[Twisted-Python] twisted.python.sendmsg segfault - looking for confirmation

[Amber "Hawkie" Brown]

> On 28 Dec. 2016, at 14:15, ex vito <ex.vitorino at gmail.com> wrote:
> 
> Dear all,
> 
> While working on http://twistedmatrix.com/trac/ticket/8912 and the associated PR at https://github.com/twisted/twisted/pull/647 I hit a segmentation fault on a test I was preparing.
> 
> I think narrowed it down Linux + Python 2, but I'm not 100% sure. Can anyone please confirm the  code below segfaults on such environment? For completeness, from my tests, it works fine on Linux + Python 3, and fails with socket.error/OSError on Mac OS 10.9.5 + Python 2/3. No BSDs at hand to try out...
> 
> As far as I can tell, no code in Twisted uses sendmsg in the way the code is using it. However, twisted.python.sendmsg is a public API so someone somewhere may hit this like I just did.
> 
> Other than looking for confirmation, I have a fix which I included two isolated commits in the PR. Twisted project members: if this is confirmed, maybe a ticket should be created in Trac and an independent PR created, correct?
> 
> Thanks in advance for any input.
> Regards,
> --
> exvito
> 
> -[ cut here ]----------------------------------------------------------------------
> 
> import os
> import socket
> import struct
> import sys
> 
> from twisted.python import sendmsg
> 
> if __name__ == '__main__':
> 
>    # This code segfaults when running with twisted/trunk commit
>    # 98a3df200968b78bd3b985dfd4fb10a5b415d6fc on Linux Python 2
>    # due to a bug in src/twisted/python/_sendmsg.c and glibc's
>    # CMSG_NXTHDR implementation.
> 
>    # get connected sockets
>    s1, s2 = socket.socketpair(socket.AF_UNIX)
> 
>    # two CMSGs in ancillary data (no segfault with just one)
>    ancillary = [
>        (socket.SOL_SOCKET, sendmsg.SCM_RIGHTS, struct.pack('i', s1.fileno())),
>        (socket.SOL_SOCKET, sendmsg.SCM_RIGHTS, struct.pack('i', s2.fileno())),
>    ]
> 
>    expected = {
>        2: 'Expecting to segfault.',
>        3: 'Should not segfault.',
>    }
>    print expected.get(sys.version_info.major, 'Unexpected Python version.')
>    retval = sendmsg.sendmsg(s1, data=b'some data', ancillary=ancillary)
>    print 'Did not segfault.'
> 
> -[ cut here ]----------------------------------------------------------------------
> 
> 
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FWIW, for people wanting to track this down, this may be a bug in our C extension (src/twisted/python/_sendmsg.c). Python 3 uses the stdlib's functionality, which is probably why it isn't running into this.

- Amber