cgroups: fix API thinko

Add cgroup_attach_task_all()

The existing cgroup_attach_task_current_cg() API is called by a thread to
attach another thread to all of its cgroups; this is unsuitable for cases
where a privileged task wants to attach itself to the cgroups of a less
privileged one, since the call must be made from the context of the target
task.

This patch adds a more generic cgroup_attach_task_all() API that allows
both the source task and to-be-moved task to be specified.
cgroup_attach_task_current_cg() becomes a specialization of the more
generic new function.

[menage@google.com: rewrote changelog]
[akpm@linux-foundation.org: address reviewer comments]
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Ben Blum <bblum@google.com>
Cc: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 18 insertions, 7 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ed3e92e41c6e..0c991023ee47 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -578,7 +578,12 @@ struct task_struct *cgroup_iter_next(struct cgroup *cgrp,
 void cgroup_iter_end(struct cgroup *cgrp, struct cgroup_iter *it);
 int cgroup_scan_tasks(struct cgroup_scanner *scan);
 int cgroup_attach_task(struct cgroup *, struct task_struct *);
-int cgroup_attach_task_current_cg(struct task_struct *);
+int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
+
+static inline int cgroup_attach_task_current_cg(struct task_struct *tsk)
+{
+        return cgroup_attach_task_all(current, tsk);
+}
 
 /*
  * CSS ID is ID for cgroup_subsys_state structs under subsys. This only works
@@ -636,6 +641,11 @@ static inline int cgroupstats_build(struct cgroupstats *stats,
 }
 
 /* No cgroups - nothing to do */
+static inline int cgroup_attach_task_all(struct task_struct *from,
+                                         struct task_struct *t)
+{
+        return 0;
+}
 static inline int cgroup_attach_task_current_cg(struct task_struct *t)
 {
         return 0;
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 192f88c5b0f9..c9483d8f6140 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1791,19 +1791,20 @@ out:
 }
 
 /**
- * cgroup_attach_task_current_cg - attach task 'tsk' to current task's cgroup
+ * cgroup_attach_task_all - attach task 'tsk' to all cgroups of task 'from'
+ * @from: attach to all cgroups of a given task
  * @tsk: the task to be attached
  */
-int cgroup_attach_task_current_cg(struct task_struct *tsk)
+int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
 {
         struct cgroupfs_root *root;
-        struct cgroup *cur_cg;
         int retval = 0;
 
         cgroup_lock();
         for_each_active_root(root) {
-                cur_cg = task_cgroup_from_root(current, root);
-                retval = cgroup_attach_task(cur_cg, tsk);
+                struct cgroup *from_cg = task_cgroup_from_root(from, root);
+
+                retval = cgroup_attach_task(from_cg, tsk);
                 if (retval)
                         break;
         }
@@ -1811,7 +1812,7 @@ int cgroup_attach_task_current_cg(struct task_struct *tsk)
 
         return retval;
 }
-EXPORT_SYMBOL_GPL(cgroup_attach_task_current_cg);
+EXPORT_SYMBOL_GPL(cgroup_attach_task_all);
 
 /*
  * Attach task with pid 'pid' to cgroup 'cgrp'. Call with cgroup_mutex