Re: [Qemu-devel] [PATCH] block: change default of .has_zero_init to 0

[Kevin Wolf]

Am 27.06.2013 um 15:52 hat Peter Lieven geschrieben:
> .has_zero_init defaults to 1 for all formats and protocols.
> 
> this is a dangerous default since this means that all
> new added drivers need to manually overwrite it to 0 if
> they do not ensure that a device is zero initialized
> after bdrv_create().
> 
> if a driver needs to explicitly set this value to
> 1 its easier to verify the correctness in the review process.
> 
> during review of the existing drivers it turned out
> that ssh and gluster had a wrong default of 1.
> both protocols support host_devices as backend
> which are not by default zero initialized. this
> wrong assumption will lead to possible corruption
> if qemu-img convert is used to write to such a backend.
> 
> a similar problem with the wrong default existed for
> iscsi mose likely because the driver developer did
> oversee the default value of 1.
> 
> Signed-off-by: Peter Lieven <address@hidden>
> ---
>  block.c               |    8 +++++++-
>  block/qcow.c          |    1 +
>  block/qcow2.c         |    1 +
>  block/qed.c           |    1 +
>  block/raw-posix.c     |   10 +---------
>  block/raw-win32.c     |    7 +------
>  block/rbd.c           |    1 +
>  block/sheepdog.c      |    1 +
>  block/vdi.c           |    1 +
>  block/vmdk.c          |    1 +
>  include/block/block.h |    1 +
>  11 files changed, 17 insertions(+), 16 deletions(-)

You forgot cow, which is also a simple case that can be handled in this
patch.

vpc is still easy, but a bit more complicated than a constant return 1,
because it depends on the subformat whether a new image will inherit the
has_zero_init property from the underlying storage or whether it always
produces zeros (for VHD_FIXED type images, it's basically raw + footer).
I'll send a separate patch for this.

A similar situation exists for vmdk, I think, just that the difference
can be per extent there. I guess we need to return 0 if one of the
extents is flat and has an underlying storage returning 0. I'll leave
this part to Fam. For now, please remove the bdrv_has_zero_init_1 for
vmdk from this patch as it's unsafe.

Kevin