[PATCH v2 2/4] perf-probe: Fix wrong variable warning when the probe point is not found

[Masami Hiramatsu <mhiramat@kernel.org>]

Fix a wrong "variable not found" warning when the probe point is
not found in the debuginfo.
Since the debuginfo__find_probes() can return 0 even if it does not
find given probe point in the debuginfo, fill_empty_trace_arg() can
be called with tf.ntevs == 0 and it can warn a wrong warning.
To fix this, reject ntevs == 0 in fill_empty_trace_arg().

E.g. without this patch;

  # perf probe -x /lib64/libc-2.30.so -a "memcpy arg1=%di"
  Failed to find the location of the '%di' variable at this address.
   Perhaps it has been optimized out.
   Use -V with the --range option to show '%di' location range.
  Added new events:
    probe_libc:memcpy    (on memcpy in /usr/lib64/libc-2.30.so with arg1=%di)
    probe_libc:memcpy    (on memcpy in /usr/lib64/libc-2.30.so with arg1=%di)

  You can now use it in all perf tools, such as:

  	perf record -e probe_libc:memcpy -aR sleep 1

With this;

  # perf probe -x /lib64/libc-2.30.so -a "memcpy arg1=%di"
  Added new events:
    probe_libc:memcpy    (on memcpy in /usr/lib64/libc-2.30.so with arg1=%di)
    probe_libc:memcpy    (on memcpy in /usr/lib64/libc-2.30.so with arg1=%di)

  You can now use it in all perf tools, such as:

  	perf record -e probe_libc:memcpy -aR sleep 1


Reported-by: Andi Kleen <andi@firstfloor.org>
Fixes: cb4027308570 ("perf probe: Trace a magic number if variable is not found")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
 tools/perf/util/probe-finder.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 55924255c535..9963e4e8ea20 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -1408,6 +1408,9 @@ static int fill_empty_trace_arg(struct perf_probe_event *pev,
 	char *type;
 	int i, j, ret;
 
+	if (!ntevs)
+		return -ENOENT;
+
 	for (i = 0; i < pev->nargs; i++) {
 		type = NULL;
 		for (j = 0; j < ntevs; j++) {