Unverified Commit a9d7de0f authored by Christian Brauner's avatar Christian Brauner
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Merge patch series "pidfs: ensure consistent ENOENT/ESRCH reporting" 

Christian Brauner <brauner@kernel.org> says:

In a prior patch series we tried to cleanly differentiate between:

(1) The task has already been reaped.
(2) The caller requested a pidfd for a thread-group leader but the pid
actually references a struct pid that isn't used as a thread-group
leader.

as this was causing issues for non-threaded workloads.

But there's cases where the current simple logic is wrong. Specifically,
if the pid was a leader pid and the check races with __unhash_process().
Stabilize this by using the pidfd waitqueue lock.

* patches from https://lore.kernel.org/20250411-work-pidfs-enoent-v2-0-60b2d3bb545f@kernel.org:
  pidfs: ensure consistent ENOENT/ESRCH reporting
  exit: move wake_up_all() pidfd waiters into __unhash_process()

Link: https://lore.kernel.org/20250411-work-pidfs-enoent-v2-0-60b2d3bb545f@kernel.org


Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parents 1e940fff 17f1b08a

kernel/exit.c

+5 −0
Original line number Diff line number Diff line
@@ -133,8 +133,13 @@ struct release_task_post { 
static void __unhash_process(struct release_task_post *post, struct task_struct *p,

			     bool group_dead)

{

	struct pid *pid = task_pid(p);



	nr_threads--;



	detach_pid(post->pids, p, PIDTYPE_PID);

	wake_up_all(&pid->wait_pidfd);



	if (group_dead) {

		detach_pid(post->pids, p, PIDTYPE_TGID);

		detach_pid(post->pids, p, PIDTYPE_PGID);

kernel/fork.c

+16 −18
Original line number Diff line number Diff line
@@ -2108,27 +2108,25 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re 
 */

int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)

{

	int err = 0;



	if (!(flags & PIDFD_THREAD)) {

	/*

		 * If this is struct pid isn't used as a thread-group

		 * leader pid but the caller requested to create a

		 * thread-group leader pidfd then report ENOENT to the

		 * caller as a hint.

	 * While holding the pidfd waitqueue lock removing the task

	 * linkage for the thread-group leader pid (PIDTYPE_TGID) isn't

	 * possible. Thus, if there's still task linkage for PIDTYPE_PID

	 * not having thread-group leader linkage for the pid means it

	 * wasn't a thread-group leader in the first place.

	 */

		if (!pid_has_task(pid, PIDTYPE_TGID))

			err = -ENOENT;

	}



	scoped_guard(spinlock_irq, &pid->wait_pidfd.lock) {

		/* Task has already been reaped. */

		if (!pid_has_task(pid, PIDTYPE_PID))

			return -ESRCH;

		/*

	 * If this wasn't a thread-group leader struct pid or the task

	 * got reaped in the meantime report -ESRCH to userspace.

		 * If this struct pid isn't used as a thread-group

		 * leader but the caller requested to create a

		 * thread-group leader pidfd then report ENOENT.

		 */

	if (!pid_has_task(pid, PIDTYPE_PID))

		err = -ESRCH;

	if (err)

		return err;

		if (!(flags & PIDFD_THREAD) && !pid_has_task(pid, PIDTYPE_TGID))

			return -ENOENT;

	}



	return __pidfd_prepare(pid, flags, ret);

}

kernel/pid.c

+0 −5
Original line number Diff line number Diff line
@@ -359,11 +359,6 @@ static void __change_pid(struct pid **pids, struct task_struct *task, 
	hlist_del_rcu(&task->pid_links[type]);

	*pid_ptr = new;



	if (type == PIDTYPE_PID) {

		WARN_ON_ONCE(pid_has_task(pid, PIDTYPE_PID));

		wake_up_all(&pid->wait_pidfd);

	}



	for (tmp = PIDTYPE_MAX; --tmp >= 0; )

		if (pid_has_task(pid, tmp))

			return;