diff --git a/block.c b/block.c
index 58f804228c..86f2faa266 100644
--- a/block.c
+++ b/block.c
@@ -303,13 +303,21 @@ void path_combine(char *dest, int dest_size,
     }
 }
 
+void bdrv_get_full_backing_filename_from_filename(const char *backed,
+                                                  const char *backing,
+                                                  char *dest, size_t sz)
+{
+    if (backing[0] == '\0' || path_has_protocol(backing)) {
+        pstrcpy(dest, sz, backing);
+    } else {
+        path_combine(dest, sz, backed, backing);
+    }
+}
+
 void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz)
 {
-    if (bs->backing_file[0] == '\0' || path_has_protocol(bs->backing_file)) {
-        pstrcpy(dest, sz, bs->backing_file);
-    } else {
-        path_combine(dest, sz, bs->filename, bs->backing_file);
-    }
+    bdrv_get_full_backing_filename_from_filename(bs->filename, bs->backing_file,
+                                                 dest, sz);
 }
 
 void bdrv_register(BlockDriver *bdrv)
diff --git a/include/block/block.h b/include/block/block.h
index 6e7275d95b..eac9bb0957 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -397,6 +397,9 @@ void bdrv_get_backing_filename(BlockDriverState *bs,
                                char *filename, int filename_size);
 void bdrv_get_full_backing_filename(BlockDriverState *bs,
                                     char *dest, size_t sz);
+void bdrv_get_full_backing_filename_from_filename(const char *backed,
+                                                  const char *backing,
+                                                  char *dest, size_t sz);
 int bdrv_is_snapshot(BlockDriverState *bs);
 
 int path_has_protocol(const char *path);