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

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Date:	Fri, 06 Mar 2015 16:05:49 +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.
> 
> 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++;
>  	}

Ah, I've found this is the magic...
Here, we need another fix on my series.

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 22392b06..f9c1e53 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -309,10 +309,8 @@ static int find_alternative_probe_point(struct debuginfo *d

        /* Find the address of given function */
        map__for_each_symbol_by_name(map, pp->function, sym) {
-               if (sym->binding == STB_GLOBAL || sym->binding == STB_LOCAL) {
-                       address = sym->start;
-                       break;
-               }
+               address = sym->start;
+               break;
        }
        if (!address) {
                ret = -ENOENT;
-------

With this fix, I could get variables on waitpid and calloc.
-----
# ./perf probe -x /lib64/libc-2.17.so -V waitpid
Available variables at waitpid
        @<__libc_waitpid+0>
                __pid_t pid
                int     oldtype
                int     options
                int*    stat_loc
-----

I'll update and include it my series.

Thank you!

>  
>  	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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