diff --git a/metropolis/pkg/erofs/defs_test.go b/metropolis/pkg/erofs/defs_test.go
new file mode 100644
index 0000000..e32e155
--- /dev/null
+++ b/metropolis/pkg/erofs/defs_test.go
@@ -0,0 +1,52 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package erofs
+
+import (
+	"bytes"
+	"encoding/binary"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// These test that the specified structures serialize to the same number of bytes as the ones in the
+// EROFS kernel module.
+
+func TestSuperblockSize(t *testing.T) {
+	var buf bytes.Buffer
+	if err := binary.Write(&buf, binary.LittleEndian, &superblock{}); err != nil {
+		t.Fatalf("failed to write superblock: %v", err)
+	}
+	assert.Equal(t, 128, buf.Len())
+}
+
+func TestDirectoryEntrySize(t *testing.T) {
+	var buf bytes.Buffer
+	if err := binary.Write(&buf, binary.LittleEndian, &directoryEntryRaw{}); err != nil {
+		t.Fatalf("failed to write directory entry: %v", err)
+	}
+	assert.Equal(t, 12, buf.Len())
+}
+
+func TestInodeCompactSize(t *testing.T) {
+	var buf bytes.Buffer
+	if err := binary.Write(&buf, binary.LittleEndian, &inodeCompact{}); err != nil {
+		t.Fatalf("failed to write compact inode: %v", err)
+	}
+	assert.Equal(t, 32, buf.Len())
+}