On Fri, Sep 17, 2021 at 10:37 AM Florian Weimer <fwei...@redhat.com> wrote: > > * Richard Biener: > > > On Fri, Sep 17, 2021 at 10:08 AM Florian Weimer <fwei...@redhat.com> wrote: > >> > >> * Richard Biener via Gcc: > >> > >> > On Thu, Sep 16, 2021 at 10:36 PM Joseph Myers <jos...@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.
Aww, symbol versions. Yeah, that makes it difficult ... Anyway, with a declaration available it's good enough I think. Richard. > (We should probably stop Cc:ing libc-coord because this is so very > GNU-specific at this point.) > > Thanks, > Florian >
