linux-kernel - Re: [PATCH 3/4] perf probe: Fix probing kretprobes

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Date:	Mon, 12 Jan 2015 21:22:13 +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>,
	David Ahern <dsahern@...il.com>
Subject: Re: [PATCH 3/4] perf probe: Fix probing kretprobes

(2015/01/10 19:33), Namhyung Kim wrote:
> The commit dfef99cd0b2c ("perf probe: Use ref_reloc_sym based address
> instead of the symbol name") converts kprobes to use ref_reloc_sym
> (i.e. _stext) and offset instead of using symbol's name directly.  So
> on my system, adding do_fork ends up with like below:
> 
>   $ sudo perf probe -v --add do_fork%return
>   probe-definition(0): do_fork%return
>   symbol:do_fork file:(null) line:0 offset:0 return:1 lazy:(null)
>   0 arguments
>   Looking at the vmlinux_path (7 entries long)
>   Using /lib/modules/3.17.6-1-ARCH/build/vmlinux for symbols
>   Could not open debuginfo. Try to use symbols.
>   Opening /sys/kernel/debug/tracing/kprobe_events write=1
>   Added new event:
>   Writing event: r:probe/do_fork _stext+456136
>   Failed to write event: Invalid argument
>   Error: Failed to add events. Reason: Operation not permitted (Code: -1)
> 
> As you can see, the do_fork was translated to _stext+456136.  This was
> because to support (local) symbols that have same name.  But the
> problem is that kretprobe requires to be inserted at function start
> point so it simply checks whether it's called with offset 0.  And if
> not, it'll return with -EINVAL.  You can see it with dmesg.
> 
>   $ dmesg | tail -1
>     [125621.764103] Return probe must be used without offset.
> 
> So we need to use the symbol name instead of ref_reloc_sym in case of
> return probes.

Thanks, but as I pointed in previous mail, this patch fixes just one
code path which doesn't use debuginfo. With debuginfo, perf-probe
executes try_to_find_probe_trace_events() to analyze debuginfo.
We can just skip calling it for retprobes, but I prefer to keep
it works. So just skipping conversion in post_process_probe_trace_events()
is better.

Thank you,

> 
> Reported-by: Jiri Olsa <jolsa@...hat.com>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> ---
>  tools/perf/util/probe-event.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index e5af16988791..6fe5aa357efc 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -2321,7 +2321,7 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
>  		goto out;
>  	}
>  
> -	if (!pev->uprobes) {
> +	if (!pev->uprobes && !pp->retprobe) {
>  		kmap = map__kmap(map);
>  		reloc_sym = kmap->ref_reloc_sym;
>  		if (!reloc_sym) {
> 


-- 
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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