// SPDX-License-Identifier: GPL-2.0
#include <stddef.h>
#include <stdint.h>
#include <stdbool.h>

#include <linux/bpf.h>
#include <linux/stddef.h>
#include <linux/pkt_cls.h>
#include <linux/if_ether.h>
#include <linux/in.h>
#include <linux/ip.h>
#include <linux/ipv6.h>

#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>

#ifndef ctx_ptr
# define ctx_ptr(field)         (void *)(long)(field)
#endif

#define ip4_src                 0xac100164 /* 172.16.1.100 */
#define ip4_dst                 0xac100264 /* 172.16.2.100 */

#define ip6_src                 { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, \
                                  0x00, 0x01, 0xde, 0xad, 0xbe, 0xef, 0xca, 0xfe }
#define ip6_dst                 { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, \
                                  0x00, 0x02, 0xde, 0xad, 0xbe, 0xef, 0xca, 0xfe }

#ifndef v6_equal
# define v6_equal(a, b)         (a.s6_addr32[0] == b.s6_addr32[0] && \
                                 a.s6_addr32[1] == b.s6_addr32[1] && \
                                 a.s6_addr32[2] == b.s6_addr32[2] && \
                                 a.s6_addr32[3] == b.s6_addr32[3])
#endif

volatile const __u32 IFINDEX_SRC;
volatile const __u32 IFINDEX_DST;

static __always_inline bool is_remote_ep_v4(struct __sk_buff *skb,
                                            __be32 addr)
{
        void *data_end = ctx_ptr(skb->data_end);
        void *data = ctx_ptr(skb->data);
        struct iphdr *ip4h;

        if (data + sizeof(struct ethhdr) > data_end)
                return false;

        ip4h = (struct iphdr *)(data + sizeof(struct ethhdr));
        if ((void *)(ip4h + 1) > data_end)
                return false;

        return ip4h->daddr == addr;
}

static __always_inline bool is_remote_ep_v6(struct __sk_buff *skb,
                                            struct in6_addr addr)
{
        void *data_end = ctx_ptr(skb->data_end);
        void *data = ctx_ptr(skb->data);
        struct ipv6hdr *ip6h;

        if (data + sizeof(struct ethhdr) > data_end)
                return false;

        ip6h = (struct ipv6hdr *)(data + sizeof(struct ethhdr));
        if ((void *)(ip6h + 1) > data_end)
                return false;

        return v6_equal(ip6h->daddr, addr);
}

SEC("tc")
int tc_chk(struct __sk_buff *skb)
{
        void *data_end = ctx_ptr(skb->data_end);
        void *data = ctx_ptr(skb->data);
        __u32 *raw = data;

        if (data + sizeof(struct ethhdr) > data_end)
                return TC_ACT_SHOT;

        return !raw[0] && !raw[1] && !raw[2] ? TC_ACT_SHOT : TC_ACT_OK;
}

SEC("tc")
int tc_dst(struct __sk_buff *skb)
{
        __u8 zero[ETH_ALEN * 2];
        bool redirect = false;

        switch (skb->protocol) {
        case __bpf_constant_htons(ETH_P_IP):
                redirect = is_remote_ep_v4(skb, __bpf_constant_htonl(ip4_src));
                break;
        case __bpf_constant_htons(ETH_P_IPV6):
                redirect = is_remote_ep_v6(skb, (struct in6_addr){{ip6_src}});
                break;
        }

        if (!redirect)
                return TC_ACT_OK;

        __builtin_memset(&zero, 0, sizeof(zero));
        if (bpf_skb_store_bytes(skb, 0, &zero, sizeof(zero), 0) < 0)
                return TC_ACT_SHOT;

        return bpf_redirect_neigh(IFINDEX_SRC, NULL, 0, 0);
}

SEC("tc")
int tc_src(struct __sk_buff *skb)
{
        __u8 zero[ETH_ALEN * 2];
        bool redirect = false;

        switch (skb->protocol) {
        case __bpf_constant_htons(ETH_P_IP):
                redirect = is_remote_ep_v4(skb, __bpf_constant_htonl(ip4_dst));
                break;
        case __bpf_constant_htons(ETH_P_IPV6):
                redirect = is_remote_ep_v6(skb, (struct in6_addr){{ip6_dst}});
                break;
        }

        if (!redirect)
                return TC_ACT_OK;

        __builtin_memset(&zero, 0, sizeof(zero));
        if (bpf_skb_store_bytes(skb, 0, &zero, sizeof(zero), 0) < 0)
                return TC_ACT_SHOT;

        return bpf_redirect_neigh(IFINDEX_DST, NULL, 0, 0);
}

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