@@ -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 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) {
+ 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");
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 | 14 ++++++++++++++ 1 file changed, 14 insertions(+)