netdev - Re: [PATCH v2 net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

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Message-ID: <CAL+tcoDURf_krTOSKxM8fhPgR9h7rGaqTPFERVai=n3v6bG-sg@mail.gmail.com>
Date: Tue, 20 Aug 2024 09:07:07 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, ncardwell@...gle.com, 
	kuniyu@...zon.com
Cc: netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>, 
	Jade Dong <jadedong@...cent.com>
Subject: Re: [PATCH v2 net-next] tcp: avoid reusing FIN_WAIT2 when trying to
 find port in connect() process

Hello Eric,

On Thu, Aug 15, 2024 at 7:37 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> We found that one close-wait socket was reset by the other side
> which is beyond our expectation, so we have to investigate the
> underlying reason. The following experiment is conducted in the
> test environment. We limit the port range from 40000 to 40010
> and delay the time to close() after receiving a fin from the
> active close side, which can help us easily reproduce like what
> happened in production.
>
> Here are three connections captured by tcpdump:
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965525191
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 2769915070
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [F.], seq 1, ack 1
> // a few seconds later, within 60 seconds
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [.], ack 2
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [R], seq 2965525193
> // later, very quickly
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 3120990805
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1
>
> As we can see, the first flow is reset because:
> 1) client starts a new connection, I mean, the second one
> 2) client tries to find a suitable port which is a timewait socket
>    (its state is timewait, substate is fin_wait2)
> 3) client occupies that timewait port to send a SYN
> 4) server finds a corresponding close-wait socket in ehash table,
>    then replies with a challenge ack
> 5) client sends an RST to terminate this old close-wait socket.
>
> I don't think the port selection algo can choose a FIN_WAIT2 socket
> when we turn on tcp_tw_reuse because on the server side there
> remain unread data. If one side haven't call close() yet, we should
> not consider it as expendable and treat it at will.
>
> Even though, sometimes, the server isn't able to call close() as soon
> as possible like what we expect, it can not be terminated easily,
> especially due to a second unrelated connection happening.
>
> After this patch, we can see the expected failure if we start a
> connection when all the ports are occupied in fin_wait2 state:
> "Ncat: Cannot assign requested address."
>
> Reported-by: Jade Dong <jadedong@...cent.com>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
> v2
> Link: https://lore.kernel.org/all/20240814035136.60796-1-kerneljasonxing@gmail.com/
> 1. change from fin_wait2 to timewait test statement, no functional
> change (Kuniyuki)
> ---
>  net/ipv4/inet_hashtables.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index 9bfcfd016e18..b95215fc15f6 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -563,7 +563,8 @@ static int __inet_check_established(struct inet_timewait_death_row *death_row,
>                         continue;
>
>                 if (likely(inet_match(net, sk2, acookie, ports, dif, sdif))) {
> -                       if (sk2->sk_state == TCP_TIME_WAIT) {
> +                       if (sk2->sk_state == TCP_TIME_WAIT &&
> +                           inet_twsk(sk2)->tw_substate == TCP_TIME_WAIT) {
>                                 tw = inet_twsk(sk2);
>                                 if (sk->sk_protocol == IPPROTO_TCP &&
>                                     tcp_twsk_unique(sk, sk2, twp))
> --
> 2.37.3
>

Not rushing you, so please please don't get me wrong. I'm a little bit
worried if this email is submerged in the mailing list. So please also
help me review this one :)

Thanks,
Jason