Re: Undefined use of weak symbols in gnulib

[Florian Weimer via Libc-alpha <libc-alpha@sourceware.org>]

* Bruno Haible:

> 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?

LD_PRELOAD and glibc 2.34 as originally planned.

> 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.

We get the occasional bug report when these things break.  We have not
seen much of that yet because our gnulib-using programs are still at
older versions for most of our users.

Here's an example of such a bug report, although not for libpthread:

  powerpc: libc segfaults when LD_PRELOADed with libgcc
  <https://sourceware.org/bugzilla/show_bug.cgi?id=26615>

I think in this bug, libc.so.6 was invoked during some build process
(which could easily run bison as well, so it has to work with
LD_PRELOAD).

>> 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.

Yeah, but that's not good enough.  So I spent today on coming up with a
workaround in glibc.

>> 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?

No, glibc 2.34 won't need LD_PRELOAD to expose the bug.  LD_PRELOAD is
just a development aid that reveals the problem with glibc 2.33 and
earlier.

Thanks,
Florian