TCP: avoid to send keepalive probes if receiving data

RFC 1122 says the following:
...
  Keep-alive packets MUST only be sent when no data or
  acknowledgement packets have been received for the
  connection within an interval.
...

The acknowledgement packet is reseting the keepalive
timer but the data packet isn't. This patch fixes it by
checking the timestamp of the last received data packet
too when the keepalive timer expires.
Signed-off-by: NFlavio Leitner <fleitner@redhat.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Acked-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1033,6 +1033,14 @@ static inline int keepalive_probes(const struct tcp_sock *tp)
 	return tp->keepalive_probes ? : sysctl_tcp_keepalive_probes;
 }
 
+static inline u32 keepalive_time_elapsed(const struct tcp_sock *tp)
+{
+	const struct inet_connection_sock *icsk = &tp->inet_conn;
+
+	return min_t(u32, tcp_time_stamp - icsk->icsk_ack.lrcvtime,
+			  tcp_time_stamp - tp->rcv_tstamp);
+}
+
 static inline int tcp_fin_time(const struct sock *sk)
 {
 	int fin_timeout = tcp_sk(sk)->linger2 ? : sysctl_tcp_fin_timeout;

net/ipv4/tcp.c

@@ -2298,7 +2298,7 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 			if (sock_flag(sk, SOCK_KEEPOPEN) &&
 			    !((1 << sk->sk_state) &
 			      (TCPF_CLOSE | TCPF_LISTEN))) {
-				__u32 elapsed = tcp_time_stamp - tp->rcv_tstamp;
+				u32 elapsed = keepalive_time_elapsed(tp);
 				if (tp->keepalive_time > elapsed)
 					elapsed = tp->keepalive_time - elapsed;
 				else

net/ipv4/tcp_timer.c

@@ -517,7 +517,7 @@ static void tcp_keepalive_timer (unsigned long data)
 	struct sock *sk = (struct sock *) data;
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
-	__u32 elapsed;
+	u32 elapsed;
 
 	/* Only process if socket is not in use. */
 	bh_lock_sock(sk);
@@ -554,7 +554,7 @@ static void tcp_keepalive_timer (unsigned long data)
 	if (tp->packets_out || tcp_send_head(sk))
 		goto resched;
 
-	elapsed = tcp_time_stamp - tp->rcv_tstamp;
+	elapsed = keepalive_time_elapsed(tp);
 
 	if (elapsed >= keepalive_time_when(tp)) {
 		if (icsk->icsk_probes_out >= keepalive_probes(tp)) {