Re: [Qemu-devel] [PATCH] block: fix the use of protocols in backing files

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Oct 28, 2010 at 09:30:09AM +0100, Stefan Hajnoczi wrote:
> On Wed, Oct 27, 2010 at 7:19 PM, Anthony Liguori <aliguori@us.ibm.com> wrote:
> > Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
> >
> > diff --git a/block.c b/block.c
> > index 1a965b2..00b6f21 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -603,10 +603,16 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
> >         BlockDriver *back_drv = NULL;
> >
> >         bs->backing_hd = bdrv_new("");
> > -        path_combine(backing_filename, sizeof(backing_filename),
> > -                     filename, bs->backing_file);
> > -        if (bs->backing_format[0] != '\0')
> > -            back_drv = bdrv_find_format(bs->backing_format);
> > +        back_drv = bdrv_find_protocol(bs->backing_file);
> > +        if (!back_drv) {
> > +            path_combine(backing_filename, sizeof(backing_filename),
> > +                         filename, bs->backing_file);
> > +            if (bs->backing_format[0] != '\0')
> > +                back_drv = bdrv_find_format(bs->backing_format);
> > +        } else {
> > +            pstrcpy(backing_filename, sizeof(backing_filename),
> > +                    bs->backing_file);
> > +        }
> >
> >         /* backing files always opened read-only */
> >         back_flags =
> > --
> > 1.7.0.4
> 
> I think this makes sense.
> 
> Now it is possible to specify backing files that are relative to
> QEMU's current working directory using file:filename.  I don't see
> harm in this.

Shouldn't a backing file be treated as relative to the image file pointing
to it, rather than the QEMU working directory. eg so you can do

  # qemu-img create backing.img
  # qemu-img create -o backing_file=file:backing.img main.img
  
And have main.img be able to resolve backing.img in its same directory,
no matter what directory QEMU itself is executing from

Regards,
Daniel
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