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

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Thu, Oct 28, 2010 at 10:35:02AM +0100, Daniel P. Berrange wrote:
> 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

This works relative to main.img:

# qemu-img create -o backing_file=backing.img main.img

but this works relative to QEMU's cwd:

# qemu-img create -o backing_file=file:backing.img main.img

As Kevin mentioned, special-casing the file: protocol isn't ideal.  We
shouldn't make assumptions about specific protocols.

Stefan