PID namespace init releases its file locks before its children die

[Demi Marie Obenour]

I noticed that PID 1 in a PID namespace can release file locks (due
to exiting) while its children are still running for a bit.  If the
locks held by PID 1 were relied to serialize the execution of its
child processes, this could result in data corruption.

Specifically, the child processes are killed via exit_notify() ->
forget_original_parent() -> find_child_reaper() ->
zap_pid_ns_processes().  That comes *after* exit_files(), which
releases the file locks.

While it is possible to implement this with cgroups, cgroups
are quite a bit more complicated to use, at least compared to
a single call to unshare() before fork().

Is this intentional?  Changing the behavior would make supervision
trees significantly easier to properly implement.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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