selftests/cgroup: Fix cg_read_strcmp() empty string comparison

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>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/tools/testing/selftests/cgroup/lib/cgroup_util.c b/tools/testing/selftests/cgroup/lib/cgroup_util.c
index 6a7295347e90b1cc1298aa80aa01a8dbfa21e936..42f54936f4bbd34ac8099d2315688480089042df 100644 (file)
--- 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)