[PATCH] Disable AF_UNIX handshake with setsockopt(..., SO_PEERCRED, ...)

Corinna Vinschen corinna-cygwin@cygwin.com
Sat Oct 11 18:36:00 GMT 2014

On Oct 10 20:04, Corinna Vinschen wrote:
> On Oct 10 18:36, Christian Franke wrote:
> > After a nonblocking connect(), postfix calls poll() with pollfd.events =
> > POLLIN only. If poll() succeeds, it calls recv(). This fails with ENOTCONN
> > because the state is still connect_pending.
> Oh.  So it doesn't check if the connect succeeded?  Does it check the
> poll result for POLLERR or does it explicitely check for revents==POLLIN?
> Hmm.
> [...time passes...]
> It looks like you catched a long-standing bug here.
> This isn't even AF_LOCAL specific.  The original comment in the
> write_selected branch is misleading: The AF_LOCAL specific part is just
> the call to af_local_connect, not setting the connect_state.  There was
> a previous, longer comment at one point which I shortened for no good
> reason in 2005:
>   /* eid credential transaction on successful non-blocking connect.
>      Since the read bit indicates an error, don't start transaction
>      if it's set. */
> However, If I'm not completely mistaken, your patch would only work in
> the aforementioned scenario if setsockopt(SO_PEERCRED) has been called.
> Otherwise the handshake would be skipped on the connect side and thus
> the handshake would fail on the server side.  There's also the problem
> that read_ready may indicate an error.  And POLLERR is only set if the
> socket is polled for POLLOUT so a failing connect would go unnoticed.
> In short, the whole code is written under the assumption that any sane
> application calling nonblocking connect would always call select/poll to
> check if connect succeeded in the first place.  Obviously, as postfix
> shows, this is a wrong assumption. 
> I'm not yet sure how to fix this, but I'll look into this next week.

I applied a fix which, I think, is much more elegant than the former
solution.  The af_local_connect call is now called as soon as an
FD_CONNECT event is generated and read by a call to wait_event.  It
worked for me, so I have tender hopes that I didn't miss something.

I also applied your patch on top of this new stuff and I'm just building
a new developer snapshot for testing.  In setsockopt I added a check for
socket family and type so setsockopt(SO_PEERCRED) only works for
AF_LOCAL/SOCK_STREAM sockets, just as the entire handshake stuff.  I
also added a comment to explain why we do this and a FIXME comment so we
don't forget we're still looking for a more generic solution for the
SO_PEERCRED exchange.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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