On 28 Dec. 2016, at 14:15, ex vito 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

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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.'

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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