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([2001:8a0:f932:6a00:6b6e:c7b6:c5a7:aac3]) by smtp.gmail.com with ESMTPSA id w22sm9978617wmc.13.2021.04.16.09.47.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Apr 2021 09:47:13 -0700 (PDT) Subject: Re: [PATCH glibc] nptl_db: different libpthread/ld.so load orders (bug 27744) To: Florian Weimer References: <87sg3qnrz3.fsf@oldenburg.str.redhat.com> <73b32cc6-e201-8bac-e442-e3dddcc01e0d@polymtl.ca> <625ec5fe-bd09-860a-f617-745042b94011@redhat.com> <87fszqnqi3.fsf@oldenburg.str.redhat.com> <87blaenprw.fsf@oldenburg.str.redhat.com> Message-ID: Date: Fri, 16 Apr 2021 17:47:12 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1 MIME-Version: 1.0 In-Reply-To: <87blaenprw.fsf@oldenburg.str.redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit 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: Pedro Alves via Libc-alpha Reply-To: Pedro Alves Cc: libc-alpha@sourceware.org, gdb-patches@sourceware.org, Simon Marchi , Kevin Buettner Errors-To: libc-alpha-bounces@sourceware.org Sender: "Libc-alpha" On 16/04/21 17:43, Florian Weimer wrote: > * Pedro Alves: > >> On 16/04/21 17:28, Florian Weimer wrote: >>> * Pedro Alves: >>> >>>> IIRC, the order which libraries are loaded by GDB hasn't changed. The >>>> issue is that until recently (before glibc 1daccf403b1b), the stacks >>>> lists lived in libpthread (stack_used/__stack_user), so the fact that >>>> GDB loaded libthread_db.so before ld.so's symbols were loaded didn't >>>> make a difference. Now they were moved to ld.so, so libthread_db.so >>>> can't find them until GDB reads the ld.so symbols. Is this assessment >>>> correct? >>> >>> Yes, I believe this is what happens. >>> >> >> OK, I believe what is confusing in your commit log was the reference to >> two different kinds of "loaded": >> >> "libthread_db is loaded once GDB encounters libpthread, and at this >> point, ld.so may not have been loaded yet. " >> >> The first loaded is about GDB dlopening libthread_db.so. The second loaded >> refers to reading symbols -- ld.so has been loaded by the inferior already >> at that point. >> >> It would be clearer as: >> >> "libthread_db is loaded once GDB encounters libpthread, and at this >> point, ld.so's symbols may not have been read by GDB yet. " > > I'm going to go with: > > “ > libthread_db is loaded once GDB encounters libpthread, and at this > point, ld.so may not have been processed by GDB yet. > ” Sounds good. > >> If I understood that correctly, then the following sentence is also a >> bit confusing: >> >> "As a result, _rtld_global cannot be accessed by regular means from >> libthread_db." >> >> Because that sounds to me like you were perhaps talking about some >> magic means to reference globals, some magic relocations, or some >> other magic voodoo only understood by glibc experts. > > We use the magic that GDB provides to us (ps_pglobal_lookup, I think). > I thought that this was understood by GDB experts only. 8-) LOL I skimmed the patch, and FWIW, it LGTM. Just spotted a couple typos: > +/* This test runs GDB against a forked copy of itself, to check > + whether libthreaddb can be loaded, and that access to thread-local libthreaddb -> libthread_db > +/* This function implements the subprocess un der test. It creates a "un der" -> "under" Thanks, Pedro Alves