Re: [PATCH] [BUGFIX] perf-probe: Fix to fall back to find probe point in symbols

[Namhyung Kim <namhyung@kernel.org>]

Hi Masami,

On Wed, Dec 31, 2014 at 03:27:47PM +0900, Masami Hiramatsu wrote:
> Fix to fall back to find a probe point in symbols if perf fails
> to find it in debuginfo.
> 
> This can happen when the target function is an alias of another
> function. Such alias doesn't have an entry in debuginfo but in
> symbols.
> 
> David Ahern reported this problem in https://lkml.org/lkml/2014/12/29/355
> 
> I ensured the problem and deeper investigation discovers it.
>  -----
>  eu-readelf --debug-dump=info /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.19.so | grep \"malloc\" -A6
>              name                 (strp) "malloc"
>              decl_file            (data1) 25
>              decl_line            (data2) 466
>              prototyped           (flag_present)
>              type                 (ref4) [  81b5]
>              declaration          (flag_present)
>  [  8f58]      formal_parameter
>  --
>              name                 (strp) "malloc"
>              decl_file            (data1) 23
>              decl_line            (data2) 466
>              prototyped           (flag_present)
>              type                 (ref4) [  9f4a]
>              declaration          (flag_present)
>              sibling              (ref4) [  bb29]
>  ...
>  -----
> All these entires have no instances (all of them are declarations)
> This is why the perf probe failed to find it in debuginfo.
> 
> However, there are some malloc instances in symbols.
>  -----
>  eu-readelf --symbols /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.19.so | grep malloc$
>   1181: 0000000000080700   5332 FUNC    LOCAL  DEFAULT       12 _int_malloc
>   4537: 00000000000831d0    339 FUNC    LOCAL  DEFAULT       12 __GI___libc_malloc
>   5545: 00000000000831d0    339 FUNC    LOCAL  DEFAULT       12 __malloc
>   6063: 00000000000831d0    339 FUNC    GLOBAL DEFAULT       12 malloc
>   7302: 00000000000831d0    339 FUNC    GLOBAL DEFAULT       12 __libc_malloc
>  -----
> As you an see, malloc and __libc_malloc have same address, and actually
> __libc_malloc has an entry in debuginfo. So you can set up a probe on
> __libc_malloc.
> 
> To fix this problem shortly, perf probe simply falls back to find probe
> point(malloc) in symbols if it is not found in debuginfo.

Idealy, it'd be great if it can go to the definition.  I think perf
only keeps a single symbol at a same address so if there're multiple
aliases at a specific address it might be missed even with this patch.

But that will be a rare case, so

Acked-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung


> 
> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> Reported-by: David Ahern <dsahern@gmail.com>
> ---
>  tools/perf/util/probe-event.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index 28eb141..7f9b863 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -495,9 +495,11 @@ static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
>  	}
>  
>  	if (ntevs == 0)	{	/* No error but failed to find probe point. */
> -		pr_warning("Probe point '%s' not found.\n",
> +		pr_warning("Probe point '%s' not found in debuginfo.\n",
>  			   synthesize_perf_probe_point(&pev->point));
> -		return -ENOENT;
> +		if (need_dwarf)
> +			return -ENOENT;
> +		return 0;
>  	}
>  	/* Error path : ntevs < 0 */
>  	pr_debug("An error occurred in debuginfo analysis (%d).\n", ntevs);
> 
> --
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