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 <address@hidden>
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. ;-)