// SPDX-License-Identifier: GPL-2.0
/* Copyright (c) 2026 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 had_dst = false;
bool dst_cleared = false;

SEC("tc/egress")
int dst_clear(struct __sk_buff *skb)
{
        struct sk_buff *kskb;
        struct iphdr iph;
        struct udphdr udph;
        int err;

        if (skb->protocol != __bpf_constant_htons(ETH_P_IP))
                return TC_ACT_OK;

        if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph, sizeof(iph)))
                return TC_ACT_OK;

        if (iph.protocol != IPPROTO_UDP)
                return TC_ACT_OK;

        if (bpf_skb_load_bytes(skb, ETH_HLEN + sizeof(iph), &udph, sizeof(udph)))
                return TC_ACT_OK;

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

        kskb = bpf_cast_to_kern_ctx(skb);
        had_dst = (kskb->_skb_refdst != 0);

        /* Same-protocol encap (IPIP): protocol stays IPv4, but the dst
         * from the original routing is no longer valid for the outer hdr.
         */
        err = bpf_skb_adjust_room(skb, (s32)sizeof(struct iphdr),
                                  BPF_ADJ_ROOM_MAC,
                                  BPF_F_ADJ_ROOM_FIXED_GSO |
                                  BPF_F_ADJ_ROOM_ENCAP_L3_IPV4);
        if (err)
                return TC_ACT_SHOT;

        dst_cleared = (kskb->_skb_refdst == 0);

        return TC_ACT_SHOT;
}

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