linux-kernel - Re: PID namespace init releases its file locks before its children die

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Message-ID: <20251003123828.GA26441@redhat.com>
Date: Fri, 3 Oct 2025 14:38:28 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Demi Marie Obenour <demiobenour@...il.com>,
	Christian Brauner <brauner@...nel.org>,
	Mateusz Guzik <mjguzik@...il.com>
Cc: Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: PID namespace init releases its file locks before its children
 die

Add CCs.

I can't really help, just my 2 cents...

I don't think we can change do_exit() to call exit_files() after
exit_notify().

At first glance, technically it is possible to change do_exit() so
that the exiting reaper does zap_pid_ns_processes() earlier... But
even if this is possible, I think that this complication needs more
justification.

Oleg.

On 10/02, Demi Marie Obenour wrote:
>
> 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)