Android 9.0 init 启动流程 _Android

本篇文章主要介绍 Android 开发中的部分知识点，通过阅读本篇文章，您将收获以下内容:

一、启动流程概述
二、Android启动分析
三、init 进程启动分析
四、init 启动脚本分析
五、init 进程分析
六、init 脚本执行
七、init 进程守护
八、init rc 脚本启动Zygote
九、启动分析小结

一、启动流程概述Android启动流程跟 linux启动类似，大致分为如下五个阶段。

1.开机上电，加载固化的ROM 。
2.加载BootLoader，拉起Android OS 。
3.加载Uboot，初始外设，引导Kernel启动等。
4.启动Kernel，加载驱动，硬件。
5.启动Android，挂载分区，加载驱动、服务，init进程等。

Android系统启动大致过程如下：

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【Android 9.0 init 启动流程】Android 启动过程
由于水平有限，无法深入了理解驱动层代码，本文主要对 Android上层启动流程进行分析。
二、Android启动分析Uboot启动Kernel完成系统设置后，会首先在系统中寻找init.rc文件,并启动init进程。

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Android 启动分析
三、init 进程启动分析Init进程是Android启动的第一个进程，进程号为1，是Android的系统启动的核心进程，主要用来创建Zygote、属性服务等。init.cpp 中的main 函数，是init进程的入口函数，源码主要存在systemcoreinit目录下。
常见init.xxx.rc 进程如下：

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常见init.xxx.rc 进程
init 进程主要作用

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init 进程主要作用
/system/core/init 部分内容如下：

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/system/core/init 部分内容
main 函数主要做的事情
1.创建挂载启动所需的文件系统（tmpfs、 devpts、 proc、 sysfs、 selinuxfs等）。
2.初始化并启动属性服务
3.解析init.rc 脚本配置文件，并启动Zygote 进程。
init.cpp main 函数实现代码如下：
int main(int argc, char** argv) { ... ... if (!strcmp(basename(argv[0]), "watchdogd")) { //启动看门狗函数 return watchdogd_main(argc, argv); } ... ... //启动第一阶段 if (is_first_stage) { boot_clock::time_point start_time = boot_clock::now(); // 清理 umask. umask(0); clearenv(); setenv("PATH", _PATH_DEFPATH, 1); // 在RAM内存上获取基本的文件系统，剩余的被 rc 文件所用 mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755"); mkdir("/dev/pts", 0755); mkdir("/dev/socket", 0755); mount("devpts", "/dev/pts", "devpts", 0, NULL); #define MAKE_STR(x) __STRING(x) mount("proc", "/proc", "proc", 0, "hidepid=2,gid=" MAKE_STR(AID_READPROC)); // 非特权应用不能使用 Android 命令行 chmod("/proc/cmdline", 0440); gid_t groups[] = { AID_READPROC }; setgroups(arraysize(groups), groups); mount("sysfs", "/sys", "sysfs", 0, NULL); mount("selinuxfs", "/sys/fs/selinux", "selinuxfs", 0, NULL); mknod("/dev/kmsg", S_IFCHR | 0600, makedev(1, 11)); if constexpr (WORLD_WRITABLE_KMSG) { mknod("/dev/kmsg_debug", S_IFCHR | 0622, makedev(1, 11)); } mknod("/dev/random", S_IFCHR | 0666, makedev(1, 8)); mknod("/dev/urandom", S_IFCHR | 0666, makedev(1, 9)); // Mount staging areas for devices managed by vold // See storage config details at http://source.android.com/devices/storage/ mount("tmpfs", "/mnt", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV, "mode=0755,uid=0,gid=1000"); //创建可供读写的 vendor目录 mkdir("/mnt/vendor", 0755); // 在/dev目录下挂载好 tmpfs 以及 kmsg// 这样就可以初始化 /kernel Log 系统，供用户打印log InitKernelLogging(argv); LOG(INFO) << "init first stage started!"; if (!DoFirstStageMount()) { LOG(FATAL) << "Failed to mount required partitions early ..."; } SetInitAvbVersionInRecovery(); // Enable seccomp if global boot option was passed (otherwise it is enabled in zygote). global_seccomp(); // 优先加载selinux log系统，紧接着初始化selinux SelinuxSetupKernelLogging(); SelinuxInitialize(); // 添加 selinux 是否启动成功的log if (selinux_android_restorecon("/init", 0) == -1) { PLOG(FATAL) << "restorecon failed of /init failed"; } setenv("INIT_SECOND_STAGE", "true", 1); static constexpr uint32_t kNanosecondsPerMillisecond = 1e6; uint64_t start_ms = start_time.time_since_epoch().count() / kNanosecondsPerMillisecond; setenv("INIT_STARTED_AT", std::to_string(start_ms).c_str(), 1); char* path = argv[0]; char* args[] = { path, nullptr }; execv(path, args); // execv() only returns if an error hAppened, in which case we // panic and never fall through this conditional. PLOG(FATAL) << "execv("" << path << "") failed"; } //启动第二阶段 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. keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 1); // Indicate that booting is in progress to background fw loaders, etc. close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000)); //初始化属性 property_init(); // 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); // 清空设置的环境变量 unsetenv("INIT_SECOND_STAGE"); unsetenv("INIT_STARTED_AT"); unsetenv("INIT_SELINUX_TOOK"); unsetenv("INIT_AVB_VERSION"); // 设置第二阶段的selinux SelinuxSetupKernelLogging(); SelabelInitialize(); SelinuxRestoreContext(); //创建 epoll 句柄 epoll_fd = epoll_create1(EPOLL_CLOEXEC); if (epoll_fd == -1) { PLOG(FATAL) << "epoll_create1 failed"; } //设置子进程处理函数 sigchld_handler_init(); if (!IsRebootCapable()) { // If init does not have the CAP_SYS_BOOT capability, it is running in a container. // In that case, receiving SIGTERM will cause the system to shut down. InstallSigtermHandler(); } LoadRscRoProps(); property_load_boot_defaults(); export_oem_lock_status(); //启动属性服务 start_property_service(); //为USB存储设置udc Contorller, sys/class/udc set_usb_controller(); const BuiltinFunctionMap function_map; Action::set_function_map(&function_map); subcontexts = InitializeSubcontexts(); ActionManager& am = ActionManager::GetInstance(); ServiceList& sm = ServiceList::GetInstance(); 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(); am.QueueEventTrigger("early-init"); // Queue an action that waits for coldboot done so we know ueventd has set up all of /dev... 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(MixHwrngIntoLinuxRngAction, "MixHwrngIntoLinuxRng"); am.QueueBuiltinAction(SetMmapRndBitsAction, "SetMmapRndBits"); am.QueueBuiltinAction(SetKptrRestrictAction, "SetKptrRestrict"); am.QueueBuiltinAction(keychord_init_action, "keychord_init"); am.QueueBuiltinAction(console_init_action, "console_init"); // Trigger all the boot actions to get us started. am.QueueEventTrigger("init"); // Repeat mix_hwrng_into_linux_rng in case /dev/hw_random or /dev/random // wasn't ready immediately after wait_for_coldboot_done am.QueueBuiltinAction(MixHwrngIntoLinuxRngAction, "MixHwrngIntoLinuxRng"); // Don't mount filesystems or start core system services in charger mode. std::string bootmode = GetProperty("ro.bootmode", ""); if (bootmode == "charger") { am.QueueEventTrigger("charger"); } else { am.QueueEventTrigger("late-init"); } // Run all property triggers based on current state of the properties. am.QueueBuiltinAction(queue_property_triggers_action, "queue_property_triggers"); while (true) { // By default, sleep until something happens. int epoll_timeout_ms = -1; if (do_shutdown && !shutting_down) { do_shutdown = false; if (HandlePowerctlMessage(shutdown_command)) { shutting_down = true; } } if (!(waiting_for_prop || Service::is_exec_service_running())) { am.ExecuteOneCommand(); } if (!(waiting_for_prop || Service::is_exec_service_running())) { if (!shutting_down) { auto next_process_restart_time = RestartProcesses(); // If there's a process that needs restarting, wake up in time for that. if (next_process_restart_time) { epoll_timeout_ms = std::chrono::ceil<std::chrono::milliseconds>( *next_process_restart_time - boot_clock::now()) .count(); if (epoll_timeout_ms < 0) epoll_timeout_ms = 0; } } // If there's more work to do, wake up again immediately. if (am.HasMoreCommands()) epoll_timeout_ms = 0; } epoll_event ev; int nr = TEMP_FAILURE_RETRY(epoll_wait(epoll_fd, &ev, 1, epoll_timeout_ms)); if (nr == -1) { PLOG(ERROR) << "epoll_wait failed"; } else if (nr == 1) { ((void (*)()) ev.data.ptr)(); } } return 0;}