diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index adbbc64aa9..a4fee645fd 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -144,6 +144,45 @@ static void cbw_child_perm(BlockDriverState *bs, BdrvChild *c,
     }
 }
 
+static int cbw_init(BlockDriverState *top, BlockDriverState *source,
+                    BlockDriverState *target, bool compress, Error **errp)
+{
+    BDRVCopyBeforeWriteState *state = top->opaque;
+
+    top->total_sectors = source->total_sectors;
+    top->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
+            (BDRV_REQ_FUA & source->supported_write_flags);
+    top->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
+            ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
+             source->supported_zero_flags);
+
+    bdrv_ref(target);
+    state->target = bdrv_attach_child(top, target, "target", &child_of_bds,
+                                      BDRV_CHILD_DATA, errp);
+    if (!state->target) {
+        error_prepend(errp, "Cannot attach target child: ");
+        return -EINVAL;
+    }
+
+    bdrv_ref(source);
+    top->file = bdrv_attach_child(top, source, "file", &child_of_bds,
+                                  BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
+                                  errp);
+    if (!top->file) {
+        error_prepend(errp, "Cannot attach file child: ");
+        return -EINVAL;
+    }
+
+    state->bcs = block_copy_state_new(top->file, state->target, false, compress,
+                                      errp);
+    if (!state->bcs) {
+        error_prepend(errp, "Cannot create block-copy-state: ");
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
 BlockDriver bdrv_cbw_filter = {
     .format_name = "copy-before-write",
     .instance_size = sizeof(BDRVCopyBeforeWriteState),
@@ -181,36 +220,10 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
         error_prepend(errp, "Cannot open driver: ");
         return NULL;
     }
-
     state = top->opaque;
-    top->total_sectors = source->total_sectors;
-    top->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
-            (BDRV_REQ_FUA & source->supported_write_flags);
-    top->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
-            ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
-             source->supported_zero_flags);
 
-    bdrv_ref(target);
-    state->target = bdrv_attach_child(top, target, "target", &child_of_bds,
-                                      BDRV_CHILD_DATA, errp);
-    if (!state->target) {
-        error_prepend(errp, "Cannot attach target child: ");
-        goto fail;
-    }
-
-    bdrv_ref(source);
-    top->file = bdrv_attach_child(top, source, "file", &child_of_bds,
-                                  BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
-                                  errp);
-    if (!top->file) {
-        error_prepend(errp, "Cannot attach file child: ");
-        goto fail;
-    }
-
-    state->bcs = block_copy_state_new(top->file, state->target, false, compress,
-                                      errp);
-    if (!state->bcs) {
-        error_prepend(errp, "Cannot create block-copy-state: ");
+    ret = cbw_init(top, source, target, compress, errp);
+    if (ret < 0) {
         goto fail;
     }