Avoid memcpy(. . ., NULL, 0) etc calls

[vadmium]

BPO	27570
Nosy	@benjaminp, @vadmium
Files	memcpy-null.patch memcpy-null.v2.patch memcpy-null.v3.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 = None
closed_at = <Date 2016-09-08.06:11:34.573>
created_at = <Date 2016-07-19.02:57:10.714>
labels = ['interpreter-core', 'ctypes', 'type-bug']
title = 'Avoid memcpy(. . ., NULL, 0) etc calls'
updated_at = <Date 2016-09-08.06:11:34.572>
user = 'https://github.com/vadmium'

bugs.python.org fields:

activity = <Date 2016-09-08.06:11:34.572>
actor = 'martin.panter'
assignee = 'none'
closed = True
closed_date = <Date 2016-09-08.06:11:34.573>
closer = 'martin.panter'
components = ['Interpreter Core', 'ctypes']
creation = <Date 2016-07-19.02:57:10.714>
creator = 'martin.panter'
dependencies = []
files = ['43784', '43852', '43987']
hgrepos = []
issue_num = 27570
keywords = ['patch']
message_count = 7.0
messages = ['270806', '271135', '271874', '274701', '274708', '274917', '274969']
nosy_count = 3.0
nosy_names = ['benjamin.peterson', 'python-dev', 'martin.panter']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27570'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']

[vadmium]

This patch fixes errors reported by GCC’s undefined behaviour sanitizer about calling functions with a null pointer:

./configure CC="gcc -fsanitize=undefined"

Using bpo-22605 as a precedent, I propose to avoid calling memcpy() and memmove() for zero-length copies when there may be a null pointer.

[vadmium]

V2 patch adds another fix, uncovered by recent datetime tests:

>>> a = array("B")
>>> a[:] = a
/media/disk/home/proj/python/cpython/Modules/arraymodule.c:748:5: runtime error: null pointer passed as argument 1, which is declared to never be null
/media/disk/home/proj/python/cpython/Modules/arraymodule.c:748:5: runtime error: null pointer passed as argument 2, which is declared to never be null

[vadmium]

Patch 3 fixes some more cases I found in the array module:

>>> a + a
Modules/arraymodule.c:809:5: runtime error: null pointer passed as argument 1, which is declared to never be null
Modules/arraymodule.c:809:5: runtime error: null pointer passed as argument 2, which is declared to never be null
Modules/arraymodule.c:810:5: runtime error: null pointer passed as argument 1, which is declared to never be null
Modules/arraymodule.c:810:5: runtime error: null pointer passed as argument 2, which is declared to never be null
array('B')
>>> a * 3
Modules/arraymodule.c:840:9: runtime error: null pointer passed as argument 1, which is declared to never be null
Modules/arraymodule.c:840:9: runtime error: null pointer passed as argument 2, which is declared to never be null
array('B')
>>> a += a
Modules/arraymodule.c:952:5: runtime error: null pointer passed as argument 1, which is declared to never be null
Modules/arraymodule.c:952:5: runtime error: null pointer passed as argument 2, which is declared to never be null

I wondered if there is a good argument for fixing these, or if it is only a theoretical problem. Apparently GCC can do optimizations about null pointer tests: <https://gcc.gnu.org/gcc-4.9/porting_to.html\>. I don’t think any of the cases I found are instances of this problem, but I think fixing them helps keep the UB sanitizer output clean, so any errors causing practical behaviour problems will be easier to find.

[vadmium]

Looks like revisions 5f3f6f1fb73a and ec537f9f468f may have fixed the listobject cases.

Also 66feda02f2a5 looks relevant. Benjamin, maybe you are interested in other bits of my patches :)

[benjaminp]

Sorry I missed this. The changes which I didn't already make look good. :)

[python-dev]

New changeset e231dcad3a9b by Martin Panter in branch '3.5':
Issue bpo-27570: Avoid zero-length memcpy() calls with null source pointers
https://hg.python.org/cpython/rev/e231dcad3a9b

New changeset 2d0fb659372c by Martin Panter in branch 'default':
Issue bpo-27570: Merge null pointer fixes from 3.5
https://hg.python.org/cpython/rev/2d0fb659372c

[python-dev]

New changeset d465da1e5902 by Martin Panter in branch '2.7':
Issue bpo-27570: Avoid zero-length memcpy() calls with null source pointers
https://hg.python.org/cpython/rev/d465da1e5902