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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,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 8619CC433DB for ; Thu, 28 Jan 2021 10:43:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4147C64DD8 for ; Thu, 28 Jan 2021 10:43:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231296AbhA1Knx (ORCPT ); Thu, 28 Jan 2021 05:43:53 -0500 Received: from mx2.suse.de ([195.135.220.15]:32776 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231231AbhA1Knu (ORCPT ); Thu, 28 Jan 2021 05:43:50 -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 F3195AC45; Thu, 28 Jan 2021 10:43:07 +0000 (UTC) Subject: Re: [PATCH] perf tools: Resolve symbols against debug file first To: Jiri Olsa Cc: acme@kernel.org, linux-kernel@vger.kernel.org, Peter Zijlstra , Ingo Molnar , Mark Rutland , Alexander Shishkin , Namhyung Kim References: <20210113080128.10286-1-jslaby@suse.cz> <20210113104618.GB1331835@krava> From: Jiri Slaby Message-ID: Date: Thu, 28 Jan 2021 11:43:07 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <20210113104618.GB1331835@krava> Content-Type: text/plain; charset=iso-8859-2; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13. 01. 21, 11:46, Jiri Olsa wrote: > On Wed, Jan 13, 2021 at 09:01:28AM +0100, Jiri Slaby wrote: >> With LTO, there are symbols like these: >> /usr/lib/debug/usr/lib64/libantlr4-runtime.so.4.8-4.8-1.4.x86_64.debug >> 10305: 0000000000955fa4 0 NOTYPE LOCAL DEFAULT 29 Predicate.cpp.2bc410e7 >> >> This comes from a runtime/debug split done by the standard way: >> objcopy --only-keep-debug $runtime $debug >> objcopy --add-gnu-debuglink=$debugfn -R .comment -R .GCC.command.line --strip-all $runtime >> >> perf currently cannot resolve such symbols (relicts of LTO), as section >> 29 exists only in the debug file (29 is .debug_info). And perf resolves >> symbols only against runtime file. This results in all symbols from such >> a library being unresolved: >> 0.38% main2 libantlr4-runtime.so.4.8 [.] 0x00000000000671e0 >> >> So try resolving against the debug file first. And only if it fails (the >> section has NOBITS set), try runtime file. We can do this, as "objcopy >> --only-keep-debug" per documentation preserves all sections, but clears >> data of some of them (the runtime ones) and marks them as NOBITS. >> >> The correct result is now: >> 0.38% main2 libantlr4-runtime.so.4.8 [.] antlr4::IntStream::~IntStream >> >> Note that these LTO symbols are properly skipped anyway as they belong >> neither to *text* nor to *data* (is_label && !elf_sec__filter(&shdr, >> secstrs) is true). >> >> Signed-off-by: Jiri Slaby >> Cc: Peter Zijlstra >> Cc: Ingo Molnar >> Cc: Arnaldo Carvalho de Melo >> Cc: Mark Rutland >> Cc: Alexander Shishkin >> Cc: Jiri Olsa >> Cc: Namhyung Kim >> --- >> tools/perf/util/symbol-elf.c | 10 +++++++++- >> 1 file changed, 9 insertions(+), 1 deletion(-) >> >> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c >> index f3577f7d72fe..a31b716fa61c 100644 >> --- a/tools/perf/util/symbol-elf.c >> +++ b/tools/perf/util/symbol-elf.c >> @@ -1226,12 +1226,20 @@ int dso__load_sym(struct dso *dso, struct map *map, struct symsrc *syms_ss, >> if (sym.st_shndx == SHN_ABS) >> continue; >> >> - sec = elf_getscn(runtime_ss->elf, sym.st_shndx); >> + sec = elf_getscn(syms_ss->elf, sym.st_shndx); >> if (!sec) >> goto out_elf_end; > > we iterate symbols from syms_ss, so the fix seems to be correct > to call elf_getscn on syms_ss, not on runtime_ss as we do now > > I'd think this worked only when runtime_ss == syms_ss > >> >> gelf_getshdr(sec, &shdr); >> >> + if (shdr.sh_type == SHT_NOBITS) { >> + sec = elf_getscn(runtime_ss->elf, sym.st_shndx); >> + if (!sec) >> + goto out_elf_end; >> + >> + gelf_getshdr(sec, &shdr); >> + } > > is that fallback necessary? the symbol is from syms_ss To resume this and answer: Yes, the fallback is necessary. It's because syms_ss section header has NOBITS set for the sections, so file offset is not incremented. So shdr.sh_offset (the file offset) used further in dso__load_sym has different values for syms and runtime. The syms_ss (the NOBITS) one is invalid as it has 0x1000 here. The runtime one contains good values (like 000509d0 here): .text 00082560 00000000000509d0 00000000000509d0 [-00001000-] {+000509d0+} 2**4 That is, without the fallback, the computed symbol address is wrong. thanks, -- js suse labs