[issue8623] Aliasing warnings in socketmodule.c

[Martin Panter]

Martin Panter added the comment:

The Modules/_multiprocessing/multiprocessing.c code was removed in 3.3 (Issue 
12981). Therefore I am calling this fixed.

--
resolution:  -> fixed
stage: needs patch -> resolved
status: open -> closed

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[issue8623] Aliasing warnings in socketmodule.c

[Martin Panter]

Martin Panter added the comment:

I think the time for backporting to 3.2 has passed. Is there anything else to 
do for this report?

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[issue8623] Aliasing warnings in socketmodule.c

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


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[issue8623] Aliasing warnings in socketmodule.c

[Charles-François Natali]

Charles-François Natali neolo...@free.fr added the comment:

 This change probably should be backported to 3.2 branch.

I'm not sure about this, I don't feel comfortable backporting a such
path which doesn't solve a real world problem.

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[issue8623] Aliasing warnings in socketmodule.c

[Roundup Robot]

Roundup Robot devn...@psf.upfronthosting.co.za added the comment:

New changeset 683a1b1ff15d by Charles-François Natali in branch 'default':
Issue #8623: Fix some strict-aliasing warnings. Patch by David Watson.
http://hg.python.org/cpython/rev/683a1b1ff15d

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[issue8623] Aliasing warnings in socketmodule.c

[Charles-François Natali]

Charles-François Natali neolo...@free.fr added the comment:

Thanks for your patches, David.
I've only applied the first one: looking at the second one, I don't think they 
are really problems (-Wstrict-aliasing=2 is know for generating a lot of false 
positives).

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[issue8623] Aliasing warnings in socketmodule.c

[Arfrever Frehtes Taifersar Arahesis]

Arfrever Frehtes Taifersar Arahesis arfrever@gmail.com added the comment:

This change probably should be backported to 3.2 branch.

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[issue8623] Aliasing warnings in socketmodule.c

[David Watson]

David Watson bai...@users.sourceforge.net added the comment:

For reference, the warnings are partially explained here:

http://gcc.gnu.org/onlinedocs/gcc-4.6.1/gcc/Optimize-Options.html#index-fstrict_002daliasing-825

http://gcc.gnu.org/onlinedocs/gcc-4.6.1/gcc/Warning-Options.html#index-Wstrict_002daliasing-337

I get these warnings with GCC (Ubuntu/Linaro 4.4.4-14ubuntu5)
4.4.5 [i386], plus an additional one from the new recvmsg() code.
I haven't tried GCC 4.5 or later, but as the docs imply, the
warnings will not appear in debugging builds.

I take it GCC is referring to C99 section 6.5, paragraphs 6 and 7
here, but I'm not sure exactly how much these are intended to
prohibit with regard to the (mis)use of unions, or how strictly
GCC actually enforces them.

The attached socket-aliasing-sas2sa.diff is enough to get rid of
the warnings with GCC 4.4.4 - it adds add a struct sockaddr
member to the sock_addr_t union type, changes the SAS2SA() macro
to take the address of this member instead of using a cast, and
modifies socket_gethostbyaddr() and socket_gethostbyname_ex() to
use SAS2SA() (sock_recvmsg_guts() already uses it).

Changing SAS2SA() also gets rid of most of the additional
warnings produced by the aggressive warning setting
-Wstrict-aliasing=2.  However, the gethostby* functions still
point to the union object with a pointer variable not matching
the type actually stored in it, which the GCC docs warn against.

To be more conservative, socket-aliasing-union-3.2.diff applies
on top to get rid of these pointers, and instead directly access
the union for each use other than providing a pointer argument to
a function.  socket-aliasing-union-recvmsg-3.3.diff does the same
for 3.3, and makes the complained-about line in
sock_recvmsg_guts() access the union directly as well.

One other consideration here is that the different sockaddr_*
struct types used are likely to come under the common initial
sequence rule for unions (C99 6.5.2.3, paragraph 5, or section
A8.3 of KR 2nd ed.), which might make some more questionable
uses valid.  That said, technically POSIX appears to require only
that the s*_family members of the various sockaddr struct types
have the same offset and type, not that they form part of a
common initial sequence (s*_family need not be the first
structure member - the BSDs for instance place it second,
although it can still be part of a common initial sequence).

--
keywords: +patch
nosy: +baikie
versions: +Python 3.3
Added file: http://bugs.python.org/file23186/socket-aliasing-sas2sa.diff

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[issue8623] Aliasing warnings in socketmodule.c

[David Watson]

Changes by David Watson bai...@users.sourceforge.net:


Added file: http://bugs.python.org/file23187/socket-aliasing-union-3.2.diff

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[issue8623] Aliasing warnings in socketmodule.c

[David Watson]

Changes by David Watson bai...@users.sourceforge.net:


Added file: http://bugs.python.org/file23188/socket-aliasing-union-3.3.diff

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[issue8623] Aliasing warnings in socketmodule.c

[Kristian Vlaardingerbroek]

Kristian Vlaardingerbroek kristian.vlaardingerbr...@gmail.com added the 
comment:

I can't reproduce both these cases with gcc (Ubuntu/Linaro 4.5.2-8ubuntu4) 
4.5.2 on i386 using the current default branch (70943:024827a9db64).

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[issue8623] Aliasing warnings in socketmodule.c

[Arfrever Frehtes Taifersar Arahesis]

Changes by Arfrever Frehtes Taifersar Arahesis arfrever@gmail.com:


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[issue8623] Aliasing warnings in socketmodule.c

[Éric Araujo]

Éric Araujo mer...@netwok.org added the comment:

Same with gcc 4.4.4, in socket and another file:

Modules/_multiprocessing/multiprocessing.c: In function 
‘multiprocessing_sendfd’:
Modules/_multiprocessing/multiprocessing.c:125: warning: dereferencing 
type-punned pointer will break strict-aliasing rules
Modules/_multiprocessing/multiprocessing.c: In function 
‘multiprocessing_recvfd’:
Modules/_multiprocessing/multiprocessing.c:168: warning: dereferencing 
type-punned pointer will break strict-aliasing rules

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[issue8623] Aliasing warnings in socketmodule.c

[Antoine Pitrou]

New submission from Antoine Pitrou pit...@free.fr:

This is on py3k, with gcc 4.4.3:

/home/antoine/py3k/__svn__/Modules/socketmodule.c: In function 
'socket_gethostbyaddr':
/home/antoine/py3k/__svn__/Modules/socketmodule.c:3238: warning: dereferencing 
pointer 'sa' does break strict-aliasing rules
/home/antoine/py3k/__svn__/Modules/socketmodule.c:3208: note: initialized from 
here
/home/antoine/py3k/__svn__/Modules/socketmodule.c: In function 
'socket_gethostbyname_ex':
/home/antoine/py3k/__svn__/Modules/socketmodule.c:3183: warning: dereferencing 
pointer 'sa' does break strict-aliasing rules
/home/antoine/py3k/__svn__/Modules/socketmodule.c:3181: note: initialized from 
here

--
components: Extension Modules
messages: 105033
nosy: brett.cannon, pitrou
priority: normal
severity: normal
stage: needs patch
status: open
title: Aliasing warnings in socketmodule.c
type: behavior
versions: Python 3.2

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