Add a tool to generate manifest files

This logic was previously part of the BuildManifest.apk generation but
it's split out so that it can be used in other contexts such as the
authfs test which consume the manifest.

Bug: 237384936
Test: atest AuthFsHostTest
Change-Id: I93ee4c03edc9b43c4dfbf159f70913c075b82766

fsverity/Android.bp

@@ -42,6 +42,12 @@ python_library_host {
     },
 }
 
+python_binary_host {
+    name: "fsverity_manifest_generator",
+    srcs: ["fsverity_manifest_generator.py"],
+    libs: ["fsverity_digests_proto_python"],
+}
+
 rust_protobuf {
     name: "libfsverity_digests_proto_rust",
     crate_name: "fsverity_digests_proto",

fsverity/fsverity_manifest_generator.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env python3
+#
+# Copyright 2022 Google Inc. All rights reserved.
+#
+# 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.
+
+"""
+`fsverity_manifest_generator` generates the a manifest file containing digests
+of target files.
+"""
+
+import argparse
+import os
+import subprocess
+import sys
+from fsverity_digests_pb2 import FSVerityDigests
+
+HASH_ALGORITHM = 'sha256'
+
+def _digest(fsverity_path, input_file):
+  cmd = [fsverity_path, 'digest', input_file]
+  cmd.extend(['--compact'])
+  cmd.extend(['--hash-alg', HASH_ALGORITHM])
+  out = subprocess.check_output(cmd, universal_newlines=True).strip()
+  return bytes(bytearray.fromhex(out))
+
+if __name__ == '__main__':
+  p = argparse.ArgumentParser()
+  p.add_argument(
+      '--output',
+      help='Path to the output manifest',
+      required=True)
+  p.add_argument(
+      '--fsverity-path',
+      help='path to the fsverity program',
+      required=True)
+  p.add_argument(
+      '--base-dir',
+      help='directory to use as a relative root for the inputs',
+      required=True)
+  p.add_argument(
+      'inputs',
+      nargs='+',
+      help='input file for the build manifest')
+  args = p.parse_args(sys.argv[1:])
+
+  digests = FSVerityDigests()
+  for f in sorted(args.inputs):
+    # f is a full path for now; make it relative so it starts with {mount_point}/
+    digest = digests.digests[os.path.relpath(f, args.base_dir)]
+    print(f"{os.path.relpath(f, args.base_dir)}")
+    digest.digest = _digest(args.fsverity_path, f)
+    digest.hash_alg = HASH_ALGORITHM
+
+  manifest = digests.SerializeToString()
+
+  with open(args.output, "wb") as f:
+    f.write(manifest)