Unverified Commit 17f1b08a authored by Christian Brauner's avatar Christian Brauner
Browse files

pidfs: ensure consistent ENOENT/ESRCH reporting 

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.

Link: https://lore.kernel.org/20250411-work-pidfs-enoent-v2-2-60b2d3bb545f@kernel.org


Reviewed-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 35c9701e

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);

}