netdev - Re: [PATCH v1 net] tcp: Don't drop SYN+ACK for simultaneous connect().

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Message-ID: <CANn89iKO=Y8P_tms-nymhLF8QbWmOD-g_N33DLMfA6WcO+vhbg@mail.gmail.com>
Date: Thu, 4 Jul 2024 14:23:11 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>, Dmitry Safonov <0x7f454c46@...il.com>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>, Lawrence Brakmo <brakmo@...com>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH v1 net] tcp: Don't drop SYN+ACK for simultaneous connect().

On Thu, Jul 4, 2024 at 1:16 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Wed, 2024-07-03 at 20:57 -0700, Kuniyuki Iwashima wrote:
> > RFC 9293 states that in the case of simultaneous connect(), the connection
> > gets established when SYN+ACK is received. [0]
> >
> >       TCP Peer A                                       TCP Peer B
> >
> >   1.  CLOSED                                           CLOSED
> >   2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
> >   3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
> >   4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED
> >   5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
> >   6.  ESTABLISHED  <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
> >   7.               ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED
> >
> > However, since commit 0c24604b68fc ("tcp: implement RFC 5961 4.2"), such a
> > SYN+ACK is dropped in tcp_validate_incoming() and responded with Challenge
> > ACK.
> >
> > For example, the write() syscall in the following packetdrill script fails
> > with -EAGAIN, and wrong SNMP stats get incremented.
> >
> >    0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
> >   +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
> >
> >   +0 > S  0:0(0) <mss 1460,sackOK,TS val 1000 ecr 0,nop,wscale 8>
> >   +0 < S  0:0(0) win 1000 <mss 1000>
> >   +0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 3308134035 ecr 0,nop,wscale 8>
> >   +0 < S. 0:0(0) ack 1 win 1000
> >
> >   +0 write(3, ..., 100) = 100
> >   +0 > P. 1:101(100) ack 1
> >
> >   --
> >
> >   # packetdrill cross-synack.pkt
> >   cross-synack.pkt:13: runtime error in write call: Expected result 100 but got -1 with errno 11 (Resource temporarily unavailable)
> >   # nstat
> >   ...
> >   TcpExtTCPChallengeACK           1                  0.0
> >   TcpExtTCPSYNChallenge           1                  0.0
> >
> > That said, this is no big deal because the Challenge ACK finally let the
> > connection state transition to TCP_ESTABLISHED in both directions.  If the
> > peer is not using Linux, there might be a small latency before ACK though.
> >
> > The problem is that bpf_skops_established() is triggered by the Challenge
> > ACK instead of SYN+ACK.  This causes the bpf prog to miss the chance to
> > check if the peer supports a TCP option that is expected to be exchanged
> > in SYN and SYN+ACK.
> >
> > Let's accept a bare SYN+ACK for non-TFO TCP_SYN_RECV sockets to avoid such
> > a situation.
>
> Apparently this behavior change is causing TCP AO self-tests failures:
>
> https://netdev.bots.linux.dev/contest.html?pw-n=0&branch=net-next-2024-07-04--09-00
> e.g.
> https://netdev-3.bots.linux.dev/vmksft-tcp-ao-dbg/results/668061/22-self-connect-ipv4/stdout
>

These tests seem to have broken assumptions on a kernel behavior which
are orthogonal to TCP AO.

> Could you please have a look?
>
> Thanks!
>
> Paolo
>