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

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
To: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org
Cc: berto@igalia.com, qemu-devel@nongnu.org, mreitz@redhat.com
Subject: Re: [PATCH v4 7/9] block: truncate: Don't make backing file data visible
Date: Tue, 21 Apr 2020 14:19:37 +0300	[thread overview]
Message-ID: <e679a19c-1fac-b2d7-b09e-04d5e5f1c122@virtuozzo.com> (raw)
In-Reply-To: <20200420133214.28921-8-kwolf@redhat.com>

20.04.2020 16:32, 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>
> ---
>   block/io.c | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
> 
> diff --git a/block/io.c b/block/io.c
> index 795075954e..f149dff3ba 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -3394,6 +3394,20 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset, bool exact,
>           goto out;
>       }
>   
> +    /*
> +     * If the image has a backing file that is large enough that it would
> +     * provide data for the new area, we cannot leave it unallocated because
> +     * then the backing file content would become visible. Instead, zero-fill
> +     * the area where backing file and new area overlap.

hmm, actually, you zero-fill the whole new area, not the overlap.

> +     *
> +     * Note that if the image has a backing file, but was opened without the
> +     * backing file, taking care of keeping things consistent with that backing
> +     * file is the user's responsibility.
> +     */
> +    if (new_bytes && bs->backing) {
> +        flags |= BDRV_REQ_ZERO_WRITE;
> +    }
> +
>       if (drv->bdrv_co_truncate) {
>           if (flags & ~bs->supported_truncate_flags) {
>               error_setg(errp, "Block driver does not support requested flags");
> 

-- 
Best regards,
Vladimir