Re: [Qemu-devel] [PATCH 0/3] vpc: support probing of fixed size images

[Eric Blake <eblake@redhat.com>]

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On 08/15/2014 07:37 AM, Kevin Wolf wrote:

> We can choose Markus's suggestion of using the file name to guess the
> format. I don't really like it much, but it seems like a fair compromise
> that doesn't hurt usability as much.

In other words, if a user gives a file a "known suffix", then it is
their own fault if they made that file raw and the guest then happened
to convert the file to the format matching the suffix?  Or would this
start giving warnings if the known suffix doesn't match the probed contents?

> 
> If we don't want this, we can approach the problem from a different
> angle: The problem is not probing per se, but that images probed as raw
> can be written to by guests in a way that the next time they are probed
> as something else.
> 
> What if we let the raw driver know that it was probed and then it
> enables a check that returns -EIO for any write on the first 2k if that
> write would make the image look like a different format?

Not entirely future-proof - as we add support for more formats over
time, something that passes today could fail in the future.  Worse, a
guest could exploit an older qemu to write a header that a newer qemu
would reject.  But it does sound like an interesting approach
(preventing the guest from doing something risky).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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