From: Bruno Haible <bruno@clisp.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: libc-alpha@sourceware.org, bug-gnulib@gnu.org, binutils@sourceware.org
Subject: Re: Undefined use of weak symbols in gnulib
Date: Wed, 28 Apr 2021 16:40:18 +0200 [thread overview]
Message-ID: <2800926.834q8TerIH@omega> (raw)
In-Reply-To: <875z06lu3v.fsf@oldenburg.str.redhat.com>
Hi Florian,
Thank you for the details.
> > In which situations will it crash?
> >
> > (a) when the code is in an executable, that gets linked with '-lpthread'
> > and that does not use dlopen()?
>
> The pthread_mutexattr_gettype is defined, but also pthread_once and the
> weak symbols, so there is no problem because the link editor doesn't do
> funny things.
>
> > (b) when the code is in an executable, that gets linked WITHOUT
> > '-lpthread' and that does not use dlopen()?
>
> Yes, it will crash or behave incorrectly on most architectures *if*
> pthread_mutexattr_gettype becomes available for some reason.
>
> > (c) when the code is in an executable, that gets linked WITHOUT
> > '-lpthread' and that does a dlopen("libpthread.so.X")?
>
> This will probably work because pthread_mutexattr_gettype is not rebound
> to the definition.
So, in the normal cases (link with '-lpthread', link without '-lpthread',
and even with dlopen()), everything will work fine. The only problematic
case thus is the the use of LD_PRELOAD. Right?
I think few packages in a distro will be affected. And few users are
using LD_PRELOAD on their own, because since the time when glibc
started to use 'internal' calls to system calls where possible, there
are not a lot of uses of LD_PRELOAD that still work.
> > Under which conditions will it crash?
> >
> > ($) when the executable was built before glibc 2.34 and is run
> > with glibc 2.34 ?
>
> Yes.
>
> > (%) when the executable is built against glibc 2.34 and is run
> > with glibc 2.34 ?
>
> No. glibc 2.34 will behave as if an implicit -lpthread is present on
> the linker program line.
Good. This means a bullet-proof way for a distro to avoid the problem
is to "rebuild the world" after importing glibc 2.34.
> > And if it crashes, will setting the environment variable LD_DYNAMIC_WEAK [1]
> > avoid the crash?
>
> No, it's unrelated. The crash or other undefined behavior is a
> consequence of actions of the link editor and cannot be reverted at run
> time.
In other words, the problem is that
- there are some/many binaries out there, that were produced by an 'ld'
that did not anticipate the changes in glibc 2.34, and
- these binaries have a problem not when run directly, but only when
run with LD_PRELOAD.
Right?
Bruno
next prev parent reply other threads:[~2021-04-28 14:40 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 5:53 Undefined use of weak symbols in gnulib Florian Weimer via Libc-alpha
2021-04-27 6:50 ` Paul Eggert
2021-04-27 6:58 ` Florian Weimer via Libc-alpha
2021-04-27 7:13 ` Paul Eggert
2021-04-27 7:24 ` Andreas Schwab
2021-04-27 11:06 ` Florian Weimer via Libc-alpha
2021-04-28 0:09 ` Bruno Haible
2021-04-28 2:10 ` H.J. Lu via Libc-alpha
2021-04-28 2:13 ` H.J. Lu via Libc-alpha
2021-05-05 20:31 ` Fangrui Song
2021-04-28 8:35 ` Florian Weimer via Libc-alpha
2021-04-28 13:15 ` Michael Matz
2021-04-28 7:44 ` Florian Weimer via Libc-alpha
2021-04-28 14:48 ` Bruno Haible
2021-04-28 17:44 ` Florian Weimer via Libc-alpha
2021-07-17 14:38 ` Bruno Haible
2021-07-17 14:55 ` Florian Weimer via Libc-alpha
2021-07-17 16:39 ` Bruno Haible
2021-07-27 20:02 ` Joseph Myers
2021-07-27 20:19 ` Florian Weimer via Libc-alpha
2021-07-27 23:38 ` Paul Eggert
2021-04-27 23:22 ` Bruno Haible
2021-04-27 23:47 ` Bruno Haible
2021-04-28 7:57 ` Florian Weimer via Libc-alpha
2021-04-28 14:40 ` Bruno Haible [this message]
2021-04-28 17:43 ` Florian Weimer via Libc-alpha
2021-04-29 15:15 ` Bruno Haible
2021-04-30 9:55 ` Florian Weimer via Libc-alpha
2021-04-29 6:33 ` Ben Pfaff via Libc-alpha
2021-05-03 1:44 ` Alan Modra via Libc-alpha
2021-07-12 10:04 ` Michael Hudson-Doyle via Libc-alpha
2021-07-12 15:03 ` Florian Weimer via Libc-alpha
2021-07-12 15:30 ` Matthias Klose
2021-07-12 15:37 ` Florian Weimer via Libc-alpha
2021-07-13 0:22 ` Michael Hudson-Doyle via Libc-alpha
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
List information: https://www.gnu.org/software/libc/involved.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=2800926.834q8TerIH@omega \
--to=bruno@clisp.org \
--cc=binutils@sourceware.org \
--cc=bug-gnulib@gnu.org \
--cc=fweimer@redhat.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).