xref: /linux/net/ipv4/tcp_timer.c (revision a3f5f4c2f9b6bc2aa6f5a3e8e23b7519e4f2e3e3)
1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * INET		An implementation of the TCP/IP protocol suite for the LINUX
4  *		operating system.  INET is implemented using the  BSD Socket
5  *		interface as the means of communication with the user level.
6  *
7  *		Implementation of the Transmission Control Protocol(TCP).
8  *
9  * Authors:	Ross Biro
10  *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
11  *		Mark Evans, <evansmp@uhura.aston.ac.uk>
12  *		Corey Minyard <wf-rch!minyard@relay.EU.net>
13  *		Florian La Roche, <flla@stud.uni-sb.de>
14  *		Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
15  *		Linus Torvalds, <torvalds@cs.helsinki.fi>
16  *		Alan Cox, <gw4pts@gw4pts.ampr.org>
17  *		Matthew Dillon, <dillon@apollo.west.oic.com>
18  *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
19  *		Jorge Cwik, <jorge@laser.satlink.net>
20  */
21 
22 #include <linux/module.h>
23 #include <linux/gfp.h>
24 #include <net/tcp.h>
25 #include <net/rstreason.h>
26 
27 static u32 tcp_clamp_rto_to_user_timeout(const struct sock *sk)
28 {
29 	const struct inet_connection_sock *icsk = inet_csk(sk);
30 	const struct tcp_sock *tp = tcp_sk(sk);
31 	u32 elapsed, user_timeout;
32 	s32 remaining;
33 
34 	user_timeout = READ_ONCE(icsk->icsk_user_timeout);
35 	if (!user_timeout)
36 		return icsk->icsk_rto;
37 
38 	elapsed = tcp_time_stamp_ts(tp) - tp->retrans_stamp;
39 	if (tp->tcp_usec_ts)
40 		elapsed /= USEC_PER_MSEC;
41 
42 	remaining = user_timeout - elapsed;
43 	if (remaining <= 0)
44 		return 1; /* user timeout has passed; fire ASAP */
45 
46 	return min_t(u32, icsk->icsk_rto, msecs_to_jiffies(remaining));
47 }
48 
49 u32 tcp_clamp_probe0_to_user_timeout(const struct sock *sk, u32 when)
50 {
51 	const struct inet_connection_sock *icsk = inet_csk(sk);
52 	u32 remaining, user_timeout;
53 	s32 elapsed;
54 
55 	user_timeout = READ_ONCE(icsk->icsk_user_timeout);
56 	if (!user_timeout || !icsk->icsk_probes_tstamp)
57 		return when;
58 
59 	elapsed = tcp_jiffies32 - icsk->icsk_probes_tstamp;
60 	if (unlikely(elapsed < 0))
61 		elapsed = 0;
62 	remaining = msecs_to_jiffies(user_timeout) - elapsed;
63 	remaining = max_t(u32, remaining, TCP_TIMEOUT_MIN);
64 
65 	return min_t(u32, remaining, when);
66 }
67 
68 /**
69  *  tcp_write_err() - close socket and save error info
70  *  @sk:  The socket the error has appeared on.
71  *
72  *  Returns: Nothing (void)
73  */
74 
75 static void tcp_write_err(struct sock *sk)
76 {
77 	tcp_done_with_error(sk, READ_ONCE(sk->sk_err_soft) ? : ETIMEDOUT);
78 	__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONTIMEOUT);
79 }
80 
81 /**
82  *  tcp_out_of_resources() - Close socket if out of resources
83  *  @sk:        pointer to current socket
84  *  @do_reset:  send a last packet with reset flag
85  *
86  *  Do not allow orphaned sockets to eat all our resources.
87  *  This is direct violation of TCP specs, but it is required
88  *  to prevent DoS attacks. It is called when a retransmission timeout
89  *  or zero probe timeout occurs on orphaned socket.
90  *
91  *  Also close if our net namespace is exiting; in that case there is no
92  *  hope of ever communicating again since all netns interfaces are already
93  *  down (or about to be down), and we need to release our dst references,
94  *  which have been moved to the netns loopback interface, so the namespace
95  *  can finish exiting.  This condition is only possible if we are a kernel
96  *  socket, as those do not hold references to the namespace.
97  *
98  *  Criteria is still not confirmed experimentally and may change.
99  *  We kill the socket, if:
100  *  1. If number of orphaned sockets exceeds an administratively configured
101  *     limit.
102  *  2. If we have strong memory pressure.
103  *  3. If our net namespace is exiting.
104  */
105 static int tcp_out_of_resources(struct sock *sk, bool do_reset)
106 {
107 	struct tcp_sock *tp = tcp_sk(sk);
108 	int shift = 0;
109 
110 	/* If peer does not open window for long time, or did not transmit
111 	 * anything for long time, penalize it. */
112 	if ((s32)(tcp_jiffies32 - tp->lsndtime) > 2*TCP_RTO_MAX || !do_reset)
113 		shift++;
114 
115 	/* If some dubious ICMP arrived, penalize even more. */
116 	if (READ_ONCE(sk->sk_err_soft))
117 		shift++;
118 
119 	if (tcp_check_oom(sk, shift)) {
120 		/* Catch exceptional cases, when connection requires reset.
121 		 *      1. Last segment was sent recently. */
122 		if ((s32)(tcp_jiffies32 - tp->lsndtime) <= TCP_TIMEWAIT_LEN ||
123 		    /*  2. Window is closed. */
124 		    (!tp->snd_wnd && !tp->packets_out))
125 			do_reset = true;
126 		if (do_reset)
127 			tcp_send_active_reset(sk, GFP_ATOMIC,
128 					      SK_RST_REASON_TCP_ABORT_ON_MEMORY);
129 		tcp_done(sk);
130 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONMEMORY);
131 		return 1;
132 	}
133 
134 	if (!check_net(sock_net(sk))) {
135 		/* Not possible to send reset; just close */
136 		tcp_done(sk);
137 		return 1;
138 	}
139 
140 	return 0;
141 }
142 
143 /**
144  *  tcp_orphan_retries() - Returns maximal number of retries on an orphaned socket
145  *  @sk:    Pointer to the current socket.
146  *  @alive: bool, socket alive state
147  */
148 static int tcp_orphan_retries(struct sock *sk, bool alive)
149 {
150 	int retries = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_orphan_retries); /* May be zero. */
151 
152 	/* We know from an ICMP that something is wrong. */
153 	if (READ_ONCE(sk->sk_err_soft) && !alive)
154 		retries = 0;
155 
156 	/* However, if socket sent something recently, select some safe
157 	 * number of retries. 8 corresponds to >100 seconds with minimal
158 	 * RTO of 200msec. */
159 	if (retries == 0 && alive)
160 		retries = 8;
161 	return retries;
162 }
163 
164 static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
165 {
166 	const struct net *net = sock_net(sk);
167 	int mss;
168 
169 	/* Black hole detection */
170 	if (!READ_ONCE(net->ipv4.sysctl_tcp_mtu_probing))
171 		return;
172 
173 	if (!icsk->icsk_mtup.enabled) {
174 		icsk->icsk_mtup.enabled = 1;
175 		icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
176 	} else {
177 		mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
178 		mss = min(READ_ONCE(net->ipv4.sysctl_tcp_base_mss), mss);
179 		mss = max(mss, READ_ONCE(net->ipv4.sysctl_tcp_mtu_probe_floor));
180 		mss = max(mss, READ_ONCE(net->ipv4.sysctl_tcp_min_snd_mss));
181 		icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
182 	}
183 	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
184 }
185 
186 static unsigned int tcp_model_timeout(struct sock *sk,
187 				      unsigned int boundary,
188 				      unsigned int rto_base)
189 {
190 	unsigned int linear_backoff_thresh, timeout;
191 
192 	linear_backoff_thresh = ilog2(TCP_RTO_MAX / rto_base);
193 	if (boundary <= linear_backoff_thresh)
194 		timeout = ((2 << boundary) - 1) * rto_base;
195 	else
196 		timeout = ((2 << linear_backoff_thresh) - 1) * rto_base +
197 			(boundary - linear_backoff_thresh) * TCP_RTO_MAX;
198 	return jiffies_to_msecs(timeout);
199 }
200 /**
201  *  retransmits_timed_out() - returns true if this connection has timed out
202  *  @sk:       The current socket
203  *  @boundary: max number of retransmissions
204  *  @timeout:  A custom timeout value.
205  *             If set to 0 the default timeout is calculated and used.
206  *             Using TCP_RTO_MIN and the number of unsuccessful retransmits.
207  *
208  * The default "timeout" value this function can calculate and use
209  * is equivalent to the timeout of a TCP Connection
210  * after "boundary" unsuccessful, exponentially backed-off
211  * retransmissions with an initial RTO of TCP_RTO_MIN.
212  */
213 static bool retransmits_timed_out(struct sock *sk,
214 				  unsigned int boundary,
215 				  unsigned int timeout)
216 {
217 	struct tcp_sock *tp = tcp_sk(sk);
218 	unsigned int start_ts, delta;
219 
220 	if (!inet_csk(sk)->icsk_retransmits)
221 		return false;
222 
223 	start_ts = tp->retrans_stamp;
224 	if (likely(timeout == 0)) {
225 		unsigned int rto_base = TCP_RTO_MIN;
226 
227 		if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))
228 			rto_base = tcp_timeout_init(sk);
229 		timeout = tcp_model_timeout(sk, boundary, rto_base);
230 	}
231 
232 	if (tp->tcp_usec_ts) {
233 		/* delta maybe off up to a jiffy due to timer granularity. */
234 		delta = tp->tcp_mstamp - start_ts + jiffies_to_usecs(1);
235 		return (s32)(delta - timeout * USEC_PER_MSEC) >= 0;
236 	}
237 	return (s32)(tcp_time_stamp_ts(tp) - start_ts - timeout) >= 0;
238 }
239 
240 /* A write timeout has occurred. Process the after effects. */
241 static int tcp_write_timeout(struct sock *sk)
242 {
243 	struct inet_connection_sock *icsk = inet_csk(sk);
244 	struct tcp_sock *tp = tcp_sk(sk);
245 	struct net *net = sock_net(sk);
246 	bool expired = false, do_reset;
247 	int retry_until, max_retransmits;
248 
249 	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
250 		if (icsk->icsk_retransmits)
251 			__dst_negative_advice(sk);
252 		/* Paired with WRITE_ONCE() in tcp_sock_set_syncnt() */
253 		retry_until = READ_ONCE(icsk->icsk_syn_retries) ? :
254 			READ_ONCE(net->ipv4.sysctl_tcp_syn_retries);
255 
256 		max_retransmits = retry_until;
257 		if (sk->sk_state == TCP_SYN_SENT)
258 			max_retransmits += READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts);
259 
260 		expired = icsk->icsk_retransmits >= max_retransmits;
261 	} else {
262 		if (retransmits_timed_out(sk, READ_ONCE(net->ipv4.sysctl_tcp_retries1), 0)) {
263 			/* Black hole detection */
264 			tcp_mtu_probing(icsk, sk);
265 
266 			__dst_negative_advice(sk);
267 		}
268 
269 		retry_until = READ_ONCE(net->ipv4.sysctl_tcp_retries2);
270 		if (sock_flag(sk, SOCK_DEAD)) {
271 			const bool alive = icsk->icsk_rto < TCP_RTO_MAX;
272 
273 			retry_until = tcp_orphan_retries(sk, alive);
274 			do_reset = alive ||
275 				!retransmits_timed_out(sk, retry_until, 0);
276 
277 			if (tcp_out_of_resources(sk, do_reset))
278 				return 1;
279 		}
280 	}
281 	if (!expired)
282 		expired = retransmits_timed_out(sk, retry_until,
283 						READ_ONCE(icsk->icsk_user_timeout));
284 	tcp_fastopen_active_detect_blackhole(sk, expired);
285 	mptcp_active_detect_blackhole(sk, expired);
286 
287 	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RTO_CB_FLAG))
288 		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RTO_CB,
289 				  icsk->icsk_retransmits,
290 				  icsk->icsk_rto, (int)expired);
291 
292 	if (expired) {
293 		/* Has it gone just too far? */
294 		tcp_write_err(sk);
295 		return 1;
296 	}
297 
298 	if (sk_rethink_txhash(sk)) {
299 		tp->timeout_rehash++;
300 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEOUTREHASH);
301 	}
302 
303 	return 0;
304 }
305 
306 /* Called with BH disabled */
307 void tcp_delack_timer_handler(struct sock *sk)
308 {
309 	struct inet_connection_sock *icsk = inet_csk(sk);
310 	struct tcp_sock *tp = tcp_sk(sk);
311 
312 	if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
313 		return;
314 
315 	/* Handling the sack compression case */
316 	if (tp->compressed_ack) {
317 		tcp_mstamp_refresh(tp);
318 		tcp_sack_compress_send_ack(sk);
319 		return;
320 	}
321 
322 	if (!(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
323 		return;
324 
325 	if (time_after(icsk->icsk_ack.timeout, jiffies)) {
326 		sk_reset_timer(sk, &icsk->icsk_delack_timer, icsk->icsk_ack.timeout);
327 		return;
328 	}
329 	icsk->icsk_ack.pending &= ~ICSK_ACK_TIMER;
330 
331 	if (inet_csk_ack_scheduled(sk)) {
332 		if (!inet_csk_in_pingpong_mode(sk)) {
333 			/* Delayed ACK missed: inflate ATO. */
334 			icsk->icsk_ack.ato = min_t(u32, icsk->icsk_ack.ato << 1, icsk->icsk_rto);
335 		} else {
336 			/* Delayed ACK missed: leave pingpong mode and
337 			 * deflate ATO.
338 			 */
339 			inet_csk_exit_pingpong_mode(sk);
340 			icsk->icsk_ack.ato      = TCP_ATO_MIN;
341 		}
342 		tcp_mstamp_refresh(tp);
343 		tcp_send_ack(sk);
344 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKS);
345 	}
346 }
347 
348 
349 /**
350  *  tcp_delack_timer() - The TCP delayed ACK timeout handler
351  *  @t:  Pointer to the timer. (gets casted to struct sock *)
352  *
353  *  This function gets (indirectly) called when the kernel timer for a TCP packet
354  *  of this socket expires. Calls tcp_delack_timer_handler() to do the actual work.
355  *
356  *  Returns: Nothing (void)
357  */
358 static void tcp_delack_timer(struct timer_list *t)
359 {
360 	struct inet_connection_sock *icsk =
361 			from_timer(icsk, t, icsk_delack_timer);
362 	struct sock *sk = &icsk->icsk_inet.sk;
363 
364 	/* Avoid taking socket spinlock if there is no ACK to send.
365 	 * The compressed_ack check is racy, but a separate hrtimer
366 	 * will take care of it eventually.
367 	 */
368 	if (!(smp_load_acquire(&icsk->icsk_ack.pending) & ICSK_ACK_TIMER) &&
369 	    !READ_ONCE(tcp_sk(sk)->compressed_ack))
370 		goto out;
371 
372 	bh_lock_sock(sk);
373 	if (!sock_owned_by_user(sk)) {
374 		tcp_delack_timer_handler(sk);
375 	} else {
376 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
377 		/* deleguate our work to tcp_release_cb() */
378 		if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED, &sk->sk_tsq_flags))
379 			sock_hold(sk);
380 	}
381 	bh_unlock_sock(sk);
382 out:
383 	sock_put(sk);
384 }
385 
386 static void tcp_probe_timer(struct sock *sk)
387 {
388 	struct inet_connection_sock *icsk = inet_csk(sk);
389 	struct sk_buff *skb = tcp_send_head(sk);
390 	struct tcp_sock *tp = tcp_sk(sk);
391 	int max_probes;
392 
393 	if (tp->packets_out || !skb) {
394 		icsk->icsk_probes_out = 0;
395 		icsk->icsk_probes_tstamp = 0;
396 		return;
397 	}
398 
399 	/* RFC 1122 4.2.2.17 requires the sender to stay open indefinitely as
400 	 * long as the receiver continues to respond probes. We support this by
401 	 * default and reset icsk_probes_out with incoming ACKs. But if the
402 	 * socket is orphaned or the user specifies TCP_USER_TIMEOUT, we
403 	 * kill the socket when the retry count and the time exceeds the
404 	 * corresponding system limit. We also implement similar policy when
405 	 * we use RTO to probe window in tcp_retransmit_timer().
406 	 */
407 	if (!icsk->icsk_probes_tstamp) {
408 		icsk->icsk_probes_tstamp = tcp_jiffies32;
409 	} else {
410 		u32 user_timeout = READ_ONCE(icsk->icsk_user_timeout);
411 
412 		if (user_timeout &&
413 		    (s32)(tcp_jiffies32 - icsk->icsk_probes_tstamp) >=
414 		     msecs_to_jiffies(user_timeout))
415 			goto abort;
416 	}
417 	max_probes = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_retries2);
418 	if (sock_flag(sk, SOCK_DEAD)) {
419 		const bool alive = inet_csk_rto_backoff(icsk, TCP_RTO_MAX) < TCP_RTO_MAX;
420 
421 		max_probes = tcp_orphan_retries(sk, alive);
422 		if (!alive && icsk->icsk_backoff >= max_probes)
423 			goto abort;
424 		if (tcp_out_of_resources(sk, true))
425 			return;
426 	}
427 
428 	if (icsk->icsk_probes_out >= max_probes) {
429 abort:		tcp_write_err(sk);
430 	} else {
431 		/* Only send another probe if we didn't close things up. */
432 		tcp_send_probe0(sk);
433 	}
434 }
435 
436 static void tcp_update_rto_stats(struct sock *sk)
437 {
438 	struct inet_connection_sock *icsk = inet_csk(sk);
439 	struct tcp_sock *tp = tcp_sk(sk);
440 
441 	if (!icsk->icsk_retransmits) {
442 		tp->total_rto_recoveries++;
443 		tp->rto_stamp = tcp_time_stamp_ms(tp);
444 	}
445 	icsk->icsk_retransmits++;
446 	tp->total_rto++;
447 }
448 
449 /*
450  *	Timer for Fast Open socket to retransmit SYNACK. Note that the
451  *	sk here is the child socket, not the parent (listener) socket.
452  */
453 static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req)
454 {
455 	struct inet_connection_sock *icsk = inet_csk(sk);
456 	struct tcp_sock *tp = tcp_sk(sk);
457 	int max_retries;
458 
459 	req->rsk_ops->syn_ack_timeout(req);
460 
461 	/* Add one more retry for fastopen.
462 	 * Paired with WRITE_ONCE() in tcp_sock_set_syncnt()
463 	 */
464 	max_retries = READ_ONCE(icsk->icsk_syn_retries) ? :
465 		READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_synack_retries) + 1;
466 
467 	if (req->num_timeout >= max_retries) {
468 		tcp_write_err(sk);
469 		return;
470 	}
471 	/* Lower cwnd after certain SYNACK timeout like tcp_init_transfer() */
472 	if (icsk->icsk_retransmits == 1)
473 		tcp_enter_loss(sk);
474 	/* XXX (TFO) - Unlike regular SYN-ACK retransmit, we ignore error
475 	 * returned from rtx_syn_ack() to make it more persistent like
476 	 * regular retransmit because if the child socket has been accepted
477 	 * it's not good to give up too easily.
478 	 */
479 	inet_rtx_syn_ack(sk, req);
480 	req->num_timeout++;
481 	tcp_update_rto_stats(sk);
482 	if (!tp->retrans_stamp)
483 		tp->retrans_stamp = tcp_time_stamp_ts(tp);
484 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
485 			  req->timeout << req->num_timeout, TCP_RTO_MAX);
486 }
487 
488 static bool tcp_rtx_probe0_timed_out(const struct sock *sk,
489 				     const struct sk_buff *skb,
490 				     u32 rtx_delta)
491 {
492 	const struct inet_connection_sock *icsk = inet_csk(sk);
493 	u32 user_timeout = READ_ONCE(icsk->icsk_user_timeout);
494 	const struct tcp_sock *tp = tcp_sk(sk);
495 	int timeout = TCP_RTO_MAX * 2;
496 	s32 rcv_delta;
497 
498 	if (user_timeout) {
499 		/* If user application specified a TCP_USER_TIMEOUT,
500 		 * it does not want win 0 packets to 'reset the timer'
501 		 * while retransmits are not making progress.
502 		 */
503 		if (rtx_delta > user_timeout)
504 			return true;
505 		timeout = min_t(u32, timeout, msecs_to_jiffies(user_timeout));
506 	}
507 	/* Note: timer interrupt might have been delayed by at least one jiffy,
508 	 * and tp->rcv_tstamp might very well have been written recently.
509 	 * rcv_delta can thus be negative.
510 	 */
511 	rcv_delta = icsk->icsk_timeout - tp->rcv_tstamp;
512 	if (rcv_delta <= timeout)
513 		return false;
514 
515 	return msecs_to_jiffies(rtx_delta) > timeout;
516 }
517 
518 /**
519  *  tcp_retransmit_timer() - The TCP retransmit timeout handler
520  *  @sk:  Pointer to the current socket.
521  *
522  *  This function gets called when the kernel timer for a TCP packet
523  *  of this socket expires.
524  *
525  *  It handles retransmission, timer adjustment and other necessary measures.
526  *
527  *  Returns: Nothing (void)
528  */
529 void tcp_retransmit_timer(struct sock *sk)
530 {
531 	struct tcp_sock *tp = tcp_sk(sk);
532 	struct net *net = sock_net(sk);
533 	struct inet_connection_sock *icsk = inet_csk(sk);
534 	struct request_sock *req;
535 	struct sk_buff *skb;
536 
537 	req = rcu_dereference_protected(tp->fastopen_rsk,
538 					lockdep_sock_is_held(sk));
539 	if (req) {
540 		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
541 			     sk->sk_state != TCP_FIN_WAIT1);
542 		tcp_fastopen_synack_timer(sk, req);
543 		/* Before we receive ACK to our SYN-ACK don't retransmit
544 		 * anything else (e.g., data or FIN segments).
545 		 */
546 		return;
547 	}
548 
549 	if (!tp->packets_out)
550 		return;
551 
552 	skb = tcp_rtx_queue_head(sk);
553 	if (WARN_ON_ONCE(!skb))
554 		return;
555 
556 	if (!tp->snd_wnd && !sock_flag(sk, SOCK_DEAD) &&
557 	    !((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))) {
558 		/* Receiver dastardly shrinks window. Our retransmits
559 		 * become zero probes, but we should not timeout this
560 		 * connection. If the socket is an orphan, time it out,
561 		 * we cannot allow such beasts to hang infinitely.
562 		 */
563 		struct inet_sock *inet = inet_sk(sk);
564 		u32 rtx_delta;
565 
566 		rtx_delta = tcp_time_stamp_ts(tp) - (tp->retrans_stamp ?:
567 				tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb));
568 		if (tp->tcp_usec_ts)
569 			rtx_delta /= USEC_PER_MSEC;
570 
571 		if (sk->sk_family == AF_INET) {
572 			net_dbg_ratelimited("Probing zero-window on %pI4:%u/%u, seq=%u:%u, recv %ums ago, lasting %ums\n",
573 				&inet->inet_daddr, ntohs(inet->inet_dport),
574 				inet->inet_num, tp->snd_una, tp->snd_nxt,
575 				jiffies_to_msecs(jiffies - tp->rcv_tstamp),
576 				rtx_delta);
577 		}
578 #if IS_ENABLED(CONFIG_IPV6)
579 		else if (sk->sk_family == AF_INET6) {
580 			net_dbg_ratelimited("Probing zero-window on %pI6:%u/%u, seq=%u:%u, recv %ums ago, lasting %ums\n",
581 				&sk->sk_v6_daddr, ntohs(inet->inet_dport),
582 				inet->inet_num, tp->snd_una, tp->snd_nxt,
583 				jiffies_to_msecs(jiffies - tp->rcv_tstamp),
584 				rtx_delta);
585 		}
586 #endif
587 		if (tcp_rtx_probe0_timed_out(sk, skb, rtx_delta)) {
588 			tcp_write_err(sk);
589 			goto out;
590 		}
591 		tcp_enter_loss(sk);
592 		tcp_retransmit_skb(sk, skb, 1);
593 		__sk_dst_reset(sk);
594 		goto out_reset_timer;
595 	}
596 
597 	__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEOUTS);
598 	if (tcp_write_timeout(sk))
599 		goto out;
600 
601 	if (icsk->icsk_retransmits == 0) {
602 		int mib_idx = 0;
603 
604 		if (icsk->icsk_ca_state == TCP_CA_Recovery) {
605 			if (tcp_is_sack(tp))
606 				mib_idx = LINUX_MIB_TCPSACKRECOVERYFAIL;
607 			else
608 				mib_idx = LINUX_MIB_TCPRENORECOVERYFAIL;
609 		} else if (icsk->icsk_ca_state == TCP_CA_Loss) {
610 			mib_idx = LINUX_MIB_TCPLOSSFAILURES;
611 		} else if ((icsk->icsk_ca_state == TCP_CA_Disorder) ||
612 			   tp->sacked_out) {
613 			if (tcp_is_sack(tp))
614 				mib_idx = LINUX_MIB_TCPSACKFAILURES;
615 			else
616 				mib_idx = LINUX_MIB_TCPRENOFAILURES;
617 		}
618 		if (mib_idx)
619 			__NET_INC_STATS(sock_net(sk), mib_idx);
620 	}
621 
622 	tcp_enter_loss(sk);
623 
624 	tcp_update_rto_stats(sk);
625 	if (tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1) > 0) {
626 		/* Retransmission failed because of local congestion,
627 		 * Let senders fight for local resources conservatively.
628 		 */
629 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
630 					  TCP_RESOURCE_PROBE_INTERVAL,
631 					  TCP_RTO_MAX);
632 		goto out;
633 	}
634 
635 	/* Increase the timeout each time we retransmit.  Note that
636 	 * we do not increase the rtt estimate.  rto is initialized
637 	 * from rtt, but increases here.  Jacobson (SIGCOMM 88) suggests
638 	 * that doubling rto each time is the least we can get away with.
639 	 * In KA9Q, Karn uses this for the first few times, and then
640 	 * goes to quadratic.  netBSD doubles, but only goes up to *64,
641 	 * and clamps at 1 to 64 sec afterwards.  Note that 120 sec is
642 	 * defined in the protocol as the maximum possible RTT.  I guess
643 	 * we'll have to use something other than TCP to talk to the
644 	 * University of Mars.
645 	 *
646 	 * PAWS allows us longer timeouts and large windows, so once
647 	 * implemented ftp to mars will work nicely. We will have to fix
648 	 * the 120 second clamps though!
649 	 */
650 
651 out_reset_timer:
652 	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
653 	 * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
654 	 * might be increased if the stream oscillates between thin and thick,
655 	 * thus the old value might already be too high compared to the value
656 	 * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
657 	 * backoff. Limit to TCP_THIN_LINEAR_RETRIES before initiating
658 	 * exponential backoff behaviour to avoid continue hammering
659 	 * linear-timeout retransmissions into a black hole
660 	 */
661 	if (sk->sk_state == TCP_ESTABLISHED &&
662 	    (tp->thin_lto || READ_ONCE(net->ipv4.sysctl_tcp_thin_linear_timeouts)) &&
663 	    tcp_stream_is_thin(tp) &&
664 	    icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
665 		icsk->icsk_backoff = 0;
666 		icsk->icsk_rto = clamp(__tcp_set_rto(tp),
667 				       tcp_rto_min(sk),
668 				       TCP_RTO_MAX);
669 	} else if (sk->sk_state != TCP_SYN_SENT ||
670 		   tp->total_rto >
671 		   READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts)) {
672 		/* Use normal (exponential) backoff unless linear timeouts are
673 		 * activated.
674 		 */
675 		icsk->icsk_backoff++;
676 		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
677 	}
678 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
679 				  tcp_clamp_rto_to_user_timeout(sk), TCP_RTO_MAX);
680 	if (retransmits_timed_out(sk, READ_ONCE(net->ipv4.sysctl_tcp_retries1) + 1, 0))
681 		__sk_dst_reset(sk);
682 
683 out:;
684 }
685 
686 /* Called with bottom-half processing disabled.
687    Called by tcp_write_timer() */
688 void tcp_write_timer_handler(struct sock *sk)
689 {
690 	struct inet_connection_sock *icsk = inet_csk(sk);
691 	int event;
692 
693 	if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
694 	    !icsk->icsk_pending)
695 		return;
696 
697 	if (time_after(icsk->icsk_timeout, jiffies)) {
698 		sk_reset_timer(sk, &icsk->icsk_retransmit_timer, icsk->icsk_timeout);
699 		return;
700 	}
701 
702 	tcp_mstamp_refresh(tcp_sk(sk));
703 	event = icsk->icsk_pending;
704 
705 	switch (event) {
706 	case ICSK_TIME_REO_TIMEOUT:
707 		tcp_rack_reo_timeout(sk);
708 		break;
709 	case ICSK_TIME_LOSS_PROBE:
710 		tcp_send_loss_probe(sk);
711 		break;
712 	case ICSK_TIME_RETRANS:
713 		smp_store_release(&icsk->icsk_pending, 0);
714 		tcp_retransmit_timer(sk);
715 		break;
716 	case ICSK_TIME_PROBE0:
717 		smp_store_release(&icsk->icsk_pending, 0);
718 		tcp_probe_timer(sk);
719 		break;
720 	}
721 }
722 
723 static void tcp_write_timer(struct timer_list *t)
724 {
725 	struct inet_connection_sock *icsk =
726 			from_timer(icsk, t, icsk_retransmit_timer);
727 	struct sock *sk = &icsk->icsk_inet.sk;
728 
729 	/* Avoid locking the socket when there is no pending event. */
730 	if (!smp_load_acquire(&icsk->icsk_pending))
731 		goto out;
732 
733 	bh_lock_sock(sk);
734 	if (!sock_owned_by_user(sk)) {
735 		tcp_write_timer_handler(sk);
736 	} else {
737 		/* delegate our work to tcp_release_cb() */
738 		if (!test_and_set_bit(TCP_WRITE_TIMER_DEFERRED, &sk->sk_tsq_flags))
739 			sock_hold(sk);
740 	}
741 	bh_unlock_sock(sk);
742 out:
743 	sock_put(sk);
744 }
745 
746 void tcp_syn_ack_timeout(const struct request_sock *req)
747 {
748 	struct net *net = read_pnet(&inet_rsk(req)->ireq_net);
749 
750 	__NET_INC_STATS(net, LINUX_MIB_TCPTIMEOUTS);
751 }
752 EXPORT_SYMBOL(tcp_syn_ack_timeout);
753 
754 void tcp_set_keepalive(struct sock *sk, int val)
755 {
756 	if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
757 		return;
758 
759 	if (val && !sock_flag(sk, SOCK_KEEPOPEN))
760 		inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tcp_sk(sk)));
761 	else if (!val)
762 		inet_csk_delete_keepalive_timer(sk);
763 }
764 EXPORT_SYMBOL_GPL(tcp_set_keepalive);
765 
766 
767 static void tcp_keepalive_timer (struct timer_list *t)
768 {
769 	struct sock *sk = from_timer(sk, t, sk_timer);
770 	struct inet_connection_sock *icsk = inet_csk(sk);
771 	struct tcp_sock *tp = tcp_sk(sk);
772 	u32 elapsed;
773 
774 	/* Only process if socket is not in use. */
775 	bh_lock_sock(sk);
776 	if (sock_owned_by_user(sk)) {
777 		/* Try again later. */
778 		inet_csk_reset_keepalive_timer (sk, HZ/20);
779 		goto out;
780 	}
781 
782 	if (sk->sk_state == TCP_LISTEN) {
783 		pr_err("Hmm... keepalive on a LISTEN ???\n");
784 		goto out;
785 	}
786 
787 	tcp_mstamp_refresh(tp);
788 	if (sk->sk_state == TCP_FIN_WAIT2 && sock_flag(sk, SOCK_DEAD)) {
789 		if (READ_ONCE(tp->linger2) >= 0) {
790 			const int tmo = tcp_fin_time(sk) - TCP_TIMEWAIT_LEN;
791 
792 			if (tmo > 0) {
793 				tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
794 				goto out;
795 			}
796 		}
797 		tcp_send_active_reset(sk, GFP_ATOMIC, SK_RST_REASON_TCP_STATE);
798 		goto death;
799 	}
800 
801 	if (!sock_flag(sk, SOCK_KEEPOPEN) ||
802 	    ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_SYN_SENT)))
803 		goto out;
804 
805 	elapsed = keepalive_time_when(tp);
806 
807 	/* It is alive without keepalive 8) */
808 	if (tp->packets_out || !tcp_write_queue_empty(sk))
809 		goto resched;
810 
811 	elapsed = keepalive_time_elapsed(tp);
812 
813 	if (elapsed >= keepalive_time_when(tp)) {
814 		u32 user_timeout = READ_ONCE(icsk->icsk_user_timeout);
815 
816 		/* If the TCP_USER_TIMEOUT option is enabled, use that
817 		 * to determine when to timeout instead.
818 		 */
819 		if ((user_timeout != 0 &&
820 		    elapsed >= msecs_to_jiffies(user_timeout) &&
821 		    icsk->icsk_probes_out > 0) ||
822 		    (user_timeout == 0 &&
823 		    icsk->icsk_probes_out >= keepalive_probes(tp))) {
824 			tcp_send_active_reset(sk, GFP_ATOMIC,
825 					      SK_RST_REASON_TCP_KEEPALIVE_TIMEOUT);
826 			tcp_write_err(sk);
827 			goto out;
828 		}
829 		if (tcp_write_wakeup(sk, LINUX_MIB_TCPKEEPALIVE) <= 0) {
830 			icsk->icsk_probes_out++;
831 			elapsed = keepalive_intvl_when(tp);
832 		} else {
833 			/* If keepalive was lost due to local congestion,
834 			 * try harder.
835 			 */
836 			elapsed = TCP_RESOURCE_PROBE_INTERVAL;
837 		}
838 	} else {
839 		/* It is tp->rcv_tstamp + keepalive_time_when(tp) */
840 		elapsed = keepalive_time_when(tp) - elapsed;
841 	}
842 
843 resched:
844 	inet_csk_reset_keepalive_timer (sk, elapsed);
845 	goto out;
846 
847 death:
848 	tcp_done(sk);
849 
850 out:
851 	bh_unlock_sock(sk);
852 	sock_put(sk);
853 }
854 
855 static enum hrtimer_restart tcp_compressed_ack_kick(struct hrtimer *timer)
856 {
857 	struct tcp_sock *tp = container_of(timer, struct tcp_sock, compressed_ack_timer);
858 	struct sock *sk = (struct sock *)tp;
859 
860 	bh_lock_sock(sk);
861 	if (!sock_owned_by_user(sk)) {
862 		if (tp->compressed_ack) {
863 			/* Since we have to send one ack finally,
864 			 * subtract one from tp->compressed_ack to keep
865 			 * LINUX_MIB_TCPACKCOMPRESSED accurate.
866 			 */
867 			tp->compressed_ack--;
868 			tcp_mstamp_refresh(tp);
869 			tcp_send_ack(sk);
870 		}
871 	} else {
872 		if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED,
873 				      &sk->sk_tsq_flags))
874 			sock_hold(sk);
875 	}
876 	bh_unlock_sock(sk);
877 
878 	sock_put(sk);
879 
880 	return HRTIMER_NORESTART;
881 }
882 
883 void tcp_init_xmit_timers(struct sock *sk)
884 {
885 	inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
886 				  &tcp_keepalive_timer);
887 	hrtimer_init(&tcp_sk(sk)->pacing_timer, CLOCK_MONOTONIC,
888 		     HRTIMER_MODE_ABS_PINNED_SOFT);
889 	tcp_sk(sk)->pacing_timer.function = tcp_pace_kick;
890 
891 	hrtimer_init(&tcp_sk(sk)->compressed_ack_timer, CLOCK_MONOTONIC,
892 		     HRTIMER_MODE_REL_PINNED_SOFT);
893 	tcp_sk(sk)->compressed_ack_timer.function = tcp_compressed_ack_kick;
894 }
895