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From:   Yuchung Cheng <ycheng@google.com>
Date:   Tue, 12 Jan 2021 14:48:01 -0800
Message-ID: <CAK6E8=fq6Jec94FDmDHGWhsmjtZQmt3AwQB0-tLcpJpvJ=oLgg@mail.gmail.com>
Subject: Re: [PATCH] tcp: keepalive fixes
To:     Enke Chen <enkechen2020@gmail.com>
Cc:     Eric Dumazet <edumazet@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Jakub Kicinski <kuba@kernel.org>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Neal Cardwell <ncardwell@google.com>
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On Tue, Jan 12, 2021 at 2:31 PM Enke Chen <enkechen2020@gmail.com> wrote:
>
> From: Enke Chen <enchen@paloaltonetworks.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.
hi enke
Do you have an example to demonstrate this issue -- in theory when
there is data inflight, an RTO timer should be pending (which
considers user-timeout setting). based on the user-timeout description
(man tcp), the user timeout should abort the socket per the specified
time after data commences. some data would help to understand the
issue.

>
>    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@vger.kernel.org
> Signed-off-by: Enke Chen <enchen@paloaltonetworks.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
>