netdev - [PATCH net] tcp: use signed arithmetic in tcp_rtx_probe0_timed

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Date: Fri,  7 Jun 2024 12:56:52 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, Neal Cardwell <ncardwell@...gle.com>, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>, Menglong Dong <imagedong@...cent.com>
Subject: [PATCH net] tcp: use signed arithmetic in tcp_rtx_probe0_timed_out()

Due to timer wheel implementation, a timer will usually fire
after its schedule.

For instance, for HZ=1000, a timeout between 512ms and 4s
has a granularity of 64ms.
For this range of values, the extra delay could be up to 63ms.

For TCP, this means that tp->rcv_tstamp may be after
inet_csk(sk)->icsk_timeout whenever the timer interrupt
finally triggers, if one packet came during the extra delay.

We need to make sure tcp_rtx_probe0_timed_out() handles this case.

Fixes: e89688e3e978 ("net: tcp: fix unexcepted socket die when snd_wnd is 0")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Menglong Dong <imagedong@...cent.com>
---
 net/ipv4/tcp_timer.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 83fe7f62f7f10ab111512a3ef15a97a04c79cb4a..5bfd76a31af6da6473d306d95c296180141f54e0 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -485,8 +485,12 @@ static bool tcp_rtx_probe0_timed_out(const struct sock *sk,
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	const int timeout = TCP_RTO_MAX * 2;
-	u32 rcv_delta;
+	s32 rcv_delta;
 
+	/* Note: timer interrupt might have been delayed by at least one jiffy,
+	 * and tp->rcv_tstamp might very well have been written recently.
+	 * rcv_delta can thus be negative.
+	 */
 	rcv_delta = inet_csk(sk)->icsk_timeout - tp->rcv_tstamp;
 	if (rcv_delta <= timeout)
 		return false;
-- 
2.45.2.505.gda0bf45e8d-goog