[PATCH net-next v2 3/3] net: tcp: fix unexcepted socket die when snd_wnd is 0

[menglong8 . dong]

From: Menglong Dong <imagedong@xxxxxxxxxxx>

In tcp_retransmit_timer(), a window shrunk connection will be regarded
as timeout if 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX'. This is not
right all the time.

The retransmits will become zero-window probes in tcp_retransmit_timer()
if the 'snd_wnd==0'. Therefore, the icsk->icsk_rto will come up to
TCP_RTO_MAX sooner or later.

However, the timer is not precise enough, as it base on timer wheel.
Sorry that I am not good at timer, but I know the concept of time-wheel.
The longer of the timer, the rougher it will be. So the timeout is not
triggered after TCP_RTO_MAX, but 122877ms as I tested.

Therefore, 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX' is always true
once the RTO come up to TCP_RTO_MAX, and the socket will die.

Fix this by replacing the 'tcp_jiffies32' with '(u32)icsk->icsk_timeout',
which is exact the timestamp of the timeout. Meanwhile, using
"max(tp->retrans_stamp, tp->rcv_tstamp)" as the last updated timestamp in
the receiving path, as "tp->rcv_tstamp" can restart from idle, then
tp->rcv_tstamp could already be a long time (minutes or hours) in the
past even on the first RTO.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Link: https://lore.kernel.org/netdev/CADxym3YyMiO+zMD4zj03YPM3FBi-1LHi6gSD2XT8pyAMM096pg@xxxxxxxxxxxxxx/
Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
---
v2:
- consider the case of the connection restart from idle, as Neal comment
---
 net/ipv4/tcp_timer.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index d45c96c7f5a4..e4b2d8706cae 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -454,6 +454,14 @@ static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req)
 			  req->timeout << req->num_timeout, TCP_RTO_MAX);
 }
 
+static bool tcp_rtx_probe0_timed_out(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 last_ts;
+
+	last_ts = max(tp->retrans_stamp, tp->rcv_tstamp);
+	return inet_csk(sk)->icsk_timeout - last_ts > TCP_RTO_MAX;
+}
 
 /**
  *  tcp_retransmit_timer() - The TCP retransmit timeout handler
@@ -519,7 +527,7 @@ void tcp_retransmit_timer(struct sock *sk)
 					    tp->snd_una, tp->snd_nxt);
 		}
 #endif
-		if (tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX) {
+		if (tcp_rtx_probe0_timed_out(sk)) {
 			tcp_write_err(sk);
 			goto out;
 		}
-- 
2.40.1