linux-kernel - Re: [PATCH] perf tools: Resolve symbols against debug file first

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Message-ID: <87sg73kbsq.fsf@mpe.ellerman.id.au>
Date:   Thu, 14 Jan 2021 22:17:25 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Namhyung Kim <namhyung@...nel.org>, Jiri Slaby <jslaby@...e.cz>
Cc:     Jiri Olsa <jolsa@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] perf tools: Resolve symbols against debug file first

Namhyung Kim <namhyung@...nel.org> writes:
> On Wed, Jan 13, 2021 at 8:43 PM Jiri Slaby <jslaby@...e.cz> wrote:
>>
>> 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
>> >>
...
>> >> 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
>>
>> No, because the headers are copied 1:1 from runtime_ss to syms_ss. And
>> runtime_ss is then stripped, so only .debug* sections are removed there.
>> (And syms_ss's are set as NOBITS.)
>>
>> We iterated .debug* sections in syms_ss and used runtime_ss section
>> _headers_ only to adjust symbols (sometimes). That worked.
>
> It seems PPC has an opd section only in the runtime_ss and that's why
> we use it for section headers.

At least on my system (Ubuntu 20.04.1) I see .opd in the debug file with
NOBITS set:

$ readelf -e vmlinux.debug | grep opd
  [37] .opd              NOBITS           c000000001c1f548  01202e14


But possibly that's not the case with older toolchains?

cheers