tcp: fastopen: note that a child socket was created

tcp: fastopen: note that a child socket was created

This uses up the last bit in a field of tcp_sock.

Signed-off-by: Jeremy Harris <jgh@exim.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20250423124334.4916-2-jgh@exim.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/tcp.h

@@ -385,7 +385,8 @@ struct tcp_sock {
 		syn_fastopen:1,	/* SYN includes Fast Open option */
 		syn_fastopen_exp:1,/* SYN includes Fast Open exp. option */
 		syn_fastopen_ch:1, /* Active TFO re-enabling probe */
-		syn_data_acked:1;/* data in SYN is acked by SYN-ACK */
+		syn_data_acked:1,/* data in SYN is acked by SYN-ACK */
+		syn_fastopen_child:1; /* created TFO passive child socket */
 
 	u8	keepalive_probes; /* num of allowed keep alive probes	*/
 	u32	tcp_tx_delay;	/* delay (in usec) added to TX packets */

net/ipv4/tcp.c

@@ -3409,6 +3409,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	tp->rack.reo_wnd_persist = 0;
 	tp->rack.dsack_seen = 0;
 	tp->syn_data_acked = 0;
+	tp->syn_fastopen_child = 0;
 	tp->rx_opt.saw_tstamp = 0;
 	tp->rx_opt.dsack = 0;
 	tp->rx_opt.num_sacks = 0;

net/ipv4/tcp_fastopen.c

@@ -401,6 +401,7 @@ fastopen:
 				}
 				NET_INC_STATS(sock_net(sk),
 					      LINUX_MIB_TCPFASTOPENPASSIVE);
+				tcp_sk(child)->syn_fastopen_child = 1;
 				return child;
 			}
 			NET_INC_STATS(sock_net(sk),