tools/testing/selftests/bpf/progs/test_parse_tcp_hdr_opt_dynptr.c

// SPDX-License-Identifier: GPL-2.0

/* This logic is lifted from a real-world use case of packet parsing, used in
 * the open source library katran, a layer 4 load balancer.
 *
 * This test demonstrates how to parse packet contents using dynptrs. The
 * original code (parsing without dynptrs) can be found in test_parse_tcp_hdr_opt.c
 */

#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>
#include <linux/tcp.h>
#include <stdbool.h>
#include <linux/ipv6.h>
#include <linux/if_ether.h>
#include "test_tcp_hdr_options.h"
#include "bpf_kfuncs.h"

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

/* Kind number used for experiments */
const __u32 tcp_hdr_opt_kind_tpr = 0xFD;
/* Length of the tcp header option */
const __u32 tcp_hdr_opt_len_tpr = 6;
/* maximum number of header options to check to lookup server_id */
const __u32 tcp_hdr_opt_max_opt_checks = 15;

__u32 server_id;

static int parse_hdr_opt(struct bpf_dynptr *ptr, __u32 *off, __u8 *hdr_bytes_remaining,
			 __u32 *server_id)
{
	__u8 kind, hdr_len;
	__u8 buffer[sizeof(kind) + sizeof(hdr_len) + sizeof(*server_id)];
	__u8 *data;

	__builtin_memset(buffer, 0, sizeof(buffer));

	data = bpf_dynptr_slice(ptr, *off, buffer, sizeof(buffer));
	if (!data)
		return -1;

	kind = data[0];

	if (kind == TCPOPT_EOL)
		return -1;

	if (kind == TCPOPT_NOP) {
		*off += 1;
		*hdr_bytes_remaining -= 1;
		return 0;
	}

	if (*hdr_bytes_remaining < 2)
		return -1;

	hdr_len = data[1];
	if (hdr_len > *hdr_bytes_remaining)
		return -1;

	if (kind == tcp_hdr_opt_kind_tpr) {
		if (hdr_len != tcp_hdr_opt_len_tpr)
			return -1;

		__builtin_memcpy(server_id, (__u32 *)(data + 2), sizeof(*server_id));
		return 1;
	}

	*off += hdr_len;
	*hdr_bytes_remaining -= hdr_len;
	return 0;
}

SEC("xdp")
int xdp_ingress_v6(struct xdp_md *xdp)
{
	__u8 buffer[sizeof(struct tcphdr)] = {};
	__u8 hdr_bytes_remaining;
	struct tcphdr *tcp_hdr;
	__u8 tcp_hdr_opt_len;
	int err = 0;
	__u32 off;

	struct bpf_dynptr ptr;

	bpf_dynptr_from_xdp(xdp, 0, &ptr);

	off = sizeof(struct ethhdr) + sizeof(struct ipv6hdr);

	tcp_hdr = bpf_dynptr_slice(&ptr, off, buffer, sizeof(buffer));
	if (!tcp_hdr)
		return XDP_DROP;

	tcp_hdr_opt_len = (tcp_hdr->doff * 4) - sizeof(struct tcphdr);
	if (tcp_hdr_opt_len < tcp_hdr_opt_len_tpr)
		return XDP_DROP;

	hdr_bytes_remaining = tcp_hdr_opt_len;

	off += sizeof(struct tcphdr);

	/* max number of bytes of options in tcp header is 40 bytes */
	for (int i = 0; i < tcp_hdr_opt_max_opt_checks; i++) {
		err = parse_hdr_opt(&ptr, &off, &hdr_bytes_remaining, &server_id);

		if (err || !hdr_bytes_remaining)
			break;
	}

	if (!server_id)
		return XDP_DROP;

	return XDP_PASS;
}