select.poll is not thread safe

[apexo]

BPO	8865
Nosy	@gpshead, @pitrou, @vstinner, @asvetlov, @serhiy-storchaka
Files	select_threads.py: python2 script that demonstrates the problem concurrent_poll_and_register.py: python2 script to demonstrate memory corruption issue select.patch: a patch select_concurrent_poll.patch: Updated to 2.7 for review patch select_concurrent_poll.patch issue8865-poll-multithread-gps01.diff: updated, against 3.2 issue8865_v2.diff: forbid concurrent poll() invocation issue8865_v3.diff: RuntimeError instead of TypeError test.py: crasher issue8865_test.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2013-08-20.18:23:29.244>
created_at = <Date 2010-05-31.22:20:13.881>
labels = ['library', 'type-crash']
title = 'select.poll is not thread safe'
updated_at = <Date 2013-08-20.18:23:29.243>
user = 'https://bugs.python.org/apexo'

bugs.python.org fields:

activity = <Date 2013-08-20.18:23:29.243>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2013-08-20.18:23:29.244>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2010-05-31.22:20:13.881>
creator = 'apexo'
dependencies = []
files = ['17510', '17511', '17675', '27961', '27963', '27964', '27967', '27968', '28687', '28855']
hgrepos = []
issue_num = 8865
keywords = ['patch']
message_count = 31.0
messages = ['106815', '106816', '106827', '106829', '107863', '175387', '175389', '175390', '175394', '175395', '175397', '175400', '175409', '175429', '175431', '175432', '175501', '175525', '176894', '176895', '176969', '176998', '179657', '179659', '180696', '181822', '181823', '186736', '194334', '195705', '195707']
nosy_count = 9.0
nosy_names = ['gregory.p.smith', 'exarkun', 'pitrou', 'vstinner', 'asvetlov', 'neologix', 'apexo', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue8865'
versions = ['Python 2.7', 'Python 3.3', 'Python 3.4']

[apexo]

invoking select.poll.poll() concurrently from multiple threads frequently yields garbage in one thread:

while poll_poll in thread 1 is parsing its result, another thread 2 calling poll may overwrite revents; assuming poll_result was 1 in thread 1 and thread 2 managed to clear all revents before thread 1 started scanning ufds, thread 1 would iterate straight through all ufds, past its bounds (no bound checks there), and return the first out-of-bounds entry that happens to have revents != 0

this issue needs at least documentation (although bounds-checking to prevent garbage in the result wouldn't hurt)

also, since there doesn't seem to be any locking w/ regards to ufds, it might be possible to corrupt python's heap, by concurrently invoking poll_register and poll_poll. poll_register could move the ufds array to another location while resizing it and poll_poll would subsequently overwrite memory that is not allocated anymore or allocated by someone else (did not test that)

python 2.5.5

[pitrou]

Do you have a script that reproduces it?

[apexo]

okay, I've managed to put together a script that fairly consistently reproduces the problem on my (core2 duo 2,something GHz) machine

some parameters (sleep times) are randomized in each iteration, the script runs for 30 iterations, each for 1 second, and counts the number of events where the fd looks bogus (max 1 per iteration); some other events (fd correct, revents bogus are also reported, but not counted)

example output:
poll returned [(-3, 65535)], we were looking for fd 3
poll returned [(-3, 65535)], we were looking for fd 3
poll returned [(-3, 65535)], we were looking for fd 3
poll returned [(-3, 65535)], we were looking for fd 3
poll returned [(-3, 65535)], we were looking for fd 3
poll returned [(-1, 65535)], we were looking for fd 3
poll returned [(-1, 65535)], we were looking for fd 3
poll returned [(-1627358347, 60497)], we were looking for fd 3
should not get here; poll returned [(3, 0)]
poll returned [(-3, 65535)], we were looking for fd 3
9 events in 30 iterations

[apexo]

the other issue (poll_register freeing data structures that poll_poll currently uses) can be reproduced by the attached script (at least I can)

depending on the parameters/circumstances it either segfaults or prints garbage results from poll, e.g. [(1684955474, 28192)]; the expected result would be [(3, 1)]

[apexo]

added a patch which fixes both issues

before releasing the GIL we take a copy of the ufds pointer and its len, erasing the ufds pointer in the poll object (to make sure nobody else fiddles with it); when we're done we either but it back into the object (when it's still NULL) or we free it

the update logic is modified as well, since the uptodate flag is not sufficient anymore, we now count up a tag for each modification of the poll dictionary and remember that tag together with the ufds pointer

the result is:

• for the single-threaded case we do little extra work (moving ufds from/to poll object)
• correct for the multi-threaded case, with slightly higher overhead (one additional call to each of malloc, update_ufd_array, free), probably worse than having one poll object per thread, but not worse than allocating a new poll object each time we want to poll

there still is potential for incorrect poll results (but not for memory corruption): when poll_register/poll_unregister are called exactly 2**32 times (or multiples thereof) while poll_poll is running, it will happily put back its outdated ufds pointer into the poll object when its done, this could be alleviated by changing tag to long long ... which is unlikely to wrap around anytime soon.

[serhiy-storchaka]

I was unable to apply the patch automatically, so I had to do it manually. Here is an updated patch for review. I did not consider it in detail yet, but it seems to correct these errors.