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

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Date:	Thu, 05 Mar 2015 00:57:21 +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

(2015/03/04 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)

Thank you,


> 
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> ---
>  tools/perf/util/probe-event.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index 1c570c2fa7cc..12b7d018106e 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -2339,8 +2339,7 @@ static int find_probe_functions(struct map *map, char *name)
>  	struct symbol *sym;
>  
>  	map__for_each_symbol_by_name(map, name, sym) {
> -		if (sym->binding == STB_GLOBAL || sym->binding == STB_LOCAL)
> -			found++;
> +		found++;
>  	}
>  
>  	return found;
> @@ -2708,8 +2707,7 @@ static struct strfilter *available_func_filter;
>  static int filter_available_functions(struct map *map __maybe_unused,
>  				      struct symbol *sym)
>  {
> -	if ((sym->binding == STB_GLOBAL || sym->binding == STB_LOCAL) &&
> -	    strfilter__compare(available_func_filter, sym->name))
> +	if (strfilter__compare(available_func_filter, sym->name))
>  		return 0;
>  	return 1;
>  }
> 


-- 
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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