cgroups: fix API thinko

cgroup_attach_task_current_cg API that have upstream is backwards: we
really need an API to attach to the cgroups from another process A to
the current one.

In our case (vhost), a priveledged user wants to attach it's task to cgroups
from a less priveledged one, the API makes us run it in the other
task's context, and this fails.

So let's make the API generic and just pass in 'from' and 'to' tasks.
Add an inline wrapper for cgroup_attach_task_current_cg to avoid
breaking bisect.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>

include/linux/cgroup.h

@@ -578,7 +578,11 @@ 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 +640,11 @@ 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;

kernel/cgroup.c

@@ -1789,10 +1789,11 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 }
 
 /**
- * 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;
@@ -1800,7 +1801,7 @@ int cgroup_attach_task_current_cg(struct task_struct *tsk)
 
 	cgroup_lock();
 	for_each_active_root(root) {
-		cur_cg = task_cgroup_from_root(current, root);
+		cur_cg = task_cgroup_from_root(from, root);
 		retval = cgroup_attach_task(cur_cg, tsk);
 		if (retval)
 			break;
@@ -1809,7 +1810,7 @@ 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