tcp: simplify fast open cookie processing

Consolidate various cookie checking and generation code to simplify
the fast open processing. The main goal is to reduce code duplication
in tcp_v4_conn_request() for IPv6 support.

Removes two experimental sysctl flags TFO_SERVER_ALWAYS and
TFO_SERVER_COOKIE_NOT_CHKD used primarily for developmental debugging
purposes.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Daniel Lee <longinus00@gmail.com>
Signed-off-by: Jerry Chu <hkchu@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 032fcae..5ea0949 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1273,7 +1273,6 @@
 	bool want_cookie = false;
 	struct flowi4 fl4;
 	struct tcp_fastopen_cookie foc = { .len = -1 };
-	struct tcp_fastopen_cookie valid_foc = { .len = -1 };
 	struct sk_buff *skb_synack;
 	int do_fastopen;
 
@@ -1381,7 +1380,8 @@
 		if (dst == NULL)
 			goto drop_and_free;
 	}
-	do_fastopen = tcp_fastopen_check(sk, skb, req, &foc, &valid_foc);
+	do_fastopen = !want_cookie &&
+		      tcp_fastopen_check(sk, skb, req, &foc);
 
 	/* We don't call tcp_v4_send_synack() directly because we need
 	 * to make sure a child socket can be created successfully before
@@ -1394,8 +1394,7 @@
 	 * latter to remove its dependency on the current implementation
 	 * of tcp_v4_send_synack()->tcp_select_initial_window().
 	 */
-	skb_synack = tcp_make_synack(sk, dst, req,
-	    fastopen_cookie_present(&valid_foc) ? &valid_foc : NULL);
+	skb_synack = tcp_make_synack(sk, dst, req, &foc);
 
 	if (skb_synack) {
 		__tcp_v4_send_check(skb_synack, ireq->ir_loc_addr, ireq->ir_rmt_addr);
@@ -1415,9 +1414,6 @@
 		tcp_rsk(req)->listener = NULL;
 		/* Add the request_sock to the SYN table */
 		inet_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
-		if (fastopen_cookie_present(&foc) && foc.len != 0)
-			NET_INC_STATS_BH(sock_net(sk),
-			    LINUX_MIB_TCPFASTOPENPASSIVEFAIL);
 	} else if (tcp_fastopen_create_child(sk, skb, skb_synack, req))
 		goto drop_and_release;