Switch root to /system in first stage mount
Bug: 79173823
Bug: 79758715
Test: boot hikey and observe that /system is the new root
Change-Id: I22f58f1332150ebae8e7e24eccfe780ff29eba7f
diff --git a/init/switch_root.cpp b/init/switch_root.cpp
new file mode 100644
index 0000000..0e59b57
--- /dev/null
+++ b/init/switch_root.cpp
@@ -0,0 +1,151 @@
+/*
+ * Copyright (C) 2018 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 "switch_root.h"
+
+#include <dirent.h>
+#include <fcntl.h>
+#include <mntent.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include <android-base/logging.h>
+#include <android-base/strings.h>
+
+using android::base::StartsWith;
+
+using namespace std::literals;
+
+namespace android {
+namespace init {
+
+namespace {
+
+void FreeRamdisk(DIR* dir, dev_t dev) {
+ int dfd = dirfd(dir);
+
+ dirent* de;
+ while ((de = readdir(dir)) != nullptr) {
+ if (de->d_name == "."s || de->d_name == ".."s) {
+ continue;
+ }
+
+ bool is_dir = false;
+
+ if (de->d_type == DT_DIR || de->d_type == DT_UNKNOWN) {
+ struct stat info;
+ if (fstatat(dfd, de->d_name, &info, AT_SYMLINK_NOFOLLOW) != 0) {
+ continue;
+ }
+
+ if (info.st_dev != dev) {
+ continue;
+ }
+
+ if (S_ISDIR(info.st_mode)) {
+ is_dir = true;
+ auto fd = openat(dfd, de->d_name, O_RDONLY | O_DIRECTORY);
+ if (fd >= 0) {
+ auto subdir =
+ std::unique_ptr<DIR, decltype(&closedir)>{fdopendir(fd), closedir};
+ if (subdir) {
+ FreeRamdisk(subdir.get(), dev);
+ } else {
+ close(fd);
+ }
+ }
+ }
+ }
+ unlinkat(dfd, de->d_name, is_dir ? AT_REMOVEDIR : 0);
+ }
+}
+
+std::vector<std::string> GetMounts(const std::string& new_root) {
+ auto fp = std::unique_ptr<std::FILE, decltype(&endmntent)>{setmntent("/proc/mounts", "re"),
+ endmntent};
+ if (fp == nullptr) {
+ PLOG(FATAL) << "Failed to open /proc/mounts";
+ }
+
+ std::vector<std::string> result;
+ mntent* mentry;
+ while ((mentry = getmntent(fp.get())) != nullptr) {
+ // We won't try to move rootfs.
+ if (mentry->mnt_dir == "/"s) {
+ continue;
+ }
+
+ // The new root mount is handled separately.
+ if (mentry->mnt_dir == new_root) {
+ continue;
+ }
+
+ // Move operates on subtrees, so do not try to move children of other mounts.
+ if (std::find_if(result.begin(), result.end(), [&mentry](const auto& older_mount) {
+ return StartsWith(mentry->mnt_dir, older_mount);
+ }) != result.end()) {
+ continue;
+ }
+
+ result.emplace_back(mentry->mnt_dir);
+ }
+
+ return result;
+}
+
+} // namespace
+
+void SwitchRoot(const std::string& new_root) {
+ auto mounts = GetMounts(new_root);
+
+ for (const auto& mount_path : mounts) {
+ auto new_mount_path = new_root + mount_path;
+ if (mount(mount_path.c_str(), new_mount_path.c_str(), nullptr, MS_MOVE, nullptr) != 0) {
+ PLOG(FATAL) << "Unable to move mount at '" << mount_path << "'";
+ }
+ }
+
+ auto old_root_dir = std::unique_ptr<DIR, decltype(&closedir)>{opendir("/"), closedir};
+ if (!old_root_dir) {
+ PLOG(ERROR) << "Could not opendir(\"/\"), not freeing ramdisk";
+ }
+
+ struct stat old_root_info;
+ if (stat("/", &old_root_info) != 0) {
+ PLOG(ERROR) << "Could not stat(\"/\"), not freeing ramdisk";
+ old_root_dir.reset();
+ }
+
+ if (chdir(new_root.c_str()) != 0) {
+ PLOG(FATAL) << "Could not chdir to new_root, '" << new_root << "'";
+ }
+
+ if (mount(new_root.c_str(), "/", nullptr, MS_MOVE, nullptr) != 0) {
+ PLOG(FATAL) << "Unable to move root mount to new_root, '" << new_root << "'";
+ }
+
+ if (chroot(".") != 0) {
+ PLOG(FATAL) << "Unable to chroot to new root";
+ }
+
+ if (old_root_dir) {
+ FreeRamdisk(old_root_dir.get(), old_root_info.st_dev);
+ }
+}
+
+} // namespace init
+} // namespace android