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

#define PACKET_ID_MASK 0x0FFFFFFF
#define VERDICT_MASK 0xF0000000

struct counter_state {
  struct bpf_spin_lock lock;
  __u32 value;
};

struct {
  __uint(type, BPF_MAP_TYPE_ARRAY);
  __uint(max_entries, 1);
  __type(key, __u32);
  __type(value, struct counter_state);
} packet_counter SEC(".maps");

SEC("classifier")
int classifier(struct __sk_buff *skb) {
  __u32 key = 0;
  struct counter_state *state = bpf_map_lookup_elem(&packet_counter, &key);
  __u32 next = 1;

  if (state) {
    bpf_spin_lock(&state->lock);
    state->value += 1;
    next = state->value;
    bpf_spin_unlock(&state->lock);
  }

  skb->mark = (skb->mark & VERDICT_MASK) | (next & PACKET_ID_MASK);
  return TC_ACT_OK;
}

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