Re: [PATCH v4 7/9] block: truncate: Don't make backing file data visible

[Alberto Garcia]

On Mon 20 Apr 2020 03:32:12 PM CEST, 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 <address@hidden>

Reviewed-by: Alberto Garcia <address@hidden>

Berto