[PATCH] perf diff: Don't crash on freeing errno-session

[Dmitry Safonov <dima@arista.com>]

__cmd_diff() sets result of perf_session__new() to d->session.
In case of failure, it's errno and perf-diff may crash with:
failed to open perf.data: Permission denied
Failed to open perf.data
Segmentation fault (core dumped)

From the coredump:
0  0x00005569a62b5955 in auxtrace__free (session=0xffffffffffffffff)
    at util/auxtrace.c:2681
1  0x00005569a626b37d in perf_session__delete (session=0xffffffffffffffff)
    at util/session.c:295
2  perf_session__delete (session=0xffffffffffffffff) at util/session.c:291
3  0x00005569a618008a in __cmd_diff () at builtin-diff.c:1239
4  cmd_diff (argc=<optimized out>, argv=<optimized out>) at builtin-diff.c:2011
[..]

Funny enough, it won't always crash. For me it crashes only if failed
file is second in cmd-line: the reason is that cmd_diff() check files for
branch-stacks [in check_file_brstack()] and if the first file doesn't
have brstacks, it doesn't proceed to try open other files from cmd-line.

Check d->session before calling perf_session__delete().

Another solution would be assigning to temporary variable, checking it,
but I find it easier to follow with IS_ERR() check in the same function.
After some time it's still obvious why the check is needed, and with
temp variable it's possible to make the same mistake.

Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 tools/perf/builtin-diff.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-diff.c b/tools/perf/builtin-diff.c
index cefc71506409..b0c57e55052d 100644
--- a/tools/perf/builtin-diff.c
+++ b/tools/perf/builtin-diff.c
@@ -1236,7 +1236,8 @@ static int __cmd_diff(void)
 
  out_delete:
 	data__for_each_file(i, d) {
-		perf_session__delete(d->session);
+		if (!IS_ERR(d->session))
+			perf_session__delete(d->session);
 		data__free(d);
 	}
 
-- 
2.30.1