Re: [PATCH 1/2] block: Remove 'backing': null from bs->{explicit_, }options

[Alberto Garcia <berto@igalia.com>]

On Fri 08 Nov 2019 09:53:11 AM CET, Kevin Wolf wrote:
> bs->options and bs->explicit_options shouldn't contain any options for
> child nodes. bdrv_open_inherited() takes care to remove any options that
> match a child name after opening the image and the same is done when
> reopening.
>
> However, we miss the case of 'backing': null, which is a child option,
> but results in no child being created. This means that a 'backing': null
> remains in bs->options and bs->explicit_options.
>
> A typical use for 'backing': null is in live snapshots: blockdev-add for
> the qcow2 overlay makes sure not to open the backing file (because it is
> already opened and blockdev-snapshot will attach it). After doing a
> blockdev-snapshot, bs->options and bs->explicit_options become
> inconsistent with the actual state (bs has a backing file now, but the
> options still say null). On the next occasion that the image is
> reopened, e.g. switching it from read-write to read-only when another
> snapshot is taken, the option will take effect again and the node
> incorrectly loses its backing file.
>
> Fix bdrv_open_inherited() to remove the 'backing' option from
> bs->options and bs->explicit_options even for the case where it
> specifies that no backing file is wanted.
>
> Reported-by: Peter Krempa <pkrempa@redhat.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>

Reviewed-by: Alberto Garcia <berto@igalia.com>

Berto