// SPDX-License-Identifier: GPL-2.0
/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */

#include "vmlinux.h"
#include "bpf_tracing_net.h"
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>

#define UDP_TEST_PORT 7777

void *bpf_cast_to_kern_ctx(void *) __ksym;
bool init_csum_partial = false;
bool final_csum_none = false;
bool broken_csum_start = false;

static unsigned int skb_headlen(const struct sk_buff *skb)
{
        return skb->len - skb->data_len;
}

static unsigned int skb_headroom(const struct sk_buff *skb)
{
        return skb->data - skb->head;
}

static int skb_checksum_start_offset(const struct sk_buff *skb)
{
        return skb->csum_start - skb_headroom(skb);
}

SEC("tc")
int decap_sanity(struct __sk_buff *skb)
{
        struct sk_buff *kskb;
        struct ipv6hdr ip6h;
        struct udphdr udph;
        int err;

        if (skb->protocol != __bpf_constant_htons(ETH_P_IPV6))
                return TC_ACT_SHOT;

        if (bpf_skb_load_bytes(skb, ETH_HLEN, &ip6h, sizeof(ip6h)))
                return TC_ACT_SHOT;

        if (ip6h.nexthdr != IPPROTO_UDP)
                return TC_ACT_SHOT;

        if (bpf_skb_load_bytes(skb, ETH_HLEN + sizeof(ip6h), &udph, sizeof(udph)))
                return TC_ACT_SHOT;

        if (udph.dest != __bpf_constant_htons(UDP_TEST_PORT))
                return TC_ACT_SHOT;

        kskb = bpf_cast_to_kern_ctx(skb);
        init_csum_partial = (kskb->ip_summed == CHECKSUM_PARTIAL);
        err = bpf_skb_adjust_room(skb, -(s32)(ETH_HLEN + sizeof(ip6h) + sizeof(udph)),
                                  1, BPF_F_ADJ_ROOM_FIXED_GSO);
        if (err)
                return TC_ACT_SHOT;
        final_csum_none = (kskb->ip_summed == CHECKSUM_NONE);
        if (kskb->ip_summed == CHECKSUM_PARTIAL &&
            (unsigned int)skb_checksum_start_offset(kskb) >= skb_headlen(kskb))
                broken_csum_start = true;

        return TC_ACT_SHOT;
}

char __license[] SEC("license") = "GPL";