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Date: Fri, 9 Dec 2016 06:41:44 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>,
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        netdev <netdev@vger.kernel.org>, Eric Dumazet <edumazet@google.com>,
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Subject: Re: fs, net: deadlock between bind/splice on af_unix
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On Thu, Dec 08, 2016 at 10:32:00PM -0800, Cong Wang wrote:

> > Why do we do autobind there, anyway, and why is it conditional on
> > SOCK_PASSCRED?  Note that e.g. for SOCK_STREAM we can bloody well get
> > to sending stuff without autobind ever done - just use socketpair()
> > to create that sucker and we won't be going through the connect()
> > at all.
> 
> In the case Dmitry reported, unix_dgram_sendmsg() calls unix_autobind(),
> not SOCK_STREAM.

Yes, I've noticed.  What I'm asking is what in there needs autobind triggered
on sendmsg and why doesn't the same need affect the SOCK_STREAM case?

> I guess some lock, perhaps the u->bindlock could be dropped before
> acquiring the next one (sb_writer), but I need to double check.

Bad idea, IMO - do you *want* autobind being able to come through while
bind(2) is busy with mknod?