Re: [libc-coord] Add new ABI '__memcmpeq()' to libc

[Florian Weimer <fweimer@redhat.com>]

* Richard Biener:

> On Fri, Sep 17, 2021 at 10:08 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * Richard Biener via Gcc:
>>
>> > On Thu, Sep 16, 2021 at 10:36 PM Joseph Myers <joseph@codesourcery.com> wrote:
>> >>
>> >> On Thu, 16 Sep 2021, Chris Kennelly wrote:
>> >>
>> >> > In terms of relying on the feature:  If __memcmpeq is ever exposed as an a
>> >> > simple alias for memcmp (since the notes mention that it's a valid
>> >> > implementation), does that open up the possibility of depending on the
>> >> > bcmp-like behavior that we were trying to escape?
>> >>
>> >> The proposal is as an ABI only (compilers would generate calls to
>> >> __memcmpeq from boolean uses of memcmp, users wouldn't write calls to
>> >> __memcmpeq directly, __memcmpeq wouldn't be declared in installed libc
>> >> headers).  If such dependence arises, that would suggest a compiler bug
>> >> wrongly generating such calls for non-boolean memcmp uses.
>> >
>> > So the compiler would emit a call to __memcmpeq and at the same time
>> > emit a weak alias of __memcmpeq to memcmp so the program links
>> > when the libc version targeted does not provide __memcmpeq?  Or would
>> > glibc through <string.h> magically communicate the availability of the new ABI
>> > without actually declaring the function?
>>
>> I do not think ELF provides that capability.
>
> I guess a weak forwarder should do the trick at the cost of a jmp.

How would this look like in practice.

The GNU tools do not support weak symbol versions, so if you have a weak
reference to __memcmpeq@GLIBC_2.35, that's still a reference to the
GLIBC_2.35 symbol version.  The glibc 2.34 dynamic loader notes that
version and rejects the binary because GLIBC_2.35 does not exist.

(We should probably stop Cc:ing libc-coord because this is so very
GNU-specific at this point.)

Thanks,
Florian