Android O系统启动流程--init篇(第二阶段)

int main(int argc, char** argv) {
    ...
// At this point we're in the second stage of init.
    // 同样屏蔽标准输入输出及定义Kernel logger
    InitKernelLogging(argv);
    LOG(INFO) << "init second stage started!";

    // Set up a session keyring that all processes will have access to. It
    // will hold things like FBE encryption keys. No process should override
    // its session keyring.
    // 最后调用syscall，设置安全相关的值
    keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 1);

    // Indicate that booting is in progress to background fw loaders, etc.
    // 这里的功能类似于“锁”
    close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));
    //初始化属性域
    property_init();
    //初始化完属性域后，以下均完成一些属性的设定
    // If arguments are passed both on the command line and in DT,
    // properties set in DT always have priority over the command-line ones.
    process_kernel_dt();
    process_kernel_cmdline();

    // Propagate the kernel variables to internal variables
    // used by init as well as the current required properties.
    export_kernel_boot_props();

    // Make the time that init started available for bootstat to log.
    property_set("ro.boottime.init", getenv("INIT_STARTED_AT"));
    property_set("ro.boottime.init.selinux", getenv("INIT_SELINUX_TOOK"));

    // Set libavb version for Framework-only OTA match in Treble build.
    const char* avb_version = getenv("INIT_AVB_VERSION");
    if (avb_version) property_set("ro.boot.avb_version", avb_version);
    // 清除掉之前使用过的环境变量
    // Clean up our environment.
    unsetenv("INIT_SECOND_STAGE");
    unsetenv("INIT_STARTED_AT");
    unsetenv("INIT_SELINUX_TOOK");
    unsetenv("INIT_AVB_VERSION");
    // 再次完成selinux相关的工作
    // Now set up SELinux for second stage.
    selinux_initialize(false);
    selinux_restore_context();
    //init进程调用epoll_create1创建epoll句柄
    epoll_fd = epoll_create1(EPOLL_CLOEXEC);
    if (epoll_fd == -1) {
        PLOG(ERROR) << "epoll_create1 failed";
        exit(1);
    }
    //init进程调用signal_handler_init装载子进程信号处理器
    signal_handler_init();
    //设置默认系统属性及启动配置属性的服务端
    property_load_boot_defaults();
    export_oem_lock_status();
    start_property_service();
    set_usb_controller();

static void selinux_initialize(bool in_kernel_domain) {
    ...
    if (in_kernel_domain) {
        //第一阶段的工作
        ...
    } else {
        //注册处理器
        selinux_init_all_handles();
    }
}

// The files and directories that were created before initial sepolicy load or
// files on ramdisk need to have their security context restored to the proper
// value. This must happen before /dev is populated by ueventd.
static void selinux_restore_context() {
    LOG(INFO) << "Running restorecon...";
    selinux_android_restorecon("/dev", 0);
    selinux_android_restorecon("/dev/kmsg", 0);
    selinux_android_restorecon("/dev/socket", 0);
    selinux_android_restorecon("/dev/random", 0);
    selinux_android_restorecon("/dev/urandom", 0);
    selinux_android_restorecon("/dev/__properties__", 0);

    selinux_android_restorecon("/plat_file_contexts", 0);
    selinux_android_restorecon("/nonplat_file_contexts", 0);
    selinux_android_restorecon("/plat_property_contexts", 0);
    selinux_android_restorecon("/nonplat_property_contexts", 0);
    selinux_android_restorecon("/plat_seapp_contexts", 0);
    selinux_android_restorecon("/nonplat_seapp_contexts", 0);
    selinux_android_restorecon("/plat_service_contexts", 0);
    selinux_android_restorecon("/nonplat_service_contexts", 0);
    selinux_android_restorecon("/plat_hwservice_contexts", 0);
    selinux_android_restorecon("/nonplat_hwservice_contexts", 0);
    selinux_android_restorecon("/sepolicy", 0);
    selinux_android_restorecon("/vndservice_contexts", 0);

    selinux_android_restorecon("/dev/block", SELINUX_ANDROID_RESTORECON_RECURSE);
    selinux_android_restorecon("/dev/device-mapper", 0);

    selinux_android_restorecon("/sbin/mke2fs_static", 0);
    selinux_android_restorecon("/sbin/e2fsdroid_static", 0);

    selinux_android_restorecon("/sbin/mkfs.f2fs", 0);
    selinux_android_restorecon("/sbin/sload.f2fs", 0);
}

void signal_handler_init() {
    // Create a signalling mechanism for SIGCHLD.
    int s[2];
    if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0, s) == -1) {
        PLOG(ERROR) << "socketpair failed";
        exit(1);
    }

    signal_write_fd = s[0];
    signal_read_fd = s[1];

    // Write to signal_write_fd if we catch SIGCHLD.
    struct sigaction act;
    memset(&act, 0, sizeof(act));
    //信号处理器对应的执行函数为SIGCHLD_handler.被存在sigaction结构体中，负责处理SIGCHLD消息
    act.sa_handler = SIGCHLD_handler;
    act.sa_flags = SA_NOCLDSTOP;
    //调用信号安装函数sigaction，将监听的信号及对应的信号处理器注册到内核中
    sigaction(SIGCHLD, &act, 0);
    //用于终止出现问题的子进程
    ServiceManager::GetInstance().ReapAnyOutstandingChildren();
    //注册信号处理函数handle_signal
    register_epoll_handler(signal_read_fd, handle_signal);
}

void Service::Reap() {
    //清理未携带SVC_ONESHOT 或 携带了SVC_RESTART标志的srvc的进程组
    if (!(flags_ & SVC_ONESHOT) || (flags_ & SVC_RESTART)) {
        KillProcessGroup(SIGKILL);
    }

    // Remove any descriptor resources we may have created.
    //清除srvc中创建出的任意描述符
    std::for_each(descriptors_.begin(), descriptors_.end(),
                  std::bind(&DescriptorInfo::Clean, std::placeholders::_1));
    //清理工作完毕后，后面决定是否重启机器或重启服务，TEMP服务不用参与这种判断
    if (flags_ & SVC_TEMPORARY) {
        return;
    }

    pid_ = 0;
    flags_ &= (~SVC_RUNNING);

    // Oneshot processes go into the disabled state on exit,
    // except when manually restarted.
    //对于携带了SVC_ONESHOT并且未携带SVC_RESTART的srvc，将这类服务的标志置为SVC_DISABLED，不再自启动
    if ((flags_ & SVC_ONESHOT) && !(flags_ & SVC_RESTART)) {
        flags_ |= SVC_DISABLED;
    }

    // Disabled and reset processes do not get restarted automatically.
    if (flags_ & (SVC_DISABLED | SVC_RESET))  {
        NotifyStateChange("stopped");
        return;
    }

    // If we crash > 4 times in 4 minutes, reboot into recovery.
    //未携带SVC_RESTART的关键服务，在规定的间隔内，crash字数过多时，会导致整机重启；
    boot_clock::time_point now = boot_clock::now();
    if ((flags_ & SVC_CRITICAL) && !(flags_ & SVC_RESTART)) {
        if (now < time_crashed_ + 4min) {
            if (++crash_count_ > 4) {
                LOG(ERROR) << "critical process '" << name_ << "' exited 4 times in 4 minutes";
                panic();
            }
        } else {
            time_crashed_ = now;
            crash_count_ = 1;
        }
    }
    //将待重启srvc的标志位置为SVC_RESTARTING（init进程将根据该标志位，重启服务）
    flags_ &= (~SVC_RESTART);
    flags_ |= SVC_RESTARTING;

    // Execute all onrestart commands for this service.
    //重启在init.rc文件中带有onrestart选项的服务
    onrestart_.ExecuteAllCommands();

    NotifyStateChange("restarting");
    return;
}