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Undefined use of weak symbols in gnulib

From: Florian Weimer
Subject: Undefined use of weak symbols in gnulib
Date: Tue, 27 Apr 2021 07:53:16 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux)

lib/glthread/lock.h has this:

| /* The way to test at runtime whether libpthread is present is to test
|    whether a function pointer's value, such as &pthread_mutex_init, is
|    non-NULL.  However, some versions of GCC have a bug through which, in
|    PIC mode, &foo != NULL always evaluates to true if there is a direct
|    call to foo(...) in the same function.  To avoid this, we test the
|    address of a function in libpthread that we don't use.  */
| #  pragma weak pthread_mutex_init
| #  pragma weak pthread_mutex_lock
| #  pragma weak pthread_mutex_unlock
| #  pragma weak pthread_mutex_destroy
| #  pragma weak pthread_rwlock_init
| #  pragma weak pthread_rwlock_rdlock
| #  pragma weak pthread_rwlock_wrlock
| #  pragma weak pthread_rwlock_unlock
| #  pragma weak pthread_rwlock_destroy
| #  pragma weak pthread_once
| […]


| #   pragma weak pthread_mutexattr_gettype
| #   define pthread_in_use() \
|       (pthread_mutexattr_gettype != NULL || c11_threads_in_use ())
| #  endif

As far as I can tell gnulib uses this macro definition to implement
gl_once on glibc targets:

| #  define glthread_once(ONCE_CONTROL, INITFUNCTION) \
|      (pthread_in_use ()                                                       
|       ? pthread_once (ONCE_CONTROL, INITFUNCTION)                             
|       : (glthread_once_singlethreaded (ONCE_CONTROL) ? (INITFUNCTION (), 0) : 

So the net effect is this:

  if (pthread_mutexattr_gettype != NULL)
    pthread_once (control, callback);

Dynamic linking with weak symbols is not very well-defined.  On x86-64,
the link editor produces the expected dynamic symbol relocation for the
pthread_once call.  On other targets (notably POWER), no dynamic
relocation is produced, and the code will crash if
pthread_mutexattr_gettype is ever defined.

There is an old thread here covering related issues:

  Specify how undefined weak symbol should be resolved in executable

On glibc targets, there is another problem: weak references do not carry
symbol versions, so they can bind to base versions unexpectedly.

This will become an urgent issue with glibc 2.34, which defines
pthread_mutexattr_gettype unconditionally.  Certain gnulib modules will
stop working until the binaries are relinked.  I expect the issue is
already visible with earlier glibc versions if libpthread is
unexpectedly present at run time.

I think we can provide an libBrokenGnulib.so preload module which
defines pthread_mutexattr_gettype to zero (as an absolute address), so
there is a kludge to keep old binaries working, but this is really
something that must be fixed in gnulib.


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