[PATCH perf/core 4/5] [BUGFIX] perf-probe: Fix a segfault bug in debuginfo_cache

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

perf probe --list will get a segfault if the first kprobe
event is on a module and the second or latter one on
the kernel.

e.g.
  ----
  # ./perf probe -q -m pcspkr pcspkr_event
  # ./perf probe -q vfs_read
  # ./perf probe -l
  Segmentation fault (core dumped)
  ----

This is because the debuginfo_cache fails to handle NULL
module name, which causes segfault on strcmp. (Note that
strcmp("something", NULL) always causes segfault)

To fix this debuginfo_cache__open always translates the
NULL module name to "kernel" (this is correct, because
NULL module name means opening the debuginfo for the kernel)

  ----
  # ./perf probe -l
    probe:pcspkr_event   (on pcspkr_event@drivers/input/misc/pcspkr.c
    in pcspkr)
    probe:vfs_read       (on vfs_read@ksrc/linux-3/fs/read_write.c)
  ----

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
---
 tools/perf/util/probe-event.c |   21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 57a8cad..bb7766ea 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -440,19 +440,22 @@ static char *debuginfo_cache_path;
 
 static struct debuginfo *debuginfo_cache__open(const char *module, bool silent)
 {
-	if ((debuginfo_cache_path && !strcmp(debuginfo_cache_path, module)) ||
-	    (!debuginfo_cache_path && !module && debuginfo_cache))
+	const char *path = module;
+
+	/* If the module is NULL, it should be the kernel. */
+	if (!module)
+		path = "kernel";
+
+	if (debuginfo_cache_path && !strcmp(debuginfo_cache_path, path))
 		goto out;
 
 	/* Copy module path */
 	free(debuginfo_cache_path);
-	if (module) {
-		debuginfo_cache_path = strdup(module);
-		if (!debuginfo_cache_path) {
-			debuginfo__delete(debuginfo_cache);
-			debuginfo_cache = NULL;
-			goto out;
-		}
+	debuginfo_cache_path = strdup(path);
+	if (!debuginfo_cache_path) {
+		debuginfo__delete(debuginfo_cache);
+		debuginfo_cache = NULL;
+		goto out;
 	}
 
 	debuginfo_cache = open_debuginfo(module, silent);