Re: [Qemu-devel] PATCH: Control over drive open modes for backing file

[Ian Jackson]

Anthony Liguori writes ("Re: [Qemu-devel] PATCH: Control over drive open modes 
for backing file"):
> Right, but my point is that ,mode=ro does not have to force QEMU to open 
> the file O_RDONLY.  It simply needs to prevent writes from happening.

Well, yes, but actually it's probably most reliable to do it that way.
Given that this is a security feature we want to avoid accidentally
`missing' a case.  So we should definitely open the underlying file(s)
O_RDONLY.

If we do that then the guest definitely won't be able to write as if
it manages to persuade qemu to try qemu will just get an error.  This
is fine I think, if we can expose the read-only status to the guest.

> But it's important to be able to expose this property to the guest, so 
> ,mode=ro should not be allowed for disks that do not support exposing 
> their read-only-ness to the guest.

I agree that it would be an unusual thing to do, to expose a ro disk
in a way that doesn't support advertising the ro flag.  But I think it
should still be possible perhaps with some kind of force option.

Ian.