Revert "pidfd: prevent creation of pidfds for kthreads"

This reverts commit 3b5bbe79.

Eric reported that systemd-shutdown gets broken by blocking the creating
of pidfds for kthreads as older versions seems to rely on being able to
create a pidfd for any process in /proc.
Reported-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/r/20240818035818.GA1929@sol.localdomainSigned-off-by: Christian Brauner <brauner@kernel.org>

kernel/fork.c

@@ -2053,23 +2053,10 @@ 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)
 {
-	if (!pid)
-		return -EINVAL;
-
-	scoped_guard(rcu) {
-		struct task_struct *tsk;
-
-		if (flags & PIDFD_THREAD)
-			tsk = pid_task(pid, PIDTYPE_PID);
-		else
-			tsk = pid_task(pid, PIDTYPE_TGID);
-		if (!tsk)
-			return -EINVAL;
+	bool thread = flags & PIDFD_THREAD;
 
-		/* Don't create pidfds for kernel threads for now. */
-		if (tsk->flags & PF_KTHREAD)
-			return -EINVAL;
-	}
+	if (!pid || !pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))
+		return -EINVAL;
 
 	return __pidfd_prepare(pid, flags, ret);
 }
@@ -2416,12 +2403,6 @@ __latent_entropy struct task_struct *copy_process(
 	if (clone_flags & CLONE_PIDFD) {
 		int flags = (clone_flags & CLONE_THREAD) ? PIDFD_THREAD : 0;
 
-		/* Don't create pidfds for kernel threads for now. */
-		if (args->kthread) {
-			retval = -EINVAL;
-			goto bad_fork_free_pid;
-		}
-
 		/* Note that no task has been attached to @pid yet. */
 		retval = __pidfd_prepare(pid, flags, &pidfile);
 		if (retval < 0)