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Date: Fri, 07 Jun 2013 08:08:37 +0200
From: Matthias Andree <mandree@FreeBSD.org>
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To: Maxim Dounin <mdounin@mdounin.ru>
Subject: Re: IN6_IS_ADDR_* macros use invalid type punning?
References: <51B0EFC2.1020406@FreeBSD.org> <20130606232925.GU72282@mdounin.ru>
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Am 07.06.2013 01:29, schrieb Maxim Dounin:

> [...]
> 
>> try.c:9:5: warning: dereferencing type-punned pointer will break
>> strict-aliasing rules [-Wstrict-aliasing]
>>      int r = IN6_IS_ADDR_V4MAPPED((&sin6.sin6_addr));
>>      ^
>> try.c:9:5: warning: dereferencing type-punned pointer will break
>> strict-aliasing rules [-Wstrict-aliasing]
> 
> [...]

> Gleb already committed a fix for this 16 months ago 
> (unfortunately, correct patch description was lost in transit):
> 
> http://svnweb.freebsd.org/base?view=revision&revision=230584

Great.  Thank you for the pointer.  I could have checked head/ first
indeed.

Looking at
<http://svnweb.freebsd.org/base/head/sys/netinet6/in6.h?r1=230584&r2=230583&pathrev=230584>:
The code committed at that time is lucky that htonl() and ntohl() are
implemented the same way; all changed macros should be changed to use ==
htonl(1) or == htonl(0x0000ffff), just to get the proper meaning across.

ntohl would have to be applied to the __u6_addr instead, but is less
efficient because it is not open to compile-time evaluation, unlike
htonl(CONSTANT_ADDRESS).


And indeed the commit log is a bit less compelling than might have
fostered the propagation.  It looks like it were only about qualifiers,
but it is also about violating aliasing rules per ISO 9899.

> Probably it's a good idea to MFC the fix.

Please let's get this MFC'd and MFS'd (while it won't make releng/8.4 at
least we can have stable/8 fixed, too) and get the system-headers
induced warning done away with on all supported branches.

Best
Matthias