#include <linux/bpf.h>
#include <linux/in.h>

#include <bpf/bpf_helpers.h>

#include "xdp_checksum.h"
#include "xdp_manip.h"
#include "xdp_parser.h"

#ifdef XDP_DEBUG
#define dbg_bpf_printk(fmt, args...) bpf_printk(fmt, ##args)
#else
#define dbg_bpf_printk(fmt, args...)
#endif

#include "common.h"

SEC("xdp")
int xdp_udp_handler(struct xdp_md *ctx) {
  void *data_end = (void *)(long)ctx->data_end;
  void *data = (void *)(long)ctx->data;
  __u32 action = XDP_PASS; /* Default action */
  struct hdr_cursor nh = {.pos = data};
  int nh_type;
  struct ethhdr *ethhdr;
  struct iphdr *iphdr = NULL;
  struct ipv6hdr *ip6hdr = NULL;
  struct udphdr *udphdr;
  static const __u8 payload_request[] = {PAYLOAD_REQUEST};
  static const __u32 payload_request_size = sizeof(payload_request);
  static const __u8 payload_response[] = {PAYLOAD_RESPONSE};
  __u32 payload_size;

  nh_type = parse_ethhdr(&nh, data_end, &ethhdr);

  switch (__constant_ntohs(nh_type)) {
  case ETH_P_IP:
    nh_type = parse_iphdr(&nh, data_end, &iphdr);
    break;
  case ETH_P_IPV6:
    nh_type = parse_ip6hdr(&nh, data_end, &ip6hdr);
    break;
  default:
    goto out;
  }

  if (nh_type != IPPROTO_UDP) {
    goto out;
  }
  if (parse_udphdr(&nh, data_end, &udphdr) < 0) {
    action = XDP_ABORTED;
    goto out;
  }
  if (udphdr->dest != __constant_htons(UDP_PORT)) {
    goto out;
  }

  payload_size = (__u8 *)data_end - (__u8 *)nh.pos;
  if (payload_size < payload_request_size) {
    goto out;
  }

  if (nh.pos_u8 + payload_request_size + 1 > (__u8 *)data_end) {
    goto out;
  }
  if (__builtin_memcmp(nh.pos_u8, payload_request, sizeof(payload_request)) !=
      0) {
    goto out;
  }

  dbg_bpf_printk("Received UDP Ping Request #%u with size %u", nh.pos_u8[11],
                 payload_size);

  /* Shrink UDP packet for Ping Reply */
  if (bpf_xdp_adjust_tail(ctx, payload_request_size - payload_size) != 0) {
    return XDP_ABORTED;
  }

  /* Set data pointers (may not be valid anymore) */
  data = (void *)(long)ctx->data;
  data_end = (void *)(long)ctx->data_end;
  nh.pos = data;

  if (parse_ethhdr(&nh, data_end, &ethhdr) < 0) {
    action = XDP_ABORTED;
    goto out;
  }

  payload_size -= 4;

  if (iphdr != NULL) {
    if (parse_iphdr(&nh, data_end, &iphdr) < 0) {
      action = XDP_ABORTED;
      goto out;
    }
    if (parse_udphdr(&nh, data_end, &udphdr) < 0) {
      action = XDP_ABORTED;
      goto out;
    }
    swap_eth_addr(ethhdr);
    udphdr->len = __constant_htons(payload_size + sizeof(*udphdr));
    iphdr->tot_len =
        __constant_htons(iphdr->ihl * 4 + payload_size + sizeof(*udphdr));
    swap_ip4_addr(iphdr);
    iphdr->check = ip_checksum(iphdr);
    if (nh.pos_u8 + sizeof(payload_response) > (__u8 *)data_end) {
      action = XDP_ABORTED;
      goto out;
    }
    __builtin_memcpy(nh.pos_u8, payload_response, sizeof(payload_response));
    swap_udp_port(udphdr);
    udphdr->check = udp_checksum(iphdr, udphdr, data_end);
  } else if (ip6hdr != NULL) {
    parse_ip6hdr(&nh, data_end, &ip6hdr);
    goto out; // TODO: Implement!
  } else {
    action = XDP_ABORTED;
    goto out;
  }

  action = XDP_TX;
out:
  return action;
}

char _license[] SEC("license") =
    "GPL"; // Please keep this to prevent tainting kernel