From: Florian Weimer via Libc-alpha <libc-alpha@sourceware.org>
To: bug-gnulib@gnu.org
Cc: libc-alpha@sourceware.org, binutils@sourceware.org
Subject: Undefined use of weak symbols in gnulib
Date: Tue, 27 Apr 2021 07:53:16 +0200 [thread overview]
Message-ID: <87o8e0p92r.fsf@oldenburg.str.redhat.com> (raw)
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
| […]
And:
| # if !PTHREAD_IN_USE_DETECTION_HARD
| # 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) : 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
<https://sourceware.org/legacy-ml/gnu-gabi/2016-q1/msg00004.html>
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.
Thanks,
Florian
next reply other threads:[~2021-04-27 5:53 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 5:53 Florian Weimer via Libc-alpha [this message]
2021-04-27 6:50 ` Undefined use of weak symbols in gnulib 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
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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