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

From: Peter Krempa
Subject: Re: [PATCH 1/2] block: Remove 'backing': null from bs->{explicit_,}options
Date: Tue, 12 Nov 2019 17:01:09 +0100
User-agent: Mutt/1.12.1 (2019-06-15)

On Fri, Nov 08, 2019 at 09:53:11 +0100, 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

Note that we also use '"backing": null' as a terminator for the last
image in the chain if the user configures the chain manually.

This is kind-of a protection from opening the backing file from the
header if it was misconfigured somehow. I think this functionality
should be kept despite probably not making practical sense.

In my testing this scenario worked properly.

> 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 <address@hidden>
> Signed-off-by: Kevin Wolf <address@hidden>

The fix looks sane-enough to me and works as expected, but since I'm not
familiar enough with this code I'm comfortable only with a:

Tested-by: Peter Krempa <address@hidden>

> ---
>  block.c | 2 ++
>  1 file changed, 2 insertions(+)

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