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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 3E1AAC433E0 for ; Mon, 11 Jan 2021 06:32:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0723222581 for ; Mon, 11 Jan 2021 06:32:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727345AbhAKGby (ORCPT ); Mon, 11 Jan 2021 01:31:54 -0500 Received: from mx2.suse.de ([195.135.220.15]:56750 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727130AbhAKGbx (ORCPT ); Mon, 11 Jan 2021 01:31:53 -0500 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 F0864AB3E; Mon, 11 Jan 2021 06:31:11 +0000 (UTC) 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" From: Jiri Slaby Subject: perf does not resolve plt symbols from libstdc++ right (.plt.sec problem) Message-ID: Date: Mon, 11 Jan 2021 07:31:11 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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