linux-kernel - Re: [RFC/PATCH 2/2] perf probe: Allow weak symbols to be probed

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Date:	Fri, 06 Mar 2015 15:36:14 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/PATCH 2/2] perf probe: Allow weak symbols to be probed

(2015/03/06 15:15), Namhyung Kim wrote:
> Hi Masami,
> 
> On Thu, Mar 05, 2015 at 12:57:21AM +0900, Masami Hiramatsu wrote:
>> (2015/03/04 22:52), Namhyung Kim wrote:
>>> It currently prevents adding probes in weak symbols.  But there're cases
>>> that given name is an only weak symbol so that we cannot add probe.
>>>
>>>   $ perf probe -x /usr/lib/libc.so.6 -a calloc
>>>   Failed to find symbol calloc in /usr/lib/libc-2.21.so
>>>     Error: Failed to add events.
>>>
>>>   $ nm /usr/lib/libc.so.6 | grep calloc
>>>   000000000007b1f0 t __calloc
>>>   000000000007b1f0 T __libc_calloc
>>>   000000000007b1f0 W calloc
>>>
>>> This change will result in duplicate probes when strong and weak symbols
>>> co-exist in a binary.  But I think it's not a big problem since probes
>>> at the weak symbol will never be hit anyway.
>>
>> Hmm, even on my previous series, I got an error with calloc and waitpid.
>>
>> $ ./perf probe -x /usr/lib64/libc-2.17.so -vvV calloc
>> probe-definition(0): calloc
>> symbol:calloc file:(null) line:0 offset:0 return:0 lazy:(null)
>> 0 arguments
>> Open Debuginfo file: /usr/lib/debug/usr/lib64/libc-2.17.so.debug
>> Searching variables at calloc
>> Failed to find the address of calloc
>>   Error: Failed to show vars. Reason: No such file or directory (Code: -2)
>>
>> However, it seems that calloc is loaded as a symbol.
>>
>> $ ./perf probe -x /usr/lib64/libc-2.17.so -vvvvV calloc
>> ...
>> symbol__new: __xstat64 0xe7340-0xe7385
>> symbol__new: calloc 0x80a90-0x80d2a
>> symbol__new: msgget 0xf7940-0xf7961
>> ...
>>
>> FYI, without these patches, I see the same result (calloc is loaded)
> 
> I'm bit confused with the English ;-).  So you mean that now you *can*
> probe calloc and waitpid with this patch, right?

Ah, sorry for confusing you. I meant that I couldn't probe it
even with your patch. I'm not sure why, since the calloc symbol
is created as above message, but can't find in the map...

Thank you,

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


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