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pidfs: ensure consistent ENOENT/ESRCH reporting

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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: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2025-04-12 14:04:53 +02:00
parent 35c9701ea7
commit 17f1b08acf
1 changed files with 17 additions and 19 deletions
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kernel/fork.c
+17 -19
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@@ -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.
*/
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.
* 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_PID))
err = -ESRCH;
if (err)
return err;
scoped_guard(spinlock_irq, &pid->wait_pidfd.lock) {
/* Task has already been reaped. */
if (!pid_has_task(pid, PIDTYPE_PID))
return -ESRCH;
/*
* 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 (!(flags & PIDFD_THREAD) && !pid_has_task(pid, PIDTYPE_TGID))
return -ENOENT;
}
return __pidfd_prepare(pid, flags, ret);
}