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>
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