_set_skb_tso_segs(skb, mss);

		TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
		if (!err)
			tcp_event_new_data_sent(sk, skb);
		return err;
	} else {
		if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
			tcp_xmit_probe_skb(sk, 1, mib);
		return tcp_xmit_probe_skb(sk, 0, mib);
	}
}

/* A window probe timeout has occurred.  If window is not closed send
 * a partial packet else a zero probe.
 */
void tcp_send_probe0(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	struct net *net = sock_net(sk);
	unsigned long timeout;
	int err;

	err = tcp_write_wakeup(sk, LINUX_MIB_TCPWINPROBE);

	if (tp->packets_out || tcp_write_queue_empty(sk)) {
		/* Cancel probe timer, if it is not required. */
		WRITE_ONCE(icsk->icsk_probes_out, 0);
		icsk->icsk_backoff = 0;
		icsk->icsk_probes_tstamp = 0;
		return;
	}

	WRITE_ONCE(icsk->icsk_probes_out, icsk->icsk_probes_out + 1);
	if (err <= 0) {
		if (icsk->icsk_backoff < READ_ONCE(net->ipv4.sysctl_tcp_retries2))
			icsk->icsk_backoff++;
		timeout = tcp_probe0_when(sk, tcp_rto_max(sk));
	} else {
		/* If packet was not sent due to local congestion,
		 * Let senders fight for local resources conservatively.
		 */
		timeout = TCP_RESOURCE_PROBE_INTERVAL;
	}

	timeout = tcp_clamp_probe0_to_user_timeout(sk, timeout);
	tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, timeout, true);
}

int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
{
	const struct tcp_request_sock_ops *af_ops = tcp_rsk(req)->af_specific;
	struct flowi fl;
	int res;

	/* Paired with WRITE_ONCE() in sock_setsockopt() */
	if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)
		WRITE_ONCE(tcp_rsk(req)->txhash, net_tx_rndhash());
	res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_RETRANS,
				  NULL);
	if (!res) {
		TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
		if (unlikely(tcp_passive_fastopen(sk))) {
			/* sk has const attribute because listeners are lockless.
			 * However in this case, we are dealing with a passive fastopen
			 * socket thus we can change total_retrans value.
			 */
			tcp_sk_rw(sk)->total_retrans++;
		}
		trace_tcp_retransmit_synack(sk, req);
		WRITE_ONCE(req->num_retrans, req->num_retrans + 1);
	}
	return res;
}
EXPORT_IPV6_MOD(tcp_rtx_synack);