// SPDX-License-Identifier: GPL-2.0-only
#include "vmlinux.h"
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>
#include "bpf_tracing_net.h"

#define NF_DROP                 0
#define NF_ACCEPT               1
#define ETH_P_IP                0x0800
#define ETH_P_IPV6              0x86DD
#define IP_MF                   0x2000
#define IP_OFFSET               0x1FFF
#define NEXTHDR_FRAGMENT        44

volatile int shootdowns = 0;

static bool is_frag_v4(struct iphdr *iph)
{
        int offset;
        int flags;

        offset = bpf_ntohs(iph->frag_off);
        flags = offset & ~IP_OFFSET;
        offset &= IP_OFFSET;
        offset <<= 3;

        return (flags & IP_MF) || offset;
}

static bool is_frag_v6(struct ipv6hdr *ip6h)
{
        /* Simplifying assumption that there are no extension headers
         * between fixed header and fragmentation header. This assumption
         * is only valid in this test case. It saves us the hassle of
         * searching all potential extension headers.
         */
        return ip6h->nexthdr == NEXTHDR_FRAGMENT;
}

static int handle_v4(struct __sk_buff *skb)
{
        struct bpf_dynptr ptr;
        u8 iph_buf[20] = {};
        struct iphdr *iph;

        if (bpf_dynptr_from_skb(skb, 0, &ptr))
                return NF_DROP;

        iph = bpf_dynptr_slice(&ptr, 0, iph_buf, sizeof(iph_buf));
        if (!iph)
                return NF_DROP;

        /* Shootdown any frags */
        if (is_frag_v4(iph)) {
                shootdowns++;
                return NF_DROP;
        }

        return NF_ACCEPT;
}

static int handle_v6(struct __sk_buff *skb)
{
        struct bpf_dynptr ptr;
        struct ipv6hdr *ip6h;
        u8 ip6h_buf[40] = {};

        if (bpf_dynptr_from_skb(skb, 0, &ptr))
                return NF_DROP;

        ip6h = bpf_dynptr_slice(&ptr, 0, ip6h_buf, sizeof(ip6h_buf));
        if (!ip6h)
                return NF_DROP;

        /* Shootdown any frags */
        if (is_frag_v6(ip6h)) {
                shootdowns++;
                return NF_DROP;
        }

        return NF_ACCEPT;
}

SEC("netfilter")
int defrag(struct bpf_nf_ctx *ctx)
{
        struct __sk_buff *skb = (struct __sk_buff *)ctx->skb;

        switch (bpf_ntohs(ctx->skb->protocol)) {
        case ETH_P_IP:
                return handle_v4(skb);
        case ETH_P_IPV6:
                return handle_v6(skb);
        default:
                return NF_ACCEPT;
        }
}

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