Re: [Qemu-devel] [PATCH] Make default invocation of block drivers safer

[Kevin Wolf]

Am 14.07.2010 20:43, schrieb Christoph Hellwig:
> Err, strong NACK.  Please don't start messing with the contents of the
> data plane, we're getting into real trouble there.  It's perfectly
> valid for a guest to create an image inside an image, and with hardware
> support for nested virtualization I guess this use case will become
> rather common, just as it already is on S/390 with VM.

But you'll hardly ever find a legitimate or even common use case for
putting qcow2 on a raw hard disk. I mean, we have all learned that qcow2
can be used in LVs (which can grow at least), but directly on a hard
disk just doesn't make any sense to me. That's the first point.

The second is that you should always specify the format explicitly,
which turns this magic off. Specify format=raw and you're free to format
as many hard disks as qcow2 as you want.

Third, if you don't specify format=raw and the guest has written a qcow2
header, next time it would get the wrong content anyway because qcow2
will be interpreted by the host and not the level 1 guest (this one is
the security problem).

Kevin