linux-kernel - Re: [Xen-devel] [PATCH 1/3] xen/pv-on-hvm kexec: prevent crash in xenwatch

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Date:	Wed, 17 Aug 2011 14:30:12 +0100
From:	Keir Fraser <keir@....org>
To:	Olaf Hering <olaf@...fle.de>,
	Ian Campbell <Ian.Campbell@...rix.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [Xen-devel] [PATCH 1/3] xen/pv-on-hvm kexec: prevent crash in
 xenwatch_thread() when stale watch events arrive

On 17/08/2011 13:51, "Olaf Hering" <olaf@...fle.de> wrote:

> On Tue, Aug 16, Ian Campbell wrote:
> 
>> On Tue, 2011-08-16 at 14:16 +0100, Olaf Hering wrote:
>>> During repeated kexec boots xenwatch_thread() can crash because
>>> xenbus_watch->callback is cleared by xenbus_watch_path() if a node/token
>>> combo for a new watch happens to match an already registered watch from
>>> an old kernel.  In this case xs_watch returns -EEXISTS, then
>>> register_xenbus_watch() does not remove the to-be-registered watch from
>>> the list of active watches but returns the -EEXISTS to the caller
>>> anyway.
>> 
>> Isn't this behaviour the root cause of the issue (which should be fixed)
>> rather than papering over it during watch processing. IOW should't
>> register_xenbus_watch cleanup after itself if xs_watch fails.
> 
> Keir, the EEXISTS case in register_xenbus_watch() was added by you 6
> years ago. Do you happen to know what it tried to solve, and do these
> conditions still apply today?  Perhaps the EEXISTS can be removed now.
> 
> http://xenbits.xen.org/hg/xen-unstable.hg/diff/8016551fde98/linux-2.6-xen-spar
> se/drivers/xen/xenbus/xenbus_xs.c

Bad me. Either remove the EEXIST check, or convert EEXIST to return code 0
in register_xenbus_watch(). You could do either, since I'm sure I added the
EEXIST check only as an attempt to theoretically robustify that function,
and looks like I got it wrong.

 K.

> Olaf


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