#include <linux/bpf.h>
#include <linux/pkt_cls.h>
#include <bpf/bpf_endian.h>
#include <bpf/bpf_helpers.h>

#define CAPTURE_MAGIC 0x4d544350
#define CAPTURE_VERSION 1

struct capture_header {
  __be32 magic;
  __u8 version;
  __u8 flags;
  __be16 header_len;
  __be32 skb_mark;
  __be32 aux_value;
} __attribute__((packed));

SEC("classifier")
int classifier(struct __sk_buff *skb) {
  struct capture_header hdr = {
    .magic = bpf_htonl(CAPTURE_MAGIC),
    .version = CAPTURE_VERSION,
    .flags = 0,
    .header_len = bpf_htons(sizeof(struct capture_header)),
    .skb_mark = bpf_htonl(skb->mark),
    .aux_value = 0,
  };

  if (!skb->mark) {
    return TC_ACT_OK;
  }
  if (bpf_skb_adjust_room(skb, sizeof(hdr), BPF_ADJ_ROOM_MAC, 0)) {
    return TC_ACT_OK;
  }
  if (bpf_skb_store_bytes(skb, 0, &hdr, sizeof(hdr), 0)) {
    return TC_ACT_OK;
  }
  return TC_ACT_OK;
}

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