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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