From: Richard Biener <richard.guenther@gmail.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: Richard Biener via Gcc <gcc@gcc.gnu.org>,
Joseph Myers <joseph@codesourcery.com>,
GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [libc-coord] Add new ABI '__memcmpeq()' to libc
Date: Fri, 17 Sep 2021 11:30:50 +0200 [thread overview]
Message-ID: <CAFiYyc0C+u5_s5XFR9eUL5Vz+ngunZb3TrtbTfO-UYEu-no9-g@mail.gmail.com> (raw)
In-Reply-To: <87lf3vha27.fsf@oldenburg.str.redhat.com>
On Fri, Sep 17, 2021 at 10:37 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * 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.
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
>
next prev parent reply other threads:[~2021-09-17 9:31 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-16 17:02 Noah Goldstein
2021-09-16 17:55 ` [libc-coord] " Chris Kennelly
2021-09-16 18:31 ` Noah Goldstein
2021-09-16 20:32 ` Chris Kennelly
2021-09-16 20:35 ` Joseph Myers
2021-09-16 20:55 ` enh
2021-09-17 7:43 ` Richard Biener
2021-09-17 8:08 ` Florian Weimer
2021-09-17 8:31 ` Richard Biener
2021-09-17 8:37 ` Florian Weimer
2021-09-17 9:30 ` Richard Biener [this message]
2021-09-17 17:40 ` Noah Goldstein
2021-09-17 9:12 ` Jakub Jelinek
2021-09-17 16:55 ` Martin Sebor
2021-09-17 14:19 ` Joseph Myers
2021-09-17 14:26 ` Florian Weimer
2021-09-21 19:53 ` Noah Goldstein
2021-09-22 17:46 ` Christoph Müllner
2021-09-22 18:15 ` Noah Goldstein
2021-09-16 21:27 ` James Y Knight
2021-09-16 21:42 ` Joseph Myers
2021-09-16 21:50 ` enh
2021-09-16 21:59 ` Noah Goldstein
2021-09-16 22:17 ` Chris Kennelly
2021-09-16 22:36 ` Joseph Myers
2021-09-16 23:24 ` Noah Goldstein
2021-09-18 1:36 ` James Y Knight
2021-10-26 22:47 ` Noah Goldstein
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAFiYyc0C+u5_s5XFR9eUL5Vz+ngunZb3TrtbTfO-UYEu-no9-g@mail.gmail.com \
--to=richard.guenther@gmail.com \
--cc=fweimer@redhat.com \
--cc=gcc@gcc.gnu.org \
--cc=joseph@codesourcery.com \
--cc=libc-alpha@sourceware.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).