// SPDX-License-Identifier: GPL-2.0

/* This parsing logic is taken from the open source library katran, a layer 4
 * load balancer.
 *
 * This code logic using dynptrs can be found in test_parse_tcp_hdr_opt_dynptr.c
 *
 * https://github.com/facebookincubator/katran/blob/main/katran/lib/bpf/pckt_parsing.h
 */

#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"

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;

struct hdr_opt_state {
        __u32 server_id;
        __u8 byte_offset;
        __u8 hdr_bytes_remaining;
};

static int parse_hdr_opt(const struct xdp_md *xdp, struct hdr_opt_state *state)
{
        const void *data = (void *)(long)xdp->data;
        const void *data_end = (void *)(long)xdp->data_end;
        __u8 *tcp_opt, kind, hdr_len;

        tcp_opt = (__u8 *)(data + state->byte_offset);
        if (tcp_opt + 1 > data_end)
                return -1;

        kind = tcp_opt[0];

        if (kind == TCPOPT_EOL)
                return -1;

        if (kind == TCPOPT_NOP) {
                state->hdr_bytes_remaining--;
                state->byte_offset++;
                return 0;
        }

        if (state->hdr_bytes_remaining < 2 ||
            tcp_opt + sizeof(__u8) + sizeof(__u8) > data_end)
                return -1;

        hdr_len = tcp_opt[1];
        if (hdr_len > state->hdr_bytes_remaining)
                return -1;

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

                if (tcp_opt + tcp_hdr_opt_len_tpr > data_end)
                        return -1;

                state->server_id = *(__u32 *)&tcp_opt[2];
                return 1;
        }

        state->hdr_bytes_remaining -= hdr_len;
        state->byte_offset += hdr_len;
        return 0;
}

SEC("xdp")
int xdp_ingress_v6(struct xdp_md *xdp)
{
        const void *data = (void *)(long)xdp->data;
        const void *data_end = (void *)(long)xdp->data_end;
        struct hdr_opt_state opt_state = {};
        __u8 tcp_hdr_opt_len = 0;
        struct tcphdr *tcp_hdr;
        __u64 tcp_offset = 0;
        int err;

        tcp_offset = sizeof(struct ethhdr) + sizeof(struct ipv6hdr);
        tcp_hdr = (struct tcphdr *)(data + tcp_offset);
        if (tcp_hdr + 1 > data_end)
                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;

        opt_state.hdr_bytes_remaining = tcp_hdr_opt_len;
        opt_state.byte_offset = sizeof(struct tcphdr) + tcp_offset;

        /* 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(xdp, &opt_state);

                if (err || !opt_state.hdr_bytes_remaining)
                        break;
        }

        if (!opt_state.server_id)
                return XDP_DROP;

        server_id = opt_state.server_id;

        return XDP_PASS;
}