From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 82808 invoked by alias); 15 Apr 2016 16:16:54 -0000 Mailing-List: contact gnu-gabi-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Post: List-Help: List-Subscribe: Sender: gnu-gabi-owner@sourceware.org Received: (qmail 82672 invoked by uid 89); 15 Apr 2016 16:16:53 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Checked: by ClamAV 0.99.1 on sourceware.org X-Virus-Found: No X-Spam-SWARE-Status: No, score=-2.3 required=5.0 tests=AWL,BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_LOW,SPF_PASS autolearn=ham version=3.3.2 spammy=standing X-Spam-Status: No, score=-2.3 required=5.0 tests=AWL,BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_LOW,SPF_PASS autolearn=ham version=3.3.2 X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on sourceware.org X-Spam-Level: X-Spam-User: qpsmtpd, 2 recipients X-HELO: mail-qk0-f179.google.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:date:message-id:subject:from:to:cc; bh=1ePOH9wjHfAIEDacYNWz6TEq873yD7yLuQ1RvR0v+Qc=; b=Ywsspdm6NswqaoHj0K1XI5yUUwGbw5nYu/20LdH3g/5OR9l6l71Sk8OObSHK1lTMNA ovmP8u2Z/XEJgg3xO5fFzOJnGnVFTfGoYcrL0kgBqju12G+42T2vI/7DbsxruqzcmhzF wyegquEU3SkcUv3FmRAHlgZVo1OVWjpkS0IwEiNozQAI7c+yRwl56ER4m/rPceyuBV58 AjfByw5Bfs+ibjsTFv5Rc9cwW0nQFsClq0YbvYvTx6VaN57og1GLKsRFUaWph4vZabKG spWUTD74/VryFvKbKbuHUqtebgEmUdylgjDcFTxxZugSxtCBalm5oGzWzRz6C9bLo+1K uK9A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:date:message-id:subject:from:to:cc; bh=1ePOH9wjHfAIEDacYNWz6TEq873yD7yLuQ1RvR0v+Qc=; b=aulfs4OEhKemi0qcRy8zzJT1+EhzRu4/8sH7221iIYX7HUZinT4q3w7uT2YUQsUxFw 9WJRiT5z566XwZp3yUxSaT1cnxDBcXhSCBaDupdyCZMJWLmItT2biL+WnKtSxxyzuoL+ dLlA61cQCUfK1hXNbE0TdiQExAQsn8pm12LA1q7786ZjON0GuFAZA34pMwU6MepGIlD9 p/0WNt16C8TPSyqKuBLERdO46Rpqgi0LTpxrwACbSqWiKEDHo7V1Zk9KoIcvcG0WJn1v M0IMeIULhgoZNi+XsZ+Mom2hWaAGUhvVokc0uZXlBuy1KPy8GxE3R/d/vZVgIQg8ADis Sl9g== X-Gm-Message-State: AOPr4FXAfpEQ54oqr8hQ2CXxuwHId2y6lsB5ehtfsgWhnbH14ltHuF3RgvPX1yTrzYbGeZ1JFQBL2lSxB3E3sg== MIME-Version: 1.0 X-Received: by 10.55.71.76 with SMTP id u73mr26769035qka.6.1460736999803; Fri, 15 Apr 2016 09:16:39 -0700 (PDT) Date: Fri, 01 Jan 2016 00:00:00 -0000 Message-ID: Subject: Re: Preventing preemption of 'protected' symbols in GNU ld 2.26 From: "H.J. Lu" To: Szabolcs Nagy , gnu-gabi@sourceware.org Cc: Ramana Radhakrishnan , Jeff Law , Cary Coutant , Joe Groff , Alan Modra , Binutils , nd Content-Type: text/plain; charset=UTF-8 X-SW-Source: 2016-q2/txt/msg00001.txt.bz2 On Fri, Apr 15, 2016 at 9:09 AM, Szabolcs Nagy wrote: > On 31/03/16 14:26, Ramana Radhakrishnan wrote: >> On Thu, Mar 31, 2016 at 1:52 AM, Jeff Law wrote: >>> On 03/30/2016 06:40 PM, Cary Coutant wrote: >>>>> >>>>> It would help me immensely on the GCC side if things if you and Alan >>>>> could >>>>> easily summarize correct behavior and the impact if we were to just >>>>> revert >>>>> HJ's change. A testcase would be amazingly helpful too. >>>> >>>> >>>> It looks like it's not just the one change. There's this patch: >>>> >>>> https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01871.html >>>> >>>> which took the idea that protected can still be pre-empted by a COPY >>>> relocation and extended it to three more targets that use COPY >>>> relocations. >>>> >>>> I wonder how many other patches have been based on the same >>>> misunderstanding? > > (sorry i missed this thread) > > this was not a misunderstanding. > > that patch is necessary for correctness (odr) in > the presence of copy relocations as described in > https://gcc.gnu.org/ml/gcc-patches/2015-09/msg02365.html > and > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55012 > > this was a long standing code gen bug in gcc and was > about time to fix it (it was also broken in glibc's > dynamic linker, but e.g. not in musl libc). > > (i don't see what is the issue with using the copy in > the main executable from a shared library, performance > is not a correctness issue, nor how it is possible to > avoid the copy relocs.) > Here is my understanding: Copy relocation and protected visibility are fundamentally incompatible. On on hand, copy relocation is the part of the psABI and is used to access global data defined in a shared object from the executable. It moves the definition of global data, which is defined in a share object, to the executable at run-time. On the other hand, protected visibility indicates that a symbol is defined locally in the shared object at run-time. Both can't be true at the same time. The current solution is to make protected symbol more or less like normal symbol, which prevents optimizing local access to protected symbol within the shared object. I propose to add GNU_PROPERTY_NO_COPY_ON_PROTECTED: https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI GNU_PROPERTY_NO_COPY_ON_PROTECTED This indicates that there should be no copy relocations against protected data symbols. If a relocat- able object contains this property, linker should treat protected data symbol as defined locally at run-time and copy this property to the output share object. Linker should add this property to the output share object if any pro- tected symbol is expected to be defined locally at run-time. Run-time loader should disallow copy relocations against protected data symbols defined in share objects with GNU_PROPERTY_NO_COPY_ON_PROTECTED prop- erty. Its PR_DATASZ should be 0. -- H.J.