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

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Date:   Thu, 28 Jan 2021 11:43:07 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     acme@...nel.org, 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>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH] perf tools: Resolve symbols against debug file first

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 <jslaby@...e.cz>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
>> Cc: Jiri Olsa <jolsa@...hat.com>
>> Cc: Namhyung Kim <namhyung@...nel.org>
>> ---
>>   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