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

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Message-ID: <20150306061530.GD7872@sejong>
Date:	Fri, 6 Mar 2015 15:15:30 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
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

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?

Thanks,
Namhyung
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