static int blacklist_modify(int fd, char *ip_string, unsigned int action)
{
	unsigned int nr_cpus = bpf_num_possible_cpus();
	__u64 values[nr_cpus];
	__u32 key; int res;

	/* Update values for all possible CPUs */
	memset(values, 0, sizeof(__u64) * nr_cpus);

	/* Convert IP-string into 32-bit network byte-order value */
	if (inet_pton(AF_INET, ip_string, &key) <=0)
		return EXIT_FAIL_IP;

	if (action == ACTION_ADD) {
		res = bpf_map_update_elem(fd, &key, values, BPF_NOEXIST);
	} else if (action == ACTION_DEL) {
		res = bpf_map_delete_elem(fd, &key);
	} else {
		return EXIT_FAIL_OPTION;
	}

	if (res != 0) { /* 0 == success */
		if (errno == 17) /* Already in blacklist */
			return EXIT_OK;
		return EXIT_FAIL_MAP_KEY;
	}
	return EXIT_OK;
}