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

[Stefan Hajnoczi]

On Tue, Aug 06, 2013 at 10:38:32AM +0800, Asias He wrote:
> 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?

I think we rely on zeroes when bs->growable is true, because
bdrv_pwrite() handles sub-sector I/O using read-modify-write.  So it
reads beyond EOF first (expects to get zeroes), then copies the
sub-sector data from the caller, then writes it back.  If we don't zero
beyond-EOF data then we would write unexpected values to the image.

Stefan