1 // SPDX-License-Identifier: GPL-2.0 2 /* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */ 3 4 #include "vmlinux.h" 5 #include "bpf_tracing_net.h" 6 #include <bpf/bpf_helpers.h> 7 #include <bpf/bpf_endian.h> 8 9 #define UDP_TEST_PORT 7777 10 11 void *bpf_cast_to_kern_ctx(void *) __ksym; 12 13 bool had_dst = false; 14 bool dst_cleared = false; 15 16 SEC("tc/egress") 17 int dst_clear(struct __sk_buff *skb) 18 { 19 struct sk_buff *kskb; 20 struct iphdr iph; 21 struct udphdr udph; 22 int err; 23 24 if (skb->protocol != __bpf_constant_htons(ETH_P_IP)) 25 return TC_ACT_OK; 26 27 if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph, sizeof(iph))) 28 return TC_ACT_OK; 29 30 if (iph.protocol != IPPROTO_UDP) 31 return TC_ACT_OK; 32 33 if (bpf_skb_load_bytes(skb, ETH_HLEN + sizeof(iph), &udph, sizeof(udph))) 34 return TC_ACT_OK; 35 36 if (udph.dest != __bpf_constant_htons(UDP_TEST_PORT)) 37 return TC_ACT_OK; 38 39 kskb = bpf_cast_to_kern_ctx(skb); 40 had_dst = (kskb->_skb_refdst != 0); 41 42 /* Same-protocol encap (IPIP): protocol stays IPv4, but the dst 43 * from the original routing is no longer valid for the outer hdr. 44 */ 45 err = bpf_skb_adjust_room(skb, (s32)sizeof(struct iphdr), 46 BPF_ADJ_ROOM_MAC, 47 BPF_F_ADJ_ROOM_FIXED_GSO | 48 BPF_F_ADJ_ROOM_ENCAP_L3_IPV4); 49 if (err) 50 return TC_ACT_SHOT; 51 52 dst_cleared = (kskb->_skb_refdst == 0); 53 54 return TC_ACT_SHOT; 55 } 56 57 char __license[] SEC("license") = "GPL"; 58