linux-kernel - [PATCH] tcp: keepalive fixes

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Date:   Tue, 12 Jan 2021 11:25:44 -0800
From:   Enke Chen <enkechen2020@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        enkechen2020@...il.com
Subject: [PATCH] tcp: keepalive fixes

From: Enke Chen <enchen@...oaltonetworks.com>

In this patch two issues with TCP keepalives are fixed:

1) TCP keepalive does not timeout when there are data waiting to be
   delivered and then the connection got broken. The TCP keepalive
   timeout is not evaluated in that condition.

   The fix is to remove the code that prevents TCP keepalive from
   being evaluated for timeout.

2) With the fix for #1, TCP keepalive can erroneously timeout after
   the 0-window probe kicks in. The 0-window probe counter is wrongly
   applied to TCP keepalives.

   The fix is to use the elapsed time instead of the 0-window probe
   counter in evaluating TCP keepalive timeout.

Cc: stable@...r.kernel.org
Signed-off-by: Enke Chen <enchen@...oaltonetworks.com>
---
 net/ipv4/tcp_timer.c | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 6c62b9ea1320..40953aa40d53 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -696,12 +696,6 @@ static void tcp_keepalive_timer (struct timer_list *t)
 	    ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_SYN_SENT)))
 		goto out;
 
-	elapsed = keepalive_time_when(tp);
-
-	/* It is alive without keepalive 8) */
-	if (tp->packets_out || !tcp_write_queue_empty(sk))
-		goto resched;
-
 	elapsed = keepalive_time_elapsed(tp);
 
 	if (elapsed >= keepalive_time_when(tp)) {
@@ -709,16 +703,15 @@ static void tcp_keepalive_timer (struct timer_list *t)
 		 * to determine when to timeout instead.
 		 */
 		if ((icsk->icsk_user_timeout != 0 &&
-		    elapsed >= msecs_to_jiffies(icsk->icsk_user_timeout) &&
-		    icsk->icsk_probes_out > 0) ||
+		     elapsed >= msecs_to_jiffies(icsk->icsk_user_timeout)) ||
 		    (icsk->icsk_user_timeout == 0 &&
-		    icsk->icsk_probes_out >= keepalive_probes(tp))) {
+		     (elapsed >= keepalive_time_when(tp) +
+		      keepalive_intvl_when(tp) * keepalive_probes(tp)))) {
 			tcp_send_active_reset(sk, GFP_ATOMIC);
 			tcp_write_err(sk);
 			goto out;
 		}
 		if (tcp_write_wakeup(sk, LINUX_MIB_TCPKEEPALIVE) <= 0) {
-			icsk->icsk_probes_out++;
 			elapsed = keepalive_intvl_when(tp);
 		} else {
 			/* If keepalive was lost due to local congestion,
@@ -732,8 +725,6 @@ static void tcp_keepalive_timer (struct timer_list *t)
 	}
 
 	sk_mem_reclaim(sk);
-
-resched:
 	inet_csk_reset_keepalive_timer (sk, elapsed);
 	goto out;
 
-- 
2.29.2