linux-kernel - Re: [RFC] Suspicious bug in module refcounting

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Message-Id: <200902101345.08580.rusty@rustcorp.com.au>
Date:	Tue, 10 Feb 2009 13:45:07 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Karsten Keil <kkeil@...e.de>, linux-kernel@...r.kernel.org,
	richard kennedy <richard@....demon.co.uk>,
	Dan Williams <dan.j.williams@...el.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	dwmw2@...radead.org, Scott Wood <scottwood@...escale.com>,
	netdev@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC] Suspicious bug in module refcounting

On Tuesday 10 February 2009 01:48:31 Michal Hocko wrote:
> Based on this change, would it make sense to update sys_accept to change
> __module_get to try_module_get like in the following patch?

I don't think so:

>  	/*
> -	 * We don't need try_module_get here, as the listening socket (sock)
> -	 * has the protocol module (sock->ops->owner) held.
> +	 * Socket's owner cannot be in unloading path because there
> +	 * must be at least one listening reference
>  	 */
> -	__module_get(newsock->ops->owner);
> +	if (unlikely(!try_module_get(newsock->ops->owner)))
> +		BUG();

rmmod --wait can make try_module_get fail even if the reference count isn't
zero.  But in this case, we should return an error from the accept call;
presumably the admin really doesn't want us to keep using the module...

Thanks,
Rusty.
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