Add an executable to create a microdroid signature

This is for testing purpose. Creating a microdroid signature and
embedding it in a payload image will be done by VirtManager in the
future.

Bug: 185069443
Test: create a signature following README
      create a payload.img with signature
      pass --disk=payload.img (check manually /dev/block/vdc1)
Change-Id: I6504dc6b3732c8e00e3bd1ffa5059995962d14b8
diff --git a/microdroid/signature/mk_microdroid_signature.cc b/microdroid/signature/mk_microdroid_signature.cc
new file mode 100644
index 0000000..d5cb1a5
--- /dev/null
+++ b/microdroid/signature/mk_microdroid_signature.cc
@@ -0,0 +1,141 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <fstream>
+#include <iostream>
+#include <string>
+
+#include <android-base/file.h>
+#include <android-base/result.h>
+#include <json/json.h>
+
+#include "microdroid/signature.h"
+
+using android::base::Dirname;
+using android::base::ErrnoError;
+using android::base::Error;
+using android::base::Result;
+using android::microdroid::MicrodroidSignature;
+using android::microdroid::WriteMicrodroidSignature;
+
+Result<uint32_t> GetFileSize(const std::string& path) {
+    struct stat st;
+    if (lstat(path.c_str(), &st) == -1) {
+        return ErrnoError() << "Can't lstat " << path;
+    }
+    return static_cast<uint32_t>(st.st_size);
+}
+
+// config JSON schema:
+// {
+//   "apexes": [
+//     {
+//       "name": string,       // the apex name
+//       "path": string,       // the path to the apex file
+//                             // absolute or relative to the config file
+//       "publicKey": string,  // optional
+//       "rootDigest": string, // optional
+//     }
+//   ]
+// }
+
+Result<MicrodroidSignature> LoadConfig(const std::string& config_file) {
+    MicrodroidSignature signature;
+    signature.set_version(1);
+
+    const std::string dirname = Dirname(config_file);
+    std::ifstream in(config_file);
+    Json::CharReaderBuilder builder;
+    Json::Value root;
+    Json::String errs;
+    if (!parseFromStream(builder, in, &root, &errs)) {
+        return Error() << "bad config: " << errs;
+    }
+
+    for (const Json::Value& apex : root["apexes"]) {
+        auto apex_signature = signature.add_apexes();
+
+        Json::Value name = apex["name"];
+        Json::Value path = apex["path"];
+        Json::Value publicKey = apex["publicKey"];
+        Json::Value rootDigest = apex["rootDigest"];
+
+        if (name.isString()) {
+            apex_signature->set_name(name.asString());
+        } else {
+            return Error() << "bad config: apexes.name should be a string: " << path;
+        }
+
+        if (path.isString()) {
+            std::string apex_path = path.asString();
+
+            // resolve path with the config_file's dirname if not absolute
+            bool is_absolute = !apex_path.empty() && apex_path[0] == '/';
+            if (!is_absolute) {
+                apex_path = dirname + "/" + apex_path;
+            }
+
+            auto file_size = GetFileSize(apex_path);
+            if (!file_size.ok()) {
+                return Error() << "I/O error: " << file_size.error();
+            }
+            apex_signature->set_size(file_size.value());
+        } else {
+            return Error() << "bad config: apexes.path should be a string: " << path;
+        }
+
+        if (publicKey.isString()) {
+            apex_signature->set_publickey(publicKey.asString());
+        } else if (!publicKey.isNull()) {
+            return Error() << "bad config: apexes.publicKey should be a string or null: "
+                           << publicKey;
+        }
+
+        if (rootDigest.isString()) {
+            apex_signature->set_rootdigest(rootDigest.asString());
+        } else if (!rootDigest.isNull()) {
+            return Error() << "bad config: apexes.rootDigest should be a string or null: "
+                           << rootDigest;
+        }
+    }
+
+    return signature;
+}
+
+int main(int argc, char** argv) {
+    if (argc != 3) {
+        std::cerr << "Usage: " << argv[0] << " <config> <output>\n";
+        return 1;
+    }
+
+    auto config = LoadConfig(argv[1]);
+    if (!config.ok()) {
+        std::cerr << config.error() << '\n';
+        return 1;
+    }
+
+    std::ofstream out(argv[2]);
+    auto result = WriteMicrodroidSignature(*config, out);
+    if (!result.ok()) {
+        std::cerr << result.error() << '\n';
+        return 1;
+    }
+    return 0;
+}
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