perf probe: Fix a segfault bug in debuginfo_cache

perf probe --list will get a segfault if the first kprobe event is on a
module and the second or latter one is 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>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20150930164135.3733.23993.stgit@localhost.localdomain
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 12 insertions, 9 deletions

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 65be284823d5..3010abc071ff 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -441,19 +441,22 @@ 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);