Re: [Qemu-devel] [PATCH 1/4] block: Correct bs->growable

[Max Reitz <mreitz@redhat.com>]

On 20.08.2014 13:40, Kevin Wolf wrote:
> Am 12.07.2014 um 00:23 hat Max Reitz geschrieben:
>> Currently, the field "growable" in a BDS is set iff the BDS is opened in
>> protocol mode (with O_BDRV_PROTOCOL). However, not every protocol block
>> driver allows growing: NBD, for instance, does not. On the other hand,
>> a non-protocol block driver may allow growing: The raw driver does.
>>
>> Fix this by correcting the "growable" field in the driver-specific open
>> function for the BDS, if necessary.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
> I'm not sure I agree with bs->growable = true for raw. It's certainly
> true that the backend can technically provide the functionality that
> writes beyond EOF grow the file. That's not the point of bs->growable,
> though.
>
> The point of it was to _forbid_ it to grow even when it's technically
> possible (non-file protocols weren't really a thing back then, apart
> from vvfat, so the assumption was that it's always technically
> possible). growable was introduced with bdrv_check_request(), which is
> supposed to reject guest requests after the end of the virtual disk (and
> this fixed a CVE, see commit 71d0770c). You're now disabling this check
> for raw.
>
> I think we need to make sure that bs->growable is only set if it is
> opened for an image that has drv->requires_growing_file set and
> therefore not directly used by a guest.
>
> Well, except that with node-name a guest will be able to use any image
> in the chain... Might this mean that it's really a BlockBackend
> property?

I guess I can make things easy for me by just introducing some 
"really_growable" or "writes_beyond_eof" field or something for the sake 
of this series. ;-)

Max