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

From: Masami Hiramatsu
Date: Wed Dec 31 2014 - 01:27:54 EST

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

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

To fix this problem shortly, perf probe simply falls back to find probe
point(malloc) in symbols if it is not found in debuginfo.

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",
- 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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