Re: [PATCH v2 1/2] qemu-img: rebase: stop when reaching EOF of old backing file

["Denis V. Lunev" <den@virtuozzo.com>]

On 5/25/23 20:02, Andrey Drobyshev wrote:
> In case when we're rebasing within one backing chain, and when target image
> is larger than old backing file, bdrv_is_allocated_above() ends up setting
> *pnum = 0.  As a result, target offset isn't getting incremented, and we
> get stuck in an infinite for loop.  Let's detect this case and proceed
> further down the loop body, as the offsets beyond the old backing size need
> to be explicitly zeroed.
>
> Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> ---
>   qemu-img.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/qemu-img.c b/qemu-img.c
> index 27f48051b0..78433f3746 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -3801,6 +3801,8 @@ static int img_rebase(int argc, char **argv)
>               }
>   
>               if (prefix_chain_bs) {
> +                uint64_t bytes = n;
> +
>                   /*
>                    * If cluster wasn't changed since prefix_chain, we don't need
>                    * to take action
> @@ -3813,9 +3815,18 @@ static int img_rebase(int argc, char **argv)
>                                    strerror(-ret));
>                       goto out;
>                   }
> -                if (!ret) {
> +                if (!ret && n) {
>                       continue;
>                   }
> +                if (!n) {
> +                    /*
> +                     * If we've reached EOF of the old backing, it means that
> +                     * offsets beyond the old backing size were read as zeroes.
> +                     * Now we will need to explicitly zero the cluster in
> +                     * order to preserve that state after the rebase.
> +                     */
> +                    n = bytes;
> +                }
>               }
>   
>               /*
Revieved-by: Denis V. Lunev <den@openvz.org>