platform_system_core/init/switch_root.cpp

/*
 * 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