Incorrect pointer alignment in _PyVectorcall_Function() of cpython/abstract.h

[cryptomilk]

BPO	40052
Nosy	@mcepl, @encukou, @eamanu, @miss-islington, @cryptomilk
PRs	bpo-40052: Fix alignment issue in PyVectorcall_Function() #19133 bpo-40052: Replace misaligned type casting with proper unions. #19179 bpo-40052: Fix alignment issue in PyVectorcall_Function() #23999 [3.9] bpo-40052: Fix alignment issue in PyVectorcall_Function() (GH-23999) #24005 [3.8] bpo-40052: Fix alignment issue in PyVectorcall_Function() (GH-23999) #24120

^{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 2021-01-14.12:19:39.772>
created_at = <Date 2020-03-24.06:58:12.620>
labels = ['expert-C-API', 'build', '3.8']
title = 'Incorrect pointer alignment in _PyVectorcall_Function() of cpython/abstract.h'
updated_at = <Date 2021-01-14.12:19:39.771>
user = 'https://github.com/cryptomilk'

bugs.python.org fields:

activity = <Date 2021-01-14.12:19:39.771>
actor = 'petr.viktorin'
assignee = 'none'
closed = True
closed_date = <Date 2021-01-14.12:19:39.772>
closer = 'petr.viktorin'
components = ['C API']
creation = <Date 2020-03-24.06:58:12.620>
creator = 'asn'
dependencies = []
files = []
hgrepos = []
issue_num = 40052
keywords = ['patch']
message_count = 7.0
messages = ['364919', '364954', '364963', '364977', '384033', '384410', '384945']
nosy_count = 6.0
nosy_names = ['mcepl', 'petr.viktorin', 'python-dev', 'eamanu', 'miss-islington', 'asn']
pr_nums = ['19133', '19179', '23999', '24005', '24120']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue40052'
versions = ['Python 3.8']

[cryptomilk]

In file included from /builds/cryptomilk/pam_wrapper/src/python/pypamtest.c:21:
In file included from /usr/include/python3.8/Python.h:147:
In file included from /usr/include/python3.8/abstract.h:837:
/usr/include/python3.8/cpython/abstract.h:91:11: error: cast from 'char *' to 'vectorcallfunc *' (aka 'struct _object *(**)(struct _object *, struct _object *const *, unsigned long, struct _object *)') increases required alignment from 1 to 8 [-Werror,-Wcast-align]
ptr = (vectorcallfunc*)(((char *)callable) + offset);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

The correct way to do it would be:

union {
   char *data;
   vectorcallfunc *ptr;
} vc;

vc.data = (char *)callable + offset;
return *vc.ptr;

[eamanu]

Hi! I cannot reproduce the error. Could you provide the way to reproduce?

thanks

[cryptomilk]

clang -Werror -Wcast-align ...

rpm -q clang9
clang9-9.0.1-8.1.x86_64

Does that help? Found in CI of

https://gitlab.com/cwrap/pam_wrapper

[cryptomilk]

I forgot, for detecting alignment issues or strict aliasing and this also falls under strict aliasing, you need to turn on optimizations.

clang -O2 -Werror -Wcast-align ...

[miss-islington]

New changeset 056c082 by Petr Viktorin in branch 'master':
bpo-40052: Fix alignment issue in PyVectorcall_Function() (GH-23999)
056c082

[encukou]

New changeset 6e72ab9 by Miss Islington (bot) in branch '3.9':
[3.9] bpo-40052: Fix alignment issue in PyVectorcall_Function() (GH-23999) (GH-24005)
6e72ab9

[encukou]

New changeset 187f76d by Petr Viktorin in branch '3.8':
[3.8] bpo-40052: Fix alignment issue in PyVectorcall_Function() (GH-23999) (GH-24120)
187f76d