Revert "pidfd: prevent creation of pidfds for kthreads"

This reverts commit 3b5bbe798b2451820e74243b738268f51901e7d0.

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.localdomain
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/kernel/fork.c b/kernel/fork.c
index 18bdc87209d0574cfca1831a7979b240a6f57eda..cc760491f20127f44ab5db96ec7e858dcce77f54 100644 (file)
--- a/kernel/fork.c
+++ b/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)