linux-kernel - Re: [RFC PATCH 5/5] kmod: Handle UMH_WAIT

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Date:	Fri, 10 Jul 2015 00:51:17 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 5/5] kmod: Handle UMH_WAIT_PROC from system unbound
	workqueue

On 07/09, Frederic Weisbecker wrote:
>
> The UMH_WAIT_PROC handler runs in its own thread for obsolete reasons.
> We couldn't launch and then wait for the exec kernel thread completion
> without blocking other usermodehelper queued jobs since khelper was
> implemented as a singlthread ordered workqueue.
>
> But now we replaced khelper with generic system unbound workqueues which
> can handle concurrent blocking jobs.
>
> So lets run it from the workqueue.

Probably this is fine, but I am a bit worried...

WQ_MAX_ACTIVE == 512, this should be enough "in practice". But nothing
protects us from creative driver(s) which spawns 512 long-living user
space tasks...

Note also that userpace can ptrace these task and "block" sys_wait()
forever.

I am worried ;)

> CHECK: I'm just worried about the signal handler that gets tweaked
> and also the call to sys_wait() that might fiddle with internals. The
> system workqueue must continue to work without surprise for other
> works.

Yes. This means that this patch is wrong without disallow_signal()
at the end.

Oleg.

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