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pidfd: improve uapi when task isn't found

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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: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2025-04-03 16:09:03 +02:00
parent 1b090949c9
commit 8cf4b738f6
1 changed files with 20 additions and 3 deletions
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23
kernel/fork.c
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@@ -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);
}