[PATCH] selftests/cgroup: Fix cg_read_strcmp() empty string comparison

[Hongfu Li <lihongfu@kylinos.cn>]

cg_read_strcmp() allocated a buffer sized to strlen(expected) + 1,
then passed it to read_text() which calls read(fd, buf, size-1).

When comparing against an empty string (""), strlen("") = 0 gives a
1-byte buffer, and read() is asked to read 0 bytes.  The file content
is never actually read, so strcmp("", buf) always returns 0 regardless
of the real content.  This caused cg_test_proc_killed() to always
report the cgroup as empty immediately, making OOM tests pass without
verifying that processes were killed.

Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
---
 tools/testing/selftests/cgroup/lib/cgroup_util.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/lib/cgroup_util.c b/tools/testing/selftests/cgroup/lib/cgroup_util.c
index 6a7295347e90..42f54936f4bb 100644
--- a/tools/testing/selftests/cgroup/lib/cgroup_util.c
+++ b/tools/testing/selftests/cgroup/lib/cgroup_util.c
@@ -106,8 +106,9 @@ int cg_read_strcmp(const char *cgroup, const char *control,
 	/* Handle the case of comparing against empty string */
 	if (!expected)
 		return -1;
-	else
-		size = strlen(expected) + 1;
+
+	/* needs size > 1, otherwise cg_read() reads 0 bytes */
+	size = (expected[0] == '\0') ? 2 : strlen(expected) + 1;
 
 	buf = malloc(size);
 	if (!buf)
-- 
2.25.1