From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS3215 2.6.0.0/16 X-Spam-Status: No, score=-4.2 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from sourceware.org (server2.sourceware.org [IPv6:2620:52:3:1:0:246e:9693:128c]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by dcvr.yhbt.net (Postfix) with ESMTPS id AA56E1F5AE for ; Tue, 27 Apr 2021 05:53:05 +0000 (UTC) Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id 39EB63980415; Tue, 27 Apr 2021 05:52:58 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 39EB63980415 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1619502778; bh=YEcFHdos/RbqzAy58LhMvPjplehsjFYYq5KA3G2dJKA=; h=To:Subject:Date:List-Id:List-Unsubscribe:List-Archive:List-Post: List-Help:List-Subscribe:From:Reply-To:Cc:From; b=O0QTJDLmtaWUc68h7YdHZhWp/DcPZ6rDWy62sZAkZTd63woi8Fg7RdKZINjbeAb8Z czeFHBqUJSX+27lSnK08oN8XdBYpLKXT62OfXFi/NH/wOZtUr4L9q4QSXW3tiyvRUu hOgYFPm8tYcTWLCRgUeG9zKbFZxrXNR69+n2xkF0= 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) To: bug-gnulib@gnu.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-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: , From: Florian Weimer via Libc-alpha Reply-To: Florian Weimer Cc: libc-alpha@sourceware.org, binutils@sourceware.org Errors-To: libc-alpha-bounces@sourceware.org Sender: "Libc-alpha" 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