linux-kernel - Re: [PATCH] xen/blkback: do not leak mode property

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Message-ID: <20121204182151.GA25878@aepfle.de>
Date:	Tue, 4 Dec 2012 19:21:51 +0100
From:	Olaf Hering <olaf@...fle.de>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	konrad.wilk@...cle.com, xen-devel@...ts.xen.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xen/blkback: do not leak mode property

On Tue, Dec 04, Jan Beulich wrote:

> This looks necessary but insufficient - there's nothing really
> preventing backend_changed() from being called more than once
> for a given device (is simply the handler of xenbus watch). Hence
> I think either that function needs to be guarded against multiple
> execution (e.g. by removing the watch from that function itself,
> if that's permitted by xenbus), or to properly deal with the
> effects this has (including but probably not limited to the leaking
> of be->mode).

If another watch does really trigger after the kfree(be) in
xen_blkbk_remove(), wouldnt backend_changed access stale memory?
So if that can really happen in practice, shouldnt the backend_watch be
a separate allocation instead being contained within backend_info?

Looking at unregister_xenbus_watch, it clears removes the watch from the
list, so that process_msg will not see it anymore.

Olaf
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