Android系统启动流程（四）——rc文件解析（基于Android13）

1 概述

rc文件由安卓初始化语言编写，可用于初始化系统服务，设置属性，创建系统资源等操作。
rc文件在init进程中完成扫描，解析，加载，执行等操作。
system/core/init/init.cpp

//执行第二阶段的启动
int SecondStageMain(int argc, char** argv) {
   

    //创建epoll
    Epoll epoll;
    if (auto result = epoll.Open(); !result.ok()) {
   
        PLOG(FATAL) << result.error();
    }

    //导入rc文件中command相关的对应函数关系
    const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();
    //将函数对应map设置到Action类的全局变量中function_map_中
    Action::set_function_map(&function_map);

    //am是rc文件中的action管理，负责触发执行。
    ActionManager& am = ActionManager::GetInstance();
    ServiceList& sm = ServiceList::GetInstance();

    //加载引导脚本，即rc文件
    LoadBootScripts(am, sm);

    // Turning this on and letting the INFO logging be discarded adds 0.2s to
    // Nexus 9 boot time, so it's disabled by default.
    if (false) DumpState();

    // Make the GSI status available before scripts start running.
    auto is_running = android::gsi::IsGsiRunning() ? "1" : "0";
    SetProperty(gsi::kGsiBootedProp, is_running);
    auto is_installed = android::gsi::IsGsiInstalled() ? "1" : "0";
    SetProperty(gsi::kGsiInstalledProp, is_installed);

    //添加内置动作触发器，其他的action在rc文件中用on声明。这里会同时添加到事件队列和action队列
    am.QueueBuiltinAction(SetupCgroupsAction, "SetupCgroups");
    am.QueueBuiltinAction(SetKptrRestrictAction, "SetKptrRestrict");
    am.QueueBuiltinAction(TestPerfEventSelinuxAction, "TestPerfEventSelinux");
    am.QueueBuiltinAction(ConnectEarlyStageSnapuserdAction, "ConnectEarlyStageSnapuserd");
    //触发rc中定义的early-init操作
    am.QueueEventTrigger("early-init");

    // Queue an action that waits for coldboot done so we know ueventd has set up all of /dev...
    //热插拔事件的触发器
    am.QueueBuiltinAction(wait_for_coldboot_done_action, "wait_for_coldboot_done");
    // ... so that we can start queuing up actions that require stuff from /dev.
    am.QueueBuiltinAction(SetMmapRndBitsAction, "SetMmapRndBits");
    /组合键触发器
    Keychords keychords;
    am.QueueBuiltinAction(
            [&epoll, &keychords](const BuiltinArguments& args) -> Result<void> {
   
                for (const auto& svc : ServiceList::GetInstance()) {
   
                    keychords.Register(svc->keycodes());
                }
                keychords.Start(&epoll, HandleKeychord);
                return {
   };
            },
            "KeychordInit");

    // Trigger all the boot actions to get us started.
    //触发rc中定义的init操作
    am.QueueEventTrigger("init");

    // Don't mount filesystems or start core system services in charger mode.
    std::string bootmode = GetProperty("ro.bootmode", "");
    if (bootmode == "charger") {
   
        //触发rc中定义的charger操作
        am.QueueEventTrigger("charger");
    } else {
   
        //触发rc中定义的late-init操作
        am.QueueEventTrigger("late-init");
    }

    // Run all property triggers based on current state of the properties.
    //根据现在的属性来触发属性触发器
    am.QueueBuiltinAction(queue_property_triggers_action, "queue_property_triggers");
	...
    while (true) {
   
		...
        //这里真正执行rc文件中定义的命令
        if (!(prop_waiter_state.MightBeWaiting() || Service::is_exec_service_running())) {
   
            am.ExecuteOneCommand();
        }
		...
    }

    return 0;
}

2 加载命令-函数map

init第二阶段启动
system/core/init/init.cpp

//导入rc文件中command相关的对应函数关系
const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();
//将函数对应map设置到Action类的全局变量中function_map_中
Action::set_function_map(&function_map);

system/core/init/action.cpp

// Builtin-function-map start
const BuiltinFunctionMap& GetBuiltinFunctionMap() {
   
    constexpr std::size_t kMax = std::numeric_limits<std::size_t>::max();
    // clang-format off
    //命令与函数对应关系，查看rc文件时，根据后面对应函数进行执行
    static const BuiltinFunctionMap builtin_functions = {
   
        {
   "bootchart",               {
   1,     1,    {
   false,  do_bootchart}}},
        {
   "chmod",                   {
   2,     2,    {
   true,   do_chmod}}},
        {
   "chown",                   {
   2,     3,    {
   true,   do_chown}}},
        {
   "class_reset",             {
   1,     1,    {
   false,  do_class_reset}}},
        {
   "class_restart",           {
   1,     2,    {
   false,  do_class_restart}}},
        {
   "class_start",             {
   1,     1,    {
   false,  do_class_start}}},
        {
   "class_stop",              {
   1,     1,    {
   false,  do_class_stop}}},
        {
   "copy",                    {
   2,     2,    {
   true,   do_copy}}},
        {
   "copy_per_line",           {
   2,     2,    {
   true,   do_copy_per_line}}},
        {
   "domainname",              {
   1,     1,    {
   true,   do_domainname}}},
        {
   "enable",                  {
   1,     1,    {
   false,  do_enable}}},
        {
   "exec",                    {
   1,     kMax, {
   false,  do_exec}}},
        {
   "exec_background",         {
   1,     kMax, {
   false,  do_exec_background}}},
        {
   "exec_start",              {
   1,     1,    {
   false,  do_exec_start}}},
        {
   "export",                  {
   2,     2,    {
   false,  do_export}}},
        {
   "hostname",                {
   1,     1,    {
   true,   do_hostname}}},
        {
   "ifup",                    {
   1,     1,    {
   true,   do_ifup}}},
        {
   "init_user0",              {
   0,     0,    {
   false,  do_init_user0}}},
        {
   "insmod",                  {
   1,     kMax, {
   true,   do_insmod}}},
        {
   "installkey",              {
   1,     1,    {
   false,  do_installkey}}},
        {
   "interface_restart",       {
   1,     1,    {
   false,  do_interface_restart}}},
        {
   "interface_start",         {
   1,     1,    {
   false,  do_interface_start}}},
        {
   "interface_stop",          {
   1,     1,    {
   false,  do_interface_stop}}},
        {
   "load_exports",            {
   1,     1,    {
   false,  do_load_exports}}},
        {
   "load_persist_props",      {
   0,     0,    {
   false,  do_load_persist_props}}},
        {
   "load_system_props",       {
   0,     0,    {
   false,  do_load_system_props}}},
        {
   "loglevel",                {
   1,     1,    {
   false,  do_loglevel}}},
        {
   "mark_post_data",          {
   0,     0,    {
   false,  do_mark_post_data}}},
        {
   "mkdir",                   {
   1,     6,    {
   true,   do_mkdir}}},
        // TODO: Do mount operations in vendor_init.
        // mount_all is currently too complex to run in vendor_init as it queues action triggers,
        // imports rc scripts, etc.  It should be simplified and run in vendor_init context.
        // mount and umount are run in the same context as mount_all for symmetry.
        {
   "mount_all",               {
   0,     kMax, {
   false,  do_mount_all}}},
        {
   "mount",                   {
   3,     kMax, {
   false,  do_mount}}},
        {
   "perform_apex_config",     {
   0,     0,    {
   false,  do_perform_apex_config}}},
        {
   "umount",                  {
   1,     1,    {
   false,  do_umount}}},
        {
   "umount_all",              {
   0,     1,    {
   false,  do_umount_all}}},
        {
   "update_linker_config",    {
   0,     0,    {
   false,  do_update_linker_config}}},
        {
   "readahead",               {
   1,     2,    {
   true,   do_readahead}}},
        {
   "remount_userdata",        {
   0,     0,    {
   false,  do_remount_userdata}}},
        {
   "restart",                 {
   1,     2,    {
   false,  do_restart}}},
        {
   "restorecon",              {
   1,     kMax, {
   true,   do_restorecon}}},
        {
   "restorecon_recursive",    {
   1,     kMax, {
   true,   do_restorecon_recursive}}},
        {
   "rm",                      {
   1,     1,    {
   true,   do_rm}}},
        {
   "rmdir",                   {
   1,     1,    {
   true,   do_rmdir}}},
        {
   "setprop",                 {
   2,     2,    {
   true,   do_setprop}}},
        {
   "setrlimit",               {
   3,     3,    {
   false,  do_setrlimit}}},
        {
   "start",                   {
   1,     1,    {
   false,  do_start}}},
        {
   "stop",                    {
   1,     1,    {
   false,  do_stop}}},
        {
   "swapon_all",              {
   0,     1,    {
   false,  do_swapon_all}}},
        {
   "enter_default_mount_ns",  {
   0,     0,    {
   false,  do_enter_default_mount_ns}}},
        {
   "symlink",                 {
   2,     2,    {
   true,   do_symlink}}},
        {
   "sysclktz",                {
   1,     1,    {
   false,  do_sysclktz}}},
        {
   "trigger",                 {
   1,     1,    {
   false,  do_trigger}}},
        {
   "verity_update_state"