int tcp_conn_request(struct request_sock_ops *rsk_ops,
		     const struct tcp_request_sock_ops *af_ops,
		     struct sock *sk, struct sk_buff *skb)
{
	struct tcp_fastopen_cookie foc = { .len = -1 };
	__u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
	struct tcp_options_received tmp_opt;
	struct tcp_sock *tp = tcp_sk(sk);
	struct sock *fastopen_sk = NULL;
	struct dst_entry *dst = NULL;
	struct request_sock *req;
	bool want_cookie = false;
	struct flowi fl;

	/* TW buckets are converted to open requests without
	 * limitations, they conserve resources and peer is
	 * evidently real one.
	 */
	if ((sysctl_tcp_syncookies == 2 ||
	     inet_csk_reqsk_queue_is_full(sk)) && !isn) {
		want_cookie = tcp_syn_flood_action(sk, skb, rsk_ops->slab_name);
		if (!want_cookie)
			goto drop;
	}


	/* Accept backlog is full. If we have already queued enough
	 * of warm entries in syn queue, drop request. It is better than
	 * clogging syn queue with openreqs with exponentially increasing
	 * timeout.
	 */
	if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
		goto drop;
	}

	req = inet_reqsk_alloc(rsk_ops, sk, !want_cookie);
	if (!req)
		goto drop;

	tcp_rsk(req)->af_specific = af_ops;

	tcp_clear_options(&tmp_opt);
	tmp_opt.mss_clamp = af_ops->mss_clamp;
	tmp_opt.user_mss  = tp->rx_opt.user_mss;
	tcp_parse_options(skb, &tmp_opt, 0, want_cookie ? NULL : &foc);

	if (want_cookie && !tmp_opt.saw_tstamp)
		tcp_clear_options(&tmp_opt);

	tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
	tcp_openreq_init(req, &tmp_opt, skb, sk);

	/* Note: tcp_v6_init_req() might override ir_iif for link locals */
	inet_rsk(req)->ir_iif = sk->sk_bound_dev_if;

	af_ops->init_req(req, sk, skb);

	if (security_inet_conn_request(sk, skb, req))
		goto drop_and_free;

	if (!want_cookie && !isn) {
		/* VJ's idea. We save last timestamp seen
		 * from the destination in peer table, when entering
		 * state TIME-WAIT, and check against it before
		 * accepting new connection request.
		 *
		 * If "isn" is not zero, this request hit alive
		 * timewait bucket, so that all the necessary checks
		 * are made in the function processing timewait state.
		 */
		if (tcp_death_row.sysctl_tw_recycle) {
			bool strict;

			dst = af_ops->route_req(sk, &fl, req, &strict);

			if (dst && strict &&
			    !tcp_peer_is_proven(req, dst, true,
						tmp_opt.saw_tstamp)) {
				NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSPASSIVEREJECTED);
				goto drop_and_release;
			}
		}
		/* Kill the following clause, if you dislike this way. */
		else if (!sysctl_tcp_syncookies &&
			 (sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
			  (sysctl_max_syn_backlog >> 2)) &&
			 !tcp_peer_is_proven(req, dst, false,
					     tmp_opt.saw_tstamp)) {
			/* Without syncookies last quarter of
			 * backlog is filled with destinations,
			 * proven to be alive.
			 * It means that we continue to communicate
			 * to destinations, already remembered
			 * to the moment of synflood.
			 */
			pr_drop_req(req, ntohs(tcp_hdr(skb)->source),
				    rsk_ops->family);
			goto drop_and_release;
		}

		isn = af_ops->init_seq(skb);
	}
	if (!dst) {
		dst = af_ops->route_req(sk, &fl, req, NULL);
		if (!dst)
			goto drop_and_free;
	}

	tcp_ecn_create_request(req, skb, sk, dst);

	if (want_cookie) {
		isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
		req->cookie_ts = tmp_opt.tstamp_ok;
		if (!tmp_opt.tstamp_ok)
			inet_rsk(req)->ecn_ok = 0;
	}

	tcp_rsk(req)->snt_isn = isn;
	tcp_rsk(req)->txhash = net_tx_rndhash();
	tcp_openreq_init_rwin(req, sk, dst);
	if (!want_cookie) {
		tcp_reqsk_record_syn(sk, req, skb);
		fastopen_sk = tcp_try_fastopen(sk, skb, req, &foc, dst);
	}
	if (fastopen_sk) {
		af_ops->send_synack(fastopen_sk, dst, &fl, req,
				    &foc, false);
		/* Add the child socket directly into the accept queue */
		inet_csk_reqsk_queue_add(sk, req, fastopen_sk);
		sk->sk_data_ready(sk);
		bh_unlock_sock(fastopen_sk);
		sock_put(fastopen_sk);
	} else {
		tcp_rsk(req)->tfo_listener = false;
		if (!want_cookie)
			inet_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
		af_ops->send_synack(sk, dst, &fl, req,
				    &foc, !want_cookie);
		if (want_cookie)
			goto drop_and_free;
	}
	reqsk_put(req);
	return 0;

drop_and_release:
	dst_release(dst);
drop_and_free:
	reqsk_free(req);
drop:
	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENDROPS);
	return 0;
}