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Date: Tue, 12 Apr 2011 10:14:58 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] To O_EXCL or not to O_EXCL open host_cdrom
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>, Ryan Harper <ryanh@us.ibm.com>, Amit Shah <amit.shah@redhat.com>, Christoph Hellwig <hch@lst.de>

On Tue, Apr 12, 2011 at 9:19 AM, Daniel P. Berrange <berrange@redhat.com> w=
rote:
> On Tue, Apr 12, 2011 at 10:10:44AM +0200, Kevin Wolf wrote:
>> Am 12.04.2011 09:52, schrieb Daniel P. Berrange:
>> > =A0- If the -drive specification has =A0readonly=3Don (thus O_RDONLY t=
o
>> > =A0 =A0open(2) call) , I expect QEMU (or the kernel) to forbid the
>> > =A0 =A0"eject" command on the host CDROM. This should prevent two gues=
ts
>> > =A0 =A0interfering seriously with each other.
>> >
>> > So I think using O_EXCL would be OK, in the case where the block
>> > driver was =A0host_cdrom and readonly=3Doff.
>>
>> This would overload readonly with a completely unrelated option (should
>> eject be allowed). Doesn't sound like a great idea.
>
> Use of the "host_cdrom" block driver enables passthrough of
> commands to the host device.
>
> Use of "readonly" is what controls whether individual passthrough
> commands are actually permitted.
>
> To me, "readonly" means don't allow anything that would impact
> another guests view of the file/device. So forbidding 'eject'
> is within scope of that IMHO.

I agree with Kevin here.  You're overloading the option.

Today I doubt readonly will prevent the eject/lock ioctls but I haven't tes=
ted.

I'm becoming more convinced now that we should be using O_EXCL.  Like
you say, forcing raw gives us the option to share the device for
reading without allowing the dangerous ioctls.

Stefan