Re: [Qemu-devel] [PATCH v2] block: Produce zeros when protocols reading beyond end of file

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Aug 06, 2013 at 09:53:40AM +0800, Asias He wrote:
> From: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
> 
> While Asias is debugging an issue creating qcow2 images on top of
> non-file protocols.  It boils down to this example using NBD:
> 
> $ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
> 
> Notice the open -g option to set bs->growable.  This means you can
> read/write beyond end of file.  Reading beyond end of file is supposed
> to produce zeroes.
> 
> We rely on this behavior in qcow2_create2() during qcow2 image
> creation.  We create a new file and then write the qcow2 header
> structure using bdrv_pwrite().  Since QCowHeader is not a multiple of
> sector size, block.c first uses bdrv_read() on the empty file to fetch
> the first sector (should be all zeroes).
> 
> Here is the output from the qemu-io NBD example above:
> 
> $ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
> 00000000:  ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab  ................
> 00000010:  ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab  ................
> 00000020:  ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab  ................
> ...
> 
> We are not zeroing the buffer!  As a result qcow2 image creation on top
> of protocols is not guaranteed to work even when file creation is
> supported by the protocol.
> 
> Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
> Signed-off-by: Asias He <asias@redhat.com>
> ---
>  block.c | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)

Applied again on top of Asias' fix so qcow2 vmstate doesn't break.

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan