cgroup: disallow rename(2) if sane_behavior

cgroup's rename(2) isn't a proper migration implementation - it can't
move the cgroup to a different parent in the hierarchy.  All it can do
is swapping the name string for that cgroup.  This isn't useful and
can mislead users to think that cgroup supports proper cgroup-level
migration.  Disallow rename(2) if sane_behavior.

v2: Fail with -EPERM instead of -EINVAL so that it matches the vfs
    return value when ->rename is not implemented as suggested by Li.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -270,6 +270,8 @@ enum {
 	 * - "release_agent" and "notify_on_release" are removed.
 	 *   Replacement notification mechanism will be implemented.
 	 *
+	 * - rename(2) is disallowed.
+	 *
 	 * - memcg: use_hierarchy is on by default and the cgroup file for
 	 *   the flag is not created.
 	 */

kernel/cgroup.c

@@ -2508,6 +2508,13 @@ static int cgroup_rename(struct inode *old_dir, struct dentry *old_dentry,
 
 	cgrp = __d_cgrp(old_dentry);
 
+	/*
+	 * This isn't a proper migration and its usefulness is very
+	 * limited.  Disallow if sane_behavior.
+	 */
+	if (cgroup_sane_behavior(cgrp))
+		return -EPERM;
+
 	name = cgroup_alloc_name(new_dentry);
 	if (!name)
 		return -ENOMEM;