From d84b9b93755ece6618ed98fa84386beeb1a0e40b Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Mon, 8 Jun 2020 15:01:36 +0100
Subject: [PATCH 08/17] block: truncate: Don't make backing file data visible

RH-Author: Kevin Wolf <kwolf@redhat.com>
Message-id: <20200608150140.38218-8-kwolf@redhat.com>
Patchwork-id: 97454
O-Subject: [RHEL-AV-8.2.1 qemu-kvm PATCH 07/11] block: truncate: Don't make backing file data visible
Bugzilla: 1780574
RH-Acked-by: Sergio Lopez Pascual <slp@redhat.com>
RH-Acked-by: Eric Blake <eblake@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>

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: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200424125448.63318-8-kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 955c7d6687fefcd903900a1e597fcbc896c661cd)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
---
 block/io.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/block/io.c b/block/io.c
index 3235ce5..6c70b56 100644
--- a/block/io.c
+++ b/block/io.c
@@ -3370,6 +3370,31 @@ 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 new area.
+     *
+     * 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) {
+        int64_t backing_len;
+
+        backing_len = bdrv_getlength(backing_bs(bs));
+        if (backing_len < 0) {
+            ret = backing_len;
+            error_setg_errno(errp, -ret, "Could not get backing file size");
+            goto out;
+        }
+
+        if (backing_len > old_size) {
+            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");
-- 
1.8.3.1