diff --git a/metropolis/node/build/mkerofs/fsspec/spec.proto b/metropolis/node/build/mkerofs/fsspec/spec.proto
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+// 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.
+
+syntax = "proto3";
+
+package metropolis.node.build.mkerofs.fsspec;
+option go_package = "source.monogon.dev/metropolis/node/build/mkerofs/fsspec";
+
+// FSSpec is the spec from which a filesystem is generated. It consists of files, directories and symbolic
+// links. Directories are also automatically inferred when required for the placement of files or symbolic
+// links. Inferred directories always have uid 0, gid 0 and permissions 0555. This can be overridden by
+// explicitly specifying a directory at a given path.
+message FSSpec {
+  repeated File file = 1;
+  repeated Directory directory = 2;
+  repeated SymbolicLink symbolic_link = 3;
+}
+
+// For internal use only. Represents all supported inodes in a oneof.
+message Inode {
+  oneof type {
+    File file = 1;
+    Directory directory = 2;
+    SymbolicLink symbolic_link = 3;
+  }
+}
+
+message File {
+  // The path where the file ends up in the filesystem.
+  string path = 1;
+  // The path on the host filesystem where the file contents should be taken from.
+  string source_path = 2;
+  // Unix permission bits
+  uint32 mode = 3;
+  // Owner uid
+  uint32 uid = 4;
+  // Owner gid
+  uint32 gid = 5;
+}
+
+message Directory {
+  // The path where the directory ends up in the filesystem.
+  string path = 1;
+  // Unix permission bits
+  uint32 mode = 2;
+  // Owner uid
+  uint32 uid = 3;
+  // Owner gid
+  uint32 gid = 4;
+}
+
+message SymbolicLink {
+  // The path where the symbolic link ends up in the filesystem.
+  string path = 1;
+  // The path to which the symbolic link resolves to.
+  string target_path = 2;
+}
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