Unverified Commit 8cf4b738 authored by Christian Brauner's avatar Christian Brauner
Browse files

pidfd: improve uapi when task isn't found 

We currently report EINVAL whenever a struct pid has no tasked attached
anymore thereby conflating two concepts:

(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.

This is causing issues for non-threaded workloads as in [1].

This patch tries to allow userspace to distinguish between (1) and (2).
This is racy of course but that shouldn't matter.

Link: https://github.com/systemd/systemd/pull/36982 [1]
Link: https://lore.kernel.org/r/20250403-work-pidfd-fixes-v1-3-a123b6ed6716@kernel.org


Reviewed-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 1b090949

kernel/fork.c

+20 −3
Original line number Diff line number Diff line
@@ -2108,10 +2108,27 @@ 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)

{

	bool thread = flags & PIDFD_THREAD;

	int err = 0;



	if (!pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))

		return -EINVAL;

	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.

		 */

		if (!pid_has_task(pid, PIDTYPE_TGID))

			err = -ENOENT;

	}



	/*

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

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

	 */

	if (!pid_has_task(pid, PIDTYPE_PID))

		err = -ESRCH;

	if (err)

		return err;



	return __pidfd_prepare(pid, flags, ret);

}