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

[Asias He]

On Tue, Aug 06, 2013 at 10:02:22AM +0800, Fam Zheng wrote:
> On Tue, 08/06 09:53, Asias He wrote:
> > From: MORITA Kazutaka <address@hidden>
> > 
> > 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.
> 
> It seems to me that "read beyond EOF" is more protocol defined than to
> be forced in block layer. Is this behavior common to all protocols, is
> it reasonable if some protocol wants other values than zero?

The reason to do this in block layer is that we do not want to duplicate
the memset in all protocols.

Do we actually have protocols that really want values other than zero?

> Thanks.
> 
> -- 
> Fam

-- 
Asias