tcp: rstreason: introduce SK_RST_REASON_TCP_KEEPALIVE_TIMEOUT for active reset

Introducing this to show the users the reason of keepalive timeout.
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/rstreason.h

@@ -21,6 +21,7 @@
 	FN(TCP_ABORT_ON_LINGER)		\
 	FN(TCP_ABORT_ON_MEMORY)		\
 	FN(TCP_STATE)			\
+	FN(TCP_KEEPALIVE_TIMEOUT)	\
 	FN(MPTCP_RST_EUNSPEC)		\
 	FN(MPTCP_RST_EMPTCP)		\
 	FN(MPTCP_RST_ERESOURCE)		\
@@ -108,6 +109,12 @@ enum sk_rst_reason {
 	 * Please see RFC 9293 for all possible reset conditions
 	 */
 	SK_RST_REASON_TCP_STATE,
+	/**
+	 * @SK_RST_REASON_TCP_KEEPALIVE_TIMEOUT: time to timeout
+	 * When we have already run out of all the chances, which means
+	 * keepalive timeout, we have to reset the connection
+	 */
+	SK_RST_REASON_TCP_KEEPALIVE_TIMEOUT,
 
 	/* Copy from include/uapi/linux/mptcp.h.
 	 * These reset fields will not be changed since they adhere to

net/ipv4/tcp_timer.c

@@ -807,7 +807,7 @@ static void tcp_keepalive_timer (struct timer_list *t)
 		    (user_timeout == 0 &&
 		    icsk->icsk_probes_out >= keepalive_probes(tp))) {
 			tcp_send_active_reset(sk, GFP_ATOMIC,
-					      SK_RST_REASON_NOT_SPECIFIED);
+					      SK_RST_REASON_TCP_KEEPALIVE_TIMEOUT);
 			tcp_write_err(sk);
 			goto out;
 		}