Re: [PATCH v6 07/10] block: truncate: Don't make backing file data visible

[Eric Blake <eblake@redhat.com>]

On 4/23/20 10:01 AM, Kevin Wolf wrote:
> When extending the size of an image that has a backing file larger than
> its old size, make sure that the backing file data doesn't become
> visible in the guest, but the added area is properly zeroed out.
> 
> Consider the following scenario where the overlay is shorter than its
> backing file:
> 
>      base.qcow2:     AAAAAAAA
>      overlay.qcow2:  BBBB
> 
> When resizing (extending) overlay.qcow2, the new blocks should not stay
> unallocated and make the additional As from base.qcow2 visible like
> before this patch, but zeros should be read.
> 
> A similar case happens with the various variants of a commit job when an
> intermediate file is short (- for unallocated):
> 
>      base.qcow2:     A-A-AAAA
>      mid.qcow2:      BB-B
>      top.qcow2:      C--C--C-
> 
> After commit top.qcow2 to mid.qcow2, the following happens:
> 
>      mid.qcow2:      CB-C00C0 (correct result)
>      mid.qcow2:      CB-C--C- (before this fix)
> 
> Without the fix, blocks that previously read as zeros on top.qcow2
> suddenly turn into A.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> Reviewed-by: Alberto Garcia <berto@igalia.com>
> ---
>   block/io.c | 24 ++++++++++++++++++++++++
>   1 file changed, 24 insertions(+)
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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