cgroups: relax ns_can_attach checks to allow attaching to grandchild cgroups

The ns_proxy cgroup allows moving processes to child cgroups only one
level deep at a time.  This commit relaxes this restriction and makes it
possible to attach tasks directly to grandchild cgroups, e.g.:

($pid is in the root cgroup)
echo $pid > /cgroup/CG1/CG2/tasks

Previously this operation would fail with -EPERM and would have to be
performed as two steps:
echo $pid > /cgroup/CG1/tasks
echo $pid > /cgroup/CG1/CG2/tasks

Also, the target cgroup no longer needs to be empty to move a task there.

Signed-off-by: Grzegorz Nosek <root@localdomain.pl>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 10 deletions

diff --git a/kernel/ns_cgroup.c b/kernel/ns_cgroup.c
index 78bc3fd..5aa854f 100644
--- a/kernel/ns_cgroup.c
+++ b/kernel/ns_cgroup.c
@@ -34,7 +34,7 @@ int ns_cgroup_clone(struct task_struct *task, struct pid *pid)
 
 /*
  * Rules:
- *   1. you can only enter a cgroup which is a child of your current
+ *   1. you can only enter a cgroup which is a descendant of your current
  *     cgroup
  *   2. you can only place another process into a cgroup if
  *     a. you have CAP_SYS_ADMIN
@@ -45,21 +45,15 @@ int ns_cgroup_clone(struct task_struct *task, struct pid *pid)
 static int ns_can_attach(struct cgroup_subsys *ss,
 		struct cgroup *new_cgroup, struct task_struct *task)
 {
-	struct cgroup *orig;
-
 	if (current != task) {
 		if (!capable(CAP_SYS_ADMIN))
 			return -EPERM;
 
-		if (!cgroup_is_descendant(new_cgroup))
+		if (!cgroup_is_descendant(new_cgroup, current))
 			return -EPERM;
 	}
 
-	if (atomic_read(&new_cgroup->count) != 0)
-		return -EPERM;
-
-	orig = task_cgroup(task, ns_subsys_id);
-	if (orig && orig != new_cgroup->parent)
+	if (!cgroup_is_descendant(new_cgroup, task))
 		return -EPERM;
 
 	return 0;
@@ -77,7 +71,7 @@ static struct cgroup_subsys_state *ns_create(struct cgroup_subsys *ss,
 
 	if (!capable(CAP_SYS_ADMIN))
 		return ERR_PTR(-EPERM);
-	if (!cgroup_is_descendant(cgroup))
+	if (!cgroup_is_descendant(cgroup, current))
 		return ERR_PTR(-EPERM);
 
 	ns_cgroup = kzalloc(sizeof(*ns_cgroup), GFP_KERNEL);