Re: Re: [PATCH] [BUGFIX] perf-probe: Fix to fall back to find probe point in symbols

[Masami Hiramatsu]

(2014/12/31 17:02), Namhyung Kim wrote:
> Hi Masami,
> 
> On Wed, Dec 31, 2014 at 03:27:47PM +0900, Masami Hiramatsu wrote:
>> Fix to fall back to find a probe point in symbols if perf fails
>> to find it in debuginfo.
>>
>> This can happen when the target function is an alias of another
>> function. Such alias doesn't have an entry in debuginfo but in
>> symbols.
>>
>> David Ahern reported this problem in https://lkml.org/lkml/2014/12/29/355
>>
>> I ensured the problem and deeper investigation discovers it.
>>  -----
>>  eu-readelf --debug-dump=info /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.19.so | grep \"malloc\" -A6
>>              name                 (strp) "malloc"
>>              decl_file            (data1) 25
>>              decl_line            (data2) 466
>>              prototyped           (flag_present)
>>              type                 (ref4) [  81b5]
>>              declaration          (flag_present)
>>  [  8f58]      formal_parameter
>>  --
>>              name                 (strp) "malloc"
>>              decl_file            (data1) 23
>>              decl_line            (data2) 466
>>              prototyped           (flag_present)
>>              type                 (ref4) [  9f4a]
>>              declaration          (flag_present)
>>              sibling              (ref4) [  bb29]
>>  ...
>>  -----
>> All these entires have no instances (all of them are declarations)
>> This is why the perf probe failed to find it in debuginfo.
>>
>> However, there are some malloc instances in symbols.
>>  -----
>>  eu-readelf --symbols /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.19.so | grep malloc$
>>   1181: 0000000000080700   5332 FUNC    LOCAL  DEFAULT       12 _int_malloc
>>   4537: 00000000000831d0    339 FUNC    LOCAL  DEFAULT       12 __GI___libc_malloc
>>   5545: 00000000000831d0    339 FUNC    LOCAL  DEFAULT       12 __malloc
>>   6063: 00000000000831d0    339 FUNC    GLOBAL DEFAULT       12 malloc
>>   7302: 00000000000831d0    339 FUNC    GLOBAL DEFAULT       12 __libc_malloc
>>  -----
>> As you an see, malloc and __libc_malloc have same address, and actually
>> __libc_malloc has an entry in debuginfo. So you can set up a probe on
>> __libc_malloc.
>>
>> To fix this problem shortly, perf probe simply falls back to find probe
>> point(malloc) in symbols if it is not found in debuginfo.
> 
> Idealy, it'd be great if it can go to the definition.  I think perf
> only keeps a single symbol at a same address so if there're multiple
> aliases at a specific address it might be missed even with this patch.

Hmm, anyway, that should be fixed by improving perf's map, instead of
introducing new symbol finder.

> But that will be a rare case, so
> 
> Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Thank you!

> 
> Thanks,
> Namhyung
> 
> 
>>
>> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
>> Reported-by: David Ahern <dsahern@xxxxxxxxx>
>> ---
>>  tools/perf/util/probe-event.c |    6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
>> index 28eb141..7f9b863 100644
>> --- a/tools/perf/util/probe-event.c
>> +++ b/tools/perf/util/probe-event.c
>> @@ -495,9 +495,11 @@ static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
>>  	}
>>  
>>  	if (ntevs == 0)	{	/* No error but failed to find probe point. */
>> -		pr_warning("Probe point '%s' not found.\n",
>> +		pr_warning("Probe point '%s' not found in debuginfo.\n",
>>  			   synthesize_perf_probe_point(&pev->point));
>> -		return -ENOENT;
>> +		if (need_dwarf)
>> +			return -ENOENT;
>> +		return 0;
>>  	}
>>  	/* Error path : ntevs < 0 */
>>  	pr_debug("An error occurred in debuginfo analysis (%d).\n", ntevs);
>>
>> --
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> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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