Re: [Qemu-devel] [PATCH] block/stream: Don't stream unbacked devices

[Paolo Bonzini <pbonzini@redhat.com>]

Il 14/11/2013 14:41, Stefan Hajnoczi ha scritto:
> Thanks for raising this, it's a bug that we don't verify that the image
> has a backing file.
> 
> I'd rather return an error that the user attempted to do something
> pointless.  It was a mistake on their part and it helps to bring this to
> their attention right away.

I like this patch because it avoids the risk of NULL-dereferencing
bs->backing_hd.

See here:

         /* Copy if allocated in the intermediate images.  Limit to the
          * known-unallocated area [sector_num, sector_num+n).  */
         ret = bdrv_is_allocated_above(bs->backing_hd, base,
                                       sector_num, n, &n);

where it's not at all documented that the first argument of
bdrv_is_allocated_above can be NULL.

But such a no-op streaming is valid, just like it is valid to
enable copy-on-read without a backing file.  Think of a system that
starts streaming a disk as soon as the VM starts, because *if* there's a
backing file it knows it is just a template on a remote/slow filesytem.
 It's easier for such a system to invoke the command even if there's no
backing file.

Paolo