1// SPDX-License-Identifier: GPL-2.0
2/* Copyright (c) 2022 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
11void *bpf_cast_to_kern_ctx(void *) __ksym;
12bool init_csum_partial = false;
13bool final_csum_none = false;
14bool broken_csum_start = false;
15
16static unsigned int skb_headlen(const struct sk_buff *skb)
17{
18	return skb->len - skb->data_len;
19}
20
21static unsigned int skb_headroom(const struct sk_buff *skb)
22{
23	return skb->data - skb->head;
24}
25
26static int skb_checksum_start_offset(const struct sk_buff *skb)
27{
28	return skb->csum_start - skb_headroom(skb);
29}
30
31SEC("tc")
32int decap_sanity(struct __sk_buff *skb)
33{
34	struct sk_buff *kskb;
35	struct ipv6hdr ip6h;
36	struct udphdr udph;
37	int err;
38
39	if (skb->protocol != __bpf_constant_htons(ETH_P_IPV6))
40		return TC_ACT_SHOT;
41
42	if (bpf_skb_load_bytes(skb, ETH_HLEN, &ip6h, sizeof(ip6h)))
43		return TC_ACT_SHOT;
44
45	if (ip6h.nexthdr != IPPROTO_UDP)
46		return TC_ACT_SHOT;
47
48	if (bpf_skb_load_bytes(skb, ETH_HLEN + sizeof(ip6h), &udph, sizeof(udph)))
49		return TC_ACT_SHOT;
50
51	if (udph.dest != __bpf_constant_htons(UDP_TEST_PORT))
52		return TC_ACT_SHOT;
53
54	kskb = bpf_cast_to_kern_ctx(skb);
55	init_csum_partial = (kskb->ip_summed == CHECKSUM_PARTIAL);
56	err = bpf_skb_adjust_room(skb, -(s32)(ETH_HLEN + sizeof(ip6h) + sizeof(udph)),
57				  1, BPF_F_ADJ_ROOM_FIXED_GSO);
58	if (err)
59		return TC_ACT_SHOT;
60	final_csum_none = (kskb->ip_summed == CHECKSUM_NONE);
61	if (kskb->ip_summed == CHECKSUM_PARTIAL &&
62	    (unsigned int)skb_checksum_start_offset(kskb) >= skb_headlen(kskb))
63		broken_csum_start = true;
64
65	return TC_ACT_SHOT;
66}
67
68char __license[] SEC("license") = "GPL";
69