From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by sourceware.org (Postfix) with ESMTP id 63F08383580D for ; Tue, 27 Apr 2021 05:52:55 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 63F08383580D Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-286-w6_SWtqYP2206xm9jIGC6g-1; Tue, 27 Apr 2021 01:52:51 -0400 X-MC-Unique: w6_SWtqYP2206xm9jIGC6g-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B433B8189C3; Tue, 27 Apr 2021 05:52:50 +0000 (UTC) Received: from oldenburg.str.redhat.com (ovpn-113-20.ams2.redhat.com [10.36.113.20]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8E11760636; Tue, 27 Apr 2021 05:52:49 +0000 (UTC) From: Florian Weimer 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 Message-ID: <87o8e0p92r.fsf@oldenburg.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-6.6 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=unavailable autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 27 Apr 2021 05:52:56 -0000 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 !=3D 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. */ |=20 | # 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 | [=E2=80=A6] And: | # if !PTHREAD_IN_USE_DETECTION_HARD | # pragma weak pthread_mutexattr_gettype | # define pthread_in_use() \ | (pthread_mutexattr_gettype !=3D 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 !=3D 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. Thanks, Florian