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Date: Fri, 17 Sep 2021 10:37:36 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Richard Biener <richard.guenther@...il.com>
Cc: Richard Biener via Gcc <gcc@....gnu.org>, Joseph Myers
<joseph@...esourcery.com>, GNU C Library <libc-alpha@...rceware.org>,
libc-coord@...ts.openwall.com
Subject: Re: Add new ABI '__memcmpeq()' to libc
* Richard Biener:
> On Fri, Sep 17, 2021 at 10:08 AM Florian Weimer <fweimer@...hat.com> wrote:
>>
>> * Richard Biener via Gcc:
>>
>> > On Thu, Sep 16, 2021 at 10:36 PM Joseph Myers <joseph@...esourcery.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@...BC_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
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