netdev - Re: [PATCH net-next] tcp: avoid RST in 3-way shakehands due to failure in tcp_timewait_state

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Message-ID: <CANn89iL5df5_EiDX7JxaFbfmZ9gDo=8ZyLXhbZs+-yp8zVD=GA@mail.gmail.com>
Date: Thu, 7 Nov 2024 09:37:09 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: Philo Lu <lulie@...ux.alibaba.com>, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, horms@...nel.org, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next] tcp: avoid RST in 3-way shakehands due to
 failure in tcp_timewait_state_process

On Thu, Nov 7, 2024 at 9:27 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Thu, Nov 7, 2024 at 4:22 PM Philo Lu <lulie@...ux.alibaba.com> wrote:
> >
> >
> >
> > On 2024/11/7 16:01, Jason Xing wrote:
> > > On Thu, Nov 7, 2024 at 3:51 PM Philo Lu <lulie@...ux.alibaba.com> wrote:
> > >>
> > >> Hi Jason,
> > >>
> > >> On 2024/11/5 10:55, Jason Xing wrote:
> > >>> From: Jason Xing <kernelxing@...cent.com>
> > >>>
> > >>> We found there are rare chances that some RST packets appear during
> > >>> the shakehands because the timewait socket cannot accept the SYN and
> > >>> doesn't return TCP_TW_SYN in tcp_timewait_state_process().
> > >>>
> > >>> Here is how things happen in production:
> > >>> Time        Client(A)        Server(B)
> > >>> 0s          SYN-->
> > >>> ...
> > >>> 132s                         <-- FIN
> > >>> ...
> > >>> 169s        FIN-->
> > >>> 169s                         <-- ACK
> > >>> 169s        SYN-->
> > >>> 169s                         <-- ACK
> > >>> 169s        RST-->
> > >>> As above picture shows, the two flows have a start time difference
> > >>> of 169 seconds. B starts to send FIN so it will finally enter into
> > >>> TIMEWAIT state. Nearly at the same time A launches a new connection
> > >>> that soon is reset by itself due to receiving a ACK.
> > >>>
> > >>> There are two key checks in tcp_timewait_state_process() when timewait
> > >>> socket in B receives the SYN packet:
> > >>> 1) after(TCP_SKB_CB(skb)->seq, rcv_nxt)
> > >>> 2) (s32)(READ_ONCE(tcptw->tw_ts_recent) - tmp_opt.rcv_tsval) < 0)
> > >>>
> > >>> Regarding the first rule, it fails as expected because in the first
> > >>> connection the seq of SYN sent from A is 1892994276, then 169s have
> > >>> passed, the second SYN has 239034613 (caused by overflow of s32).
> > >>>
> > >>> Then how about the second rule?
> > >>> It fails again!
> > >>> Let's take a look at how the tsval comes out:
> > >>> __tcp_transmit_skb()
> > >>>       -> tcp_syn_options()
> > >>>           -> opts->tsval = tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb) + tp->tsoffset;
> > >>> The timestamp depends on two things, one is skb->skb_mstamp_ns, the
> > >>> other is tp->tsoffset. The latter value is fixed, so we don't need
> > >>> to care about it. If both operations (sending FIN and then starting
> > >>> sending SYN) from A happen in 1ms, then the tsval would be the same.
> > >>> It can be clearly seen in the tcpdump log. Notice that the tsval is
> > >>> with millisecond precision.
> > >>>
> > >>> Based on the above analysis, I decided to make a small change to
> > >>> the check in tcp_timewait_state_process() so that the second flow
> > >>> would not fail.
> > >>>
> > >>
> > >> I wonder what a bad result the RST causes. As far as I know, the client
> > >> will not close the connect and return. Instead, it re-sends an SYN in
> > >> TCP_TIMEOUT_MIN(2) jiffies (implemented in
> > >> tcp_rcv_synsent_state_process). So the second connection could still be
> > >> established successfully, at the cost of a bit more delay. Like:
> > >>
> > >>    Time        Client(A)        Server(B)
> > >>    0s          SYN-->
> > >>    ...
> > >>    132s                         <-- FIN
> > >>    ...
> > >>    169s        FIN-->
> > >>    169s                         <-- ACK
> > >>    169s        SYN-->
> > >>    169s                         <-- ACK
> > >>    169s        RST-->
> > >> ~2jiffies    SYN-->
> > >>                                 <-- SYN,ACK
> > >
> > > That's exactly what I meant here :) Originally I didn't expect the
> > > application to relaunch a connection in this case.
> >
> > s/application/kernel/, right?
>
> No. Perhaps I didn't make myself clear. If the kernel doesn't silently
> drop the SYN and then send back an ACK, the application has to call
> the connect() syscall again.

My suggestion to stop the confusion:

Provide a packetdrill test.