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

From: Namhyung Kim
Date: Wed Dec 31 2014 - 03:01:22 EST


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@xxxxxxxxxx>

Thanks,
Namhyung


>
> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> Reported-by: David Ahern <dsahern@xxxxxxxxx>
> ---
> 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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