linux-kernel - Re: [PATCH 08/41] perf tools: Fix missing label symbols

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Date:	Tue, 15 Jul 2014 15:11:56 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 08/41] perf tools: Fix missing label symbols

On 07/14/2014 11:38 PM, Arnaldo Carvalho de Melo wrote:
> Em Mon, Jul 14, 2014 at 01:02:32PM +0300, Adrian Hunter escreveu:
>> label symbols are missing because elf_sec__is_a() fails
>> to find the section because the section strings do not
>> match the section headers because the sections headers
>> are from the 'runtime' object and the sections strings
>> are from the 'symbol source' object.
>>
>> Fix by getting the section strings from the 'runtime'
>> object so that they match the section headers.
> 
> Would be great to have a 'perf test' entry that fail when looking for
> those 'label symbols', i.e. looking for one such symbol in vmlinux that
> we know is always there, in some written in stone area of the kernel,
> where this label vanishing is unlikely to happen.
> 
> Can you elaborate on the case that fails? So that I can test it here,
> and then after the fix, check that it indeed fixes the problem? I may
> well write that test myself and insert it in my branch just before this
> one.

It fails with "_start" which is a label in /lib/x86_64-linux-gnu/ld-2.17.so.
It is where the program starts so it is not a function, and it is the first
user space address of an exec'ed process.

If you do:

	perf record --per-thread -T -e branches:u -c1 -d -- uname

Then, assuming the old BTS driver doesn't lose the data, it should be
pretty much the first branch.

	pref script | head

> 
> - Arnaldo
>  
>> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
>> ---
>>  tools/perf/util/symbol-elf.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
>> index 6864661..8c96817 100644
>> --- a/tools/perf/util/symbol-elf.c
>> +++ b/tools/perf/util/symbol-elf.c
>> @@ -732,7 +732,7 @@ int dso__load_sym(struct dso *dso, struct map *map,
>>  	if (symstrs == NULL)
>>  		goto out_elf_end;
>>  
>> -	sec_strndx = elf_getscn(elf, ehdr.e_shstrndx);
>> +	sec_strndx = elf_getscn(runtime_ss->elf, runtime_ss->ehdr.e_shstrndx);
>>  	if (sec_strndx == NULL)
>>  		goto out_elf_end;
>>  
>> -- 
>> 1.8.3.2
> 
> 

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