From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8BBFFC433C1 for ; Mon, 29 Mar 2021 09:10:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5ABC66187E for ; Mon, 29 Mar 2021 09:10:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232405AbhC2JJx (ORCPT ); Mon, 29 Mar 2021 05:09:53 -0400 Received: from mx2.suse.de ([195.135.220.15]:35316 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231707AbhC2JJR (ORCPT ); Mon, 29 Mar 2021 05:09:17 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id E5DE2B3C6; Mon, 29 Mar 2021 09:09:15 +0000 (UTC) Subject: Re: perf does not resolve plt symbols from libstdc++ right (.plt.sec problem) From: Jiri Slaby To: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , linux-kernel@vger.kernel.org, Richard Guenther , "H.J. Lu" References: Message-ID: <68eb3576-66dd-9155-aae2-b3cde1ebcbdf@suse.cz> Date: Mon, 29 Mar 2021 11:09:15 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=iso-8859-2; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Any ideas on this? On 11. 01. 21, 7:31, Jiri Slaby wrote: > Hi, > > this e-mails is a follow-up of my report at: > https://bugzilla.suse.com/show_bug.cgi?id=1180681 > > There is a problem with *@plt symbols in some libraries, they are > unresolved by perf (memcmp@plt in this case): > >     0.26%  main2    /usr/lib64/libstdc++.so.6.0.28            0xa51a0 >            l [.] 0x00000000000a51a0 > > On the other hand, plt symbols in other libraries are fine (memset@plt > in this case): > >     0.17%  main2    /usr/lib64/libantlr4-runtime.so.4.8       0x4ed10 >            l [.] memset@plt > > I dumped memcmp's .plt.rela entries in perf: > /usr/lib64/libantlr4-runtime.so.4.8: 154th addr=4e9d0 plt_off=4e020 > hdr=10 entry=10 > /usr/lib64/libstdc++.so.6.0.28: 772th addr=a1070 plt_off=9e020 hdr=10 > entry=10 > > The difference (offset) of stdc++'s memcmp is 0xa51a0 (correct) - > 0xa1070 (perf's computed) = 0x4130. > > The problem is perf assumes nth entry of .plt.rela to correspond to nth > function in .plt, but memcmp is in .plt.sec in libstdc++.so: > > > Relocation section '.rela.plt' at offset 0x97900 contains 1018 entries: > >     Offset             Info             Type               Symbol's > Value  Symbol's Name + Addend > > ... > > 00000000001dc838  0000007800000007 R_X86_64_JUMP_SLOT > 0000000000000000 memcmp@GLIBC_2.2.5 + 0 > > Perf does this with the rela entries: > https://github.com/torvalds/linux/blob/f5e6c330254ae691f6d7befe61c786eb5056007e/tools/perf/util/symbol-elf.c#L385 > > > It takes a symbol index from sym.r_info. Then it resolves its name from > .dynsym, appending "@plt" to it. Then this name is added to perf's > symbol table along with address which is computed as .rela.plt index > multiplied by entry size (shdr_plt.sh_entsize) plus plt header > (shdr_plt.sh_entsize on x86_64 too). > > And from this comes (almost) the offset above: > > $ objdump -h /usr/lib64/libstdc++.so.6|grep -E ' .plt(\.sec)? ' > >  12 .plt          00003fb0  000000000009e020  000000000009e020 > 0009e020  2**4 > >  14 .plt.sec      00003fa0  00000000000a2160  00000000000a2160 > 000a2160  2**4 > > 0xa2160-0x9e020 = 0x4140. I assume the 0x10 difference is that perf adds > shdr_plt.sh_entsize (0x10) to the offset to skip the first .plt entry > (header). > > Richard writes: > ====== > .plt.sec is IIRC the "second" (sec) PLT entry - the one that will be > used on the second call (and on).  This is used / emitted for ELF object > instrumented for Intel CET.  The details escape me for the moment but I > hope the x86 ABI documents this (and the constraints) in detail. > ====== > > How should perf find out whether to consider .plt or .plt.sec? Or > generally, how to properly find an address of *@plt symbols like > memcmp@plt above? > > thanks, -- js suse labs