--- /tmp/driver.py	2020-06-22 14:48:09.760186268 +0000
+++ /usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py	2020-06-25 15:26:42.344963030 +0000
@@ -2515,8 +2515,10 @@
             # If the rebased image is going to have a backing file then
             # explicitly set the backing file format to avoid any security
             # concerns related to file format auto detection.
-            backing_file = rebase_base
-            b_file_fmt = images.qemu_img_info(backing_file).file_format
+            backing_file = os.path.basename(rebase_base)  # relative path
+            volume_path = os.path.dirname(active_disk_object.source_path)
+            backing_path = os.path.join(volume_path, backing_file)
+            b_file_fmt = images.qemu_img_info(backing_path).file_format
             qemu_img_extra_arg = ['-F', b_file_fmt]
 
         qemu_img_extra_arg.append(active_disk_object.source_path)