// 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

extern int bpf_dynptr_from_skb(struct __sk_buff *skb, __u64 flags,
			      struct bpf_dynptr *ptr__uninit) __ksym;
extern void *bpf_dynptr_slice(const struct bpf_dynptr *ptr, uint32_t offset,
			      void *buffer, uint32_t buffer__sz) __ksym;

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";