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SubjectRe: Re: [PATCH] [BUGFIX] perf-probe: Fix to fall back to find probe point in symbols
(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@kernel.org>

Thank you!

>
> Thanks,
> Namhyung
>
>
>>
>> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
>> Reported-by: David Ahern <dsahern@gmail.com>
>> ---
>> 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@hitachi.com




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