Re: [PATCH v3] nilfs2: fix checkpoint root lifetime on sysfs errors

[Aldo Ariel Panzardo]

Hi Slava,

No, wait_for_completion() without a timeout is intentional here.

The kobject subsystem guarantees that the release callback will
eventually run -- CONFIG_DEBUG_KOBJECT_RELEASE only defers it, it
never drops it. So the wait is bounded in practice.

If we used wait_for_completion_killable_timeout() and the timeout
fired (or a signal arrived) before the release callback ran, we
would kfree() the root while the kobject release is still pending.
When the callback finally runs it would access freed memory -- a
use-after-free.

wait_for_completion_killable() (without timeout) has the same
problem: if the wait is interrupted by a fatal signal, we cannot
safely free the container because the release callback may still
reference it.

The unconditional wait_for_completion() is the only safe choice
when the caller must free the container of an embedded kobject.
This is the same pattern used by other subsystems (e.g.,
blk_mq_tag_set, configfs subsystems).

Aldo