Re: [PATCH] unix: avoid use-after-free in ep_remove_wait_queue (w/ Fixes:)

[Rainer Weikusat <rweikusat@mobileactivedefense.com>]

David Miller <davem@davemloft.net> writes:
> From: Rainer Weikusat <rweikusat@mobileactivedefense.com>
> Date: Mon, 16 Nov 2015 22:28:40 +0000
>
>> An AF_UNIX datagram socket being the client in an n:1 association with
>> some server socket is only allowed to send messages to the server if the
>> receive queue of this socket contains at most sk_max_ack_backlog
>> datagrams.

[...]

>> Signed-off-by: Rainer Weikusat <rweikusat@mobileactivedefense.com>
>> Fixes: ec0d215f9420 ("af_unix: fix 'poll for write'/connected DGRAM sockets")
>
> So because of a corner case of epoll handling and sender socket release,
> every single datagram sendmsg has to do a double lock now?
>
> I do not dispute the correctness of your fix at this point, but that
> added cost in the fast path is really too high.

This leaves only the option of a somewhat incorrect solution and what is
or isn't acceptable in this respect is somewhat difficult to decide. The
basic options would be

	- return EAGAIN even if sending became possible (Jason's most
          recent suggestions)

	- retry sending a limited number of times, eg, once, before
          returning EAGAIN, on the grounds that this is nicer to the
          application and that redoing all the stuff up to the _lock in
          dgram_sendmsg can possibly/ likely be avoided

Which one do you prefer?