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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


  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

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