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Date: Thu, 28 Oct 2010 09:10:30 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] block: fix the use of protocols in backing
	files
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Adam Litke <agl@us.ibm.com>

On 10/28/2010 04:51 AM, Daniel P. Berrange wrote:
>> The problem is that this wouldn't work in the general case. It's rather
>> an exception that it makes sense for file: backing files with file:
>> images. Consider this:
>>
>>    # qemu-img create -o backing_file=nbd:foo:1234 /tmp/main.img
>>
>> Without this patch, you'll end up with /tmp/nbd:foo:1234, which is
>> probably not what you wanted. With a patch that would work for file: you
>> would get a hardly better path nbd:/tmp/foo:1234
>>      
> Since we know the protocol, there could be a per-protocol function used
> for resolving the backing store URI relative to the master URI. That
> would avoid needing to special case file: in the shared generic code.
>    

Relative resolution of a backing files makes me very nervous.  Any time 
a disk image can reasonably resolve to something other than what the 
user expected is potentially a very nasty security issue.

The less obvious the resolution, the worse the problem becomes.  I think 
resolution based on current path is probably the most obvious 
implementation.

Regards,

Anthony Liguori

> Daniel
>