cgroup: update some statememt about delegation

The comment in cgroup_file_write is missing some interfaces, such as
'cgroup.threads'. All delegatable files are listed in
'/sys/kernel/cgroup/delegate', so update the comment in cgroup_file_write.
Besides, add a statement that files outside the namespace shouldn't be
visible from inside the delegated namespace.

tj: Reflowed text for consistency.
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/admin-guide/cgroup-v2.rst

@@ -533,10 +533,12 @@ cgroup namespace on namespace creation.
 Because the resource control interface files in a given directory
 control the distribution of the parent's resources, the delegatee
 shouldn't be allowed to write to them.  For the first method, this is
-achieved by not granting access to these files.  For the second, the
-kernel rejects writes to all files other than "cgroup.procs" and
-"cgroup.subtree_control" on a namespace root from inside the
-namespace.
+achieved by not granting access to these files.  For the second, files
+outside the namespace should be hidden from the delegatee by the means
+of at least mount namespacing, and the kernel rejects writes to all
+files on a namespace root from inside the cgroup namespace, except for
+those files listed in "/sys/kernel/cgroup/delegate" (including
+"cgroup.procs", "cgroup.threads", "cgroup.subtree_control", etc.).
 
 The end results are equivalent for both delegation types.  Once
 delegated, the user can build sub-hierarchy under the directory,

kernel/cgroup/cgroup.c

@@ -4124,7 +4124,7 @@ static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
 	 * If namespaces are delegation boundaries, disallow writes to
 	 * files in an non-init namespace root from inside the namespace
 	 * except for the files explicitly marked delegatable -
-	 * cgroup.procs and cgroup.subtree_control.
+	 * eg. cgroup.procs, cgroup.threads and cgroup.subtree_control.
 	 */
 	if ((cgrp->root->flags & CGRP_ROOT_NS_DELEGATE) &&
 	    !(cft->flags & CFTYPE_NS_DELEGATABLE) &&