cgroup: Handle trailing new line in cgroup.controllers

The last item in cgroup.controllers (misc or rdma in my case)
contained a new line character which caused the controller search to
fail.

This commit avoids including the newline character inside the name
comparison.

The search failure caused the "cgroup_regression_test.sh" test to fail
with a confusing error when it tries to mount a V1 subsys thus
removing the V2 and causing the available set of V2s to change between
scans.

According to the V2 docs subsys names can only include lowercase
characters and '_'. So we strictly look for those characters.

The newline (and delimiting space) is just what the kernel currently
prints. IDK if it is specified anywhere, but if it changes then the
error should be obvious.

Fixes: 310da37 ("Add new CGroups APIs")
Signed-off-by: Richard Palethorpe <[email protected]>
Reviewed-by: Petr Vorel <[email protected]>
Reviewed-by: Marius Kittler <[email protected]>

lib/tst_cgroup.c

@@ -432,9 +432,20 @@ __attribute__ ((nonnull, warn_unused_result))
 static struct cgroup_ctrl *cgroup_find_ctrl(const char *const ctrl_name)
 {
 	struct cgroup_ctrl *ctrl;
+	int l = 0;
+	char c = ctrl_name[l];
+
+	while (c == '_' || (c >= 'a' && c <= 'z'))
+		c = ctrl_name[++l];
+
+	if (l > 32)
+		tst_res(TWARN, "Subsys name len greater than max known value of MAX_CGROUP_TYPE_NAMELEN: %d > 32", l);
+
+	if (!(c == '\n' || c == '\0'))
+		tst_brk(TBROK, "Unexpected char in %s: %c", ctrl_name, c);
 
 	for_each_ctrl(ctrl) {
-		if (!strcmp(ctrl_name, ctrl->ctrl_name))
+		if (!strncmp(ctrl_name, ctrl->ctrl_name, l))
 			return ctrl;
 	}