linux-kernel - Re: [PATCH 1/1] kthread: introduce kthread_get_run() to fix __nbd

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Date:	Wed, 28 Oct 2015 16:37:07 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Markus Pargmann <mpa@...gutronix.de>,
	Tejun Heo <tj@...nel.org>, nbd-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] kthread: introduce kthread_get_run() to fix
	__nbd_ioctl()

On 10/26, Christoph Hellwig wrote:
>
> On Sun, Oct 25, 2015 at 03:27:13PM +0100, Oleg Nesterov wrote:
> > It is not safe to use the task_struct returned by kthread_run(threadfn)
> > if threadfn() can exit before the "owner" does kthread_stop(), nothing
> > protects this task_struct.
> >
> > So __nbd_ioctl() looks buggy; a killed nbd_thread_send() can exit, free
> > its task_struct, and then kthread_stop() can use the freed/reused memory.
> >
> > Add the new trivial helper, kthread_get_run(). Hopefully it will have more
> > users, this patch changes __nbd_ioctl() as an example.
>
> This looks horrible.

Do you mean the helper itself?

In fact iirc people asked for this helper before. It looks natural and simple.
kthread_run() can only be used if this kthread can't exit on its own.

> I think the real problem is that nbd is totally
> abusing signals for kthreads and that needs to go away.

I agree this code needs cleanups. And of course we can fix it without
new helper, but see above.

Oleg.

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