[Bug network/16421] IN6_IS_ADDR_UNSPECIFIED can use undefined s6_addr32

["carlos at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16421

--- Comment #13 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Octavio Alvarez from comment #12)
> Thank you. Today I have learned about statement expressions and how they are
> GCC extensions.
> 
> If we add an #ifdef __GNUC__ we would also have to add an #else that avoids
> the usage of non-ISO C extensions anyway so Clang do not throw any warnings
> or compilers that don't define __GNUC__ still have IN6_* macros available.
> Again, two implementations. Now I understand why was the original code in
> that form.

That's correct.

> If the goal is to have a universal implementation it's no use to have it
> wrapped in #ifdef __GNUC__. It just stops being universal.

Agreed, but have a __GNUC__ version optimized for GNUC is good.

> Given the above, there is no use on exposing __in6_u.__u6_addr32 for the
> purpose of fixing this bug, then.

I don't follow.

> Either we go back to patch #1, which keeps both implementations and just
> adds the test for definition of __USE_GNU and __USE_MISC macros and does not
> break strict compilers, or we remove the #ifdef __GNUC__ implementation
> altogether and leave only the #else implementation unconditionally.

Not an option. Please leave the __GNUC__ implementation for optimally
supporting those using GNU extensions.

> Sample of the #else implementation:
> # define IN6_IS_ADDR_UNSPECIFIED(a) \
>         (((const uint32_t *) (a))[0] == 0                                   
> \
>          && ((const uint32_t *) (a))[1] == 0                                
> \
>          && ((const uint32_t *) (a))[2] == 0                                
> \
>          && ((const uint32_t *) (a))[3] == 0)
> 
> I will gladly attach a patch with just the #else implementation for your
> review, but I still think the best bet to use the original patch.
> 
> Git commit aff2453d [1] (Dec 2011) shows that originally, only the #else
> implementation existed but Ulrich Drepper added a GCC-specific
> implementation, apparently to avoid warnings. Later, in commit a784e502 [2]
> (Jan 2012), he changed the usage of __const to plain const, removing pre-ISO
> C implementations.

The fix I'll accept is:

- Unconditionally define __u6_* in the union.
- Fix all the macros.
- Test that.

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