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@google.com>
Cc: Menglong Dong <imagedong@tencent.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://lore.kernel.org/r/20240607125652.1472540-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 83fe7f62f7f10ab111512a3ef15a97a04c79cb4a..5bfd76a31af6da6473d306d95c296180141f54e0 100644 (file)
--- 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;