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Commit a8ad805c authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup: fix RCU accesses around task->cgroups 



There are several places in kernel/cgroup.c where task->cgroups is
accessed and modified without going through proper RCU accessors.
None is broken as they're all lock protected accesses; however, this
still triggers sparse RCU address space warnings.

* Consistently use task_css_set() for task->cgroups dereferencing.

* Use RCU_INIT_POINTER() to clear task->cgroups to &init_css_set on
  exit.

* Remove unnecessary rcu_dereference_raw() from cset->subsys[]
  dereference in cgroup_exit().

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Reported-by: default avatarFengguang Wu <fengguang.wu@intel.com>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
parent 14611e51

kernel/cgroup.c

+13 −11
Original line number Diff line number Diff line
@@ -724,7 +724,7 @@ static struct cgroup *task_cgroup_from_root(struct task_struct *task, 
	 * task can't change groups, so the only thing that can happen

	 * is that it exits and its css is set back to init_css_set.

	 */

	cset = task->cgroups;

	cset = task_css_set(task);

	if (cset == &init_css_set) {

		res = &root->top_cgroup;

	} else {
@@ -1971,7 +1971,7 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp, 
	 * css_set to init_css_set and dropping the old one.

	 */

	WARN_ON_ONCE(tsk->flags & PF_EXITING);

	old_cset = tsk->cgroups;

	old_cset = task_css_set(tsk);



	task_lock(tsk);

	rcu_assign_pointer(tsk->cgroups, new_cset);
@@ -2094,8 +2094,11 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk, 
	 * we use find_css_set, which allocates a new one if necessary.

	 */

	for (i = 0; i < group_size; i++) {

		struct css_set *old_cset;



		tc = flex_array_get(group, i);

		tc->cg = find_css_set(tc->task->cgroups, cgrp);

		old_cset = task_css_set(tc->task);

		tc->cg = find_css_set(old_cset, cgrp);

		if (!tc->cg) {

			retval = -ENOMEM;

			goto out_put_css_set_refs;
@@ -3012,7 +3015,7 @@ static void cgroup_enable_task_cg_lists(void) 
		 * entry won't be deleted though the process has exited.

		 */

		if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list))

			list_add(&p->cg_list, &p->cgroups->tasks);

			list_add(&p->cg_list, &task_css_set(p)->tasks);

		task_unlock(p);

	} while_each_thread(g, p);

	read_unlock(&tasklist_lock);
@@ -5061,8 +5064,8 @@ static const struct file_operations proc_cgroupstats_operations = { 
void cgroup_fork(struct task_struct *child)

{

	task_lock(current);

	get_css_set(task_css_set(current));

	child->cgroups = current->cgroups;

	get_css_set(child->cgroups);

	task_unlock(current);

	INIT_LIST_HEAD(&child->cg_list);

}
@@ -5097,7 +5100,7 @@ void cgroup_post_fork(struct task_struct *child) 
		write_lock(&css_set_lock);

		task_lock(child);

		if (list_empty(&child->cg_list))

			list_add(&child->cg_list, &child->cgroups->tasks);

			list_add(&child->cg_list, &task_css_set(child)->tasks);

		task_unlock(child);

		write_unlock(&css_set_lock);

	}
@@ -5177,8 +5180,8 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks) 


	/* Reassign the task to the init_css_set. */

	task_lock(tsk);

	cset = tsk->cgroups;

	tsk->cgroups = &init_css_set;

	cset = task_css_set(tsk);

	RCU_INIT_POINTER(tsk->cgroups, &init_css_set);



	if (run_callbacks && need_forkexit_callback) {

		/*
@@ -5187,8 +5190,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks) 
		 */

		for_each_builtin_subsys(ss, i) {

			if (ss->exit) {

				struct cgroup *old_cgrp =

					rcu_dereference_raw(cset->subsys[i])->cgroup;

				struct cgroup *old_cgrp = cset->subsys[i]->cgroup;

				struct cgroup *cgrp = task_cgroup(tsk, i);



				ss->exit(cgrp, old_cgrp, tsk);
@@ -5555,7 +5557,7 @@ static u64 current_css_set_refcount_read(struct cgroup *cgrp, 
	u64 count;



	rcu_read_lock();

	count = atomic_read(&current->cgroups->refcount);

	count = atomic_read(&task_css_set(current)->refcount);

	rcu_read_unlock();

	return count;

}