Re: [PATCH net-next] tcp: do not allow to connect with the four-tuple symmetry socket

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Jason Xing <kerneljasonxing@gmail.com>
Date: Sun, 18 Aug 2024 21:50:51 +0800
> On Sun, Aug 18, 2024 at 1:16 PM Jason Xing <kerneljasonxing@gmail.com> wrote:
> >
> > On Sun, Aug 18, 2024 at 12:25 PM Jason Xing <kerneljasonxing@gmail.com> wrote:
> > >
> > > From: Jason Xing <kernelxing@tencent.com>
> > >
> > > Four-tuple symmetry here means the socket has the same remote/local
> > > port and ipaddr, like this, 127.0.0.1:8000 -> 127.0.0.1:8000.
> > > $ ss -nat | grep 8000
> > > ESTAB      0      0          127.0.0.1:8000       127.0.0.1:8000
> 
> Thanks to the failed tests appearing in patchwork, now I'm aware of
> the technical term called "self-connection" in English to describe
> this case. I will update accordingly the title, body messages,
> function name by introducing "self-connection" words like this in the
> next submission.
> 
> Following this clue, I saw many reports happening in these years, like
> [1][2]. Users are often astonished about this phenomenon and lost and
> have to find various ways to workaround it. Since, in my opinion, the
> self-connection doesn't have any advantage and usefulness,

It's useful if you want to test simultaneous connect (SYN_SENT -> SYN_RECV)
path as you see in TCP-AO tests.  See RFC 9293 and the (!ack && syn) case
in tcp_rcv_synsent_state_process().

  https://www.rfc-editor.org/rfc/rfc9293.html#section-3.5-7

So you can't remove self-connect functionality, the recent main user is
syzkaller though.