linux-kernel - Re: [PATCH net] tcp: fix forever orphan socket caused by tcp

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Message-ID: <CAL+tcoDnFCWpFvkjs=7r2C2L_1Fb_8X2J9S0pDNV1KfJKsFo+Q@mail.gmail.com>
Date: Mon, 5 Aug 2024 12:52:08 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Xueming Feng <kuro@...oa.me>, Lorenzo Colitti <lorenzo@...gle.com>, 
	"David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org, 
	Neal Cardwell <ncardwell@...gle.com>, Yuchung Cheng <ycheng@...gle.com>, 
	Soheil Hassas Yeganeh <soheil@...gle.com>, David Ahern <dsahern@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] tcp: fix forever orphan socket caused by tcp_abort

On Sat, Aug 3, 2024 at 11:48 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> Hello Eric,
>
> On Thu, Aug 1, 2024 at 9:17 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Thu, Aug 1, 2024 at 1:17 PM Xueming Feng <kuro@...oa.me> wrote:
> > >
> > > We have some problem closing zero-window fin-wait-1 tcp sockets in our
> > > environment. This patch come from the investigation.
> > >
> > > Previously tcp_abort only sends out reset and calls tcp_done when the
> > > socket is not SOCK_DEAD aka. orphan. For orphan socket, it will only
> > > purging the write queue, but not close the socket and left it to the
> > > timer.
> > >
> > > While purging the write queue, tp->packets_out and sk->sk_write_queue
> > > is cleared along the way. However tcp_retransmit_timer have early
> > > return based on !tp->packets_out and tcp_probe_timer have early
> > > return based on !sk->sk_write_queue.
> > >
> > > This caused ICSK_TIME_RETRANS and ICSK_TIME_PROBE0 not being resched
> > > and socket not being killed by the timers. Converting a zero-windowed
> > > orphan to a forever orphan.
> > >
> > > This patch removes the SOCK_DEAD check in tcp_abort, making it send
> > > reset to peer and close the socket accordingly. Preventing the
> > > timer-less orphan from happening.
> > >
> > > Fixes: e05836ac07c7 ("tcp: purge write queue upon aborting the connection")
> > > Fixes: bffd168c3fc5 ("tcp: clear tp->packets_out when purging write queue")
> > > Signed-off-by: Xueming Feng <kuro@...oa.me>
> >
> > This seems legit, but are you sure these two blamed commits added this bug ?
> >
> > Even before them, we should have called tcp_done() right away, instead
> > of waiting for a (possibly long) timer to complete the job.
> >
> > This might be important when killing millions of sockets on a busy server.
> >
> > CC Lorenzo
> >
> > Lorenzo, do you recall why your patch was testing the SOCK_DEAD flag ?
>
> I guess that one of possible reasons is to avoid double-free,
> something like this, happening in inet_csk_destroy_sock().
>
> Let me assume: if we call tcp_close() first under the memory pressure
> which means tcp_check_oom() returns true and then it will call
> inet_csk_destroy_sock() in __tcp_close(), later tcp_abort() will call
> tcp_done() to free the sk again in the inet_csk_destroy_sock() when
> not testing the SOCK_DEAD flag in tcp_abort.
>

How about this one which can prevent double calling
inet_csk_destroy_sock() when we call destroy and close nearly at the
same time under that circumstance:

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e03a342c9162..d5d3b21cc824 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4646,7 +4646,7 @@ int tcp_abort(struct sock *sk, int err)
        local_bh_disable();
        bh_lock_sock(sk);

-       if (!sock_flag(sk, SOCK_DEAD)) {
+       if (sk->sk_state != TCP_CLOSE) {
                if (tcp_need_reset(sk->sk_state))
                        tcp_send_active_reset(sk, GFP_ATOMIC,
                                              SK_RST_REASON_NOT_SPECIFIED);

Each time we call inet_csk_destroy_sock(), we must make sure we've
already set the state to TCP_CLOSE. Based on this, I think we can use
this as an indicator to avoid calling twice to destroy the socket.

Thanks,
Jason