[PATCH 5.15 256/667] selftests/resctrl: Fix null pointer dereference on open failed

[Greg Kroah-Hartman]

From: Colin Ian King <colin.i.king@xxxxxxxxx>

[ Upstream commit c7b607fa9325ccc94982774c505176677117689c ]

Currently if opening /dev/null fails to open then file pointer fp
is null and further access to fp via fprintf will cause a null
pointer dereference. Fix this by returning a negative error value
when a null fp is detected.

Detected using cppcheck static analysis:
tools/testing/selftests/resctrl/fill_buf.c:124:6: note: Assuming
that condition '!fp' is not redundant
 if (!fp)
     ^
tools/testing/selftests/resctrl/fill_buf.c:126:10: note: Null
pointer dereference
 fprintf(fp, "Sum: %d ", ret);

Fixes: a2561b12fe39 ("selftests/resctrl: Add built in benchmark")
Signed-off-by: Colin Ian King <colin.i.king@xxxxxxxxx>
Signed-off-by: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 tools/testing/selftests/resctrl/fill_buf.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c
index 51e5cf22632f..56ccbeae0638 100644
--- a/tools/testing/selftests/resctrl/fill_buf.c
+++ b/tools/testing/selftests/resctrl/fill_buf.c
@@ -121,8 +121,10 @@ static int fill_cache_read(unsigned char *start_ptr, unsigned char *end_ptr,
 
 	/* Consume read result so that reading memory is not optimized out. */
 	fp = fopen("/dev/null", "w");
-	if (!fp)
+	if (!fp) {
 		perror("Unable to write to /dev/null");
+		return -1;
+	}
 	fprintf(fp, "Sum: %d ", ret);
 	fclose(fp);
 
-- 
2.35.1