Android系统开机启动流程及init进程浅析

Android系统启动概述

Android系统开机流程基于Linux系统，总体可分为三个阶段：

Boot Loader引导程序启动
Linux内核启动
Android系统启动，Launcher/app启动

启动流程如图1形象展示：

图1 Android开机启动一般性流程

图1只简单地描述了开机启动一般性流程，“正常开机”（注意，是正常模式，不是工厂模式、recovery模式）流程为：

1. 手机、TV等android设备上电或重启后，系统硬件进行相应的复位操作，然后CPU开始执行第一条指令，该指令固化在ROM或者flash中某地址处，不可更改，由芯片制造商确定，该指令的作用就是加载引导程序到RAM执行。

2. 引导程序(boot loader)，顾名思义，引导操作系统（比如Linux、Android、windows等）启动的程序，
就是在操作系统运行之前运行的一段程序，其作用是初始化硬件设备、创建存储器空间的映射等软件运行时所需要的最小环境；加载Linux内核镜像文件（本文只针对Android、Linux）到RAM中某个地址处执行，此时引导程序的控制权就交给了内核。
各家厂商都有可能自行设计boot loader，常见的有：U-Boot、RedBoot、ARMBoot等。

3. 当内核镜像文件被加载到RAM时，通过汇编编写的程序初始化硬件、堆栈、进行一些必要的环境设置等，然后调用decompress_kernel函数解压内核镜像，再调用c语言编写的start_kernel函数启动内核，内核启动时，会进行一些列初始化工作，包括：初始化调度程序、内存管理区、日期时间、缓存、中断等，再创建init内核线程，最后调用可执行程序init执行。

4. init进程启动后，创建、挂载文件系统、设备节点，解析init.rc文件，再启动各种系统守护进程，包括Android部分最重要的Zygote、ServiceManager进程，由此进入到Android系统启动部分。无论什么Linux发行版本还是Android系统，init进程都是用户空间的第一个进程(不是内核空间)，其进程号固定为1，init进程是通向Linux、Android文件系统的大门，其他用户级进程都由init进程直接、间接创建，本文主要关注init进程的来龙去脉。

启动init进程

在kernel/init/路径下，main.c文件中的start_kernel函数就是启动内核的入口，经过一些列的初始化操作后进入到rest_init：

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static noinline void __init_refok rest_init ( void ) { int pid ; rcu_scheduler_starting ( ) ; /* * We need to spawn init first so that it obtains pid 1, however * the init task will end up wanting to create kthreads, which, if * we schedule it before we create kthreadd, will OOPS. */ kernel_thread ( kernel_init , NULL , CLONE_FS | CLONE_SIGHAND ) ; numa_default_policy ( ) ; pid = kernel_thread ( kthreadd , NULL , CLONE_FS | CLONE_FILES ) ; rcu_read_lock ( ) ; kthreadd_task = find_task_by_pid_ns ( pid , & init_pid_ns ) ; rcu_read_unlock ( ) ; complete ( & kthreadd_done ) ; /* * The boot idle thread must execute schedule() * at least once to get things moving: */ init_idle_bootup_task ( current ) ; schedule_preempt_disabled ( ) ; /* Call into cpu_idle with preempt disabled */ cpu_startup_entry ( CPUHP_ONLINE ) ; }

kernel_thread函数调用do_fork创建了2号内核线程kthreadd、1号内核线程init，他们的父进程是内核0号进程，2号内核线程kthreadd作用是管理调度其他内核线程，而init内核线程通过调用init可执行程序转变成init进程，进程号还是1，kernel_thread函数第一个参数就是kernel_init函数：

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static int __ref kernel_init ( void * unused ) { kernel_init_freeable ( ) ; /* need to finish all async __init code before freeing the memory */ async_synchronize_full ( ) ; free_initmem ( ) ; mark_rodata_ro ( ) ; system_state = SYSTEM_RUNNING ; numa_default_policy ( ) ; flush_delayed_fput ( ) ; if ( ramdisk_execute_command ) { if ( ! run_init_process ( ramdisk_execute_command ) ) return 0 ; pr_err ( "Failed to execute %s\n" , ramdisk_execute_command ) ; } /* * We try each of these until one succeeds. * * The Bourne shell can be used instead of init if we are * trying to recover a really broken machine. */ if ( execute_command ) { if ( ! run_init_process ( execute_command ) ) return 0 ; pr_err ( "Failed to execute %s. Attempting defaults...\n" , execute_command ) ; } if ( ! run_init_process ( "/sbin/init" ) || ! run_init_process ( "/etc/init" ) || ! run_init_process ( "/bin/init" ) || ! run_init_process ( "/bin/sh" ) ) return 0 ; panic ( "No init found. Try passing init= option to kernel. " "See Linux Documentation/init.txt for guidance." ) ; }

第一行就是kernel_init_freeable函数：

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static noinline void __init kernel_init_freeable ( void ) { /* * Wait until kthreadd is all set-up. */ wait_for_completion ( & kthreadd_done ) ; /* Now the scheduler is fully set up and can do blocking allocations */ gfp_allowed_mask = __GFP_BITS_MASK ; /* * init can allocate pages on any node */ set_mems_allowed ( node_states [ N_MEMORY ] ) ; /* * init can run on any cpu. */ set_cpus_allowed_ptr ( current , cpu_all_mask ) ; cad_pid = task_pid ( current ) ; smp_prepare_cpus ( setup_max_cpus ) ; do_pre_smp_initcalls ( ) ; lockup_detector_init ( ) ; smp_init ( ) ; sched_init_smp ( ) ; do_basic_setup ( ) ; /* Open the /dev/console on the rootfs, this should never fail */ if ( sys_open ( ( const char __user * ) "/dev/console" , O_RDWR , 0 ) < 0 ) pr_err ( "Warning: unable to open an initial console.\n" ) ; ( void ) sys_dup ( 0 ) ; ( void ) sys_dup ( 0 ) ; /* * check if there is an early userspace init. If yes, let it do all * the work */ if ( ! ramdisk_execute_command ) ramdisk_execute_command = "/init" ; if ( sys_access ( ( const char __user * ) ramdisk_execute_command , 0 ) != 0 ) { ramdisk_execute_command = NULL ; prepare_namespace ( ) ; } /* * Ok, we have completed the initial bootup, and * we're essentially up and running. Get rid of the * initmem segments and start the user-mode stuff.. */ /* rootfs is available now, try loading default modules */ load_default_modules ( ) ; }

wait_for_completion(&kthreadd_done);

kernel_init函数在内核线程init中执行，执行前必须等待kthreadd线程建立好后才能进行，随后进行内存页分配等工作。

do_basic_setup();

在执行此函数之前，CPU、内存管理、进程管理等已经初始化完成，但是和设备相关的工作还没有展开，而此函数就是进行设备相关的初始化，重点包括初始化设备驱动程序(编译进内核的)，这由driver_init函数完成。由此可知，在前文讨论了诸多初始化，都没有涉及到驱动，一直等到内核线程init创建时才开始。

1 2	if ( ! ramdisk_execute_command ) ramdisk_execute_command = "/init" ;

一开始，ramdisk_execute_command为空，被初始化为”/init”，默认init进程的路径在根目录下

返回到kernel_init函数继续看

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if ( ramdisk_execute_command ) { if ( ! run_init_process ( ramdisk_execute_command ) ) return 0 ; pr_err ( "Failed to execute %s\n" , ramdisk_execute_command ) ; }

ramdisk_execute_command已经被赋值为”/init”不为空，调用run_init_process处理init可执行程序，run_init_process函数中的do_execve就是系统调用execve的具体实现，作用是运行可执行程序。

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if ( ! run_init_process ( "/sbin/init" ) || ! run_init_process ( "/etc/init" ) || ! run_init_process ( "/bin/init" ) || ! run_init_process ( "/bin/sh" ) ) return 0 ; panic ( "No init found. Try passing init= option to kernel. " "See Linux Documentation/init.txt for guidance." ) ;

如果根目录下没有init可执行程序，就会在”/sbin”、”/etc”、”/bin”下查找，直到找到后执行，如果都没有找到，系统尝试建立一个交互的shell命令可执行程序，让管理员尝试修复。如果sh也没有，此时Android系统启动失败，调用panic把错误提示保存到磁盘中，待重启后显示。

注：本文关于内核部分基于3.1版本所述，与2.6有差别，比如没有init_post函数，但核心内容没有太大区别。

init进程执行过程

上文通过内核启动init可执行程序，内核把控制权交到了用户空间，开始真正的Android之旅！init进程源码所在路径：

system\core\init

查看Android.mk文件，有这两句：

LOCAL_MODULE:= init // 编译后生成的模块的名字，具体是什么模块，看include关键字编译成什么

include $(BUILD_EXECUTABLE) //编译成可执行程序

init进程主要代码是init.c，标准c语言写的程序，其main函数是入口，init.c源码如下：

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int main ( int argc , char * * argv ) { int fd_count = 0 ; struct pollfd ufds [ 4 ] ; char * tmpdev ; char * debuggable ; char tmp [ 32 ] ; int property_set_fd_init = 0 ; int signal_fd_init = 0 ; int keychord_fd_init = 0 ; bool is_charger = false ; if ( ! strcmp ( basename ( argv [ 0 ] ) , "ueventd" ) ) return ueventd_main ( argc , argv ) ; if ( ! strcmp ( basename ( argv [ 0 ] ) , "watchdogd" ) ) return watchdogd_main ( argc , argv ) ; /* clear the umask */ umask ( 0 ) ; /* Get the basic filesystem setup we need put * together in the initramdisk on / and then we'll * let the rc file figure out the rest. */ mkdir ( "/dev" , 0755 ) ; mkdir ( "/proc" , 0755 ) ; mkdir ( "/sys" , 0755 ) ; mount ( "tmpfs" , "/dev" , "tmpfs" , MS_NOSUID , "mode=0755" ) ; mkdir ( "/dev/pts" , 0755 ) ; mkdir ( "/dev/socket" , 0755 ) ; mount ( "devpts" , "/dev/pts" , "devpts" , 0 , NULL ) ; mount ( "proc" , "/proc" , "proc" , 0 , NULL ) ; mount ( "sysfs" , "/sys" , "sysfs" , 0 , NULL ) ; /* indicate that booting is in progress to background fw loaders, etc */ close ( open ( "/dev/.booting" , O_WRONLY | O_CREAT , 0000 ) ) ; /* We must have some place other than / to create the * device nodes for kmsg and null, otherwise we won't * be able to remount / read-only later on. * Now that tmpfs is mounted on /dev, we can actually * talk to the outside world. */ open_devnull_stdio ( ) ; klog_init ( ) ; property_init ( ) ; get_hardware_name ( hardware , & revision ) ; process_kernel_cmdline ( ) ; if ( is_initselinux ( ) ) { union selinux_callback cb ; cb . func_log = log_callback ; selinux_set_callback ( SELINUX_CB_LOG , cb ) ; cb . func_audit = audit_callback ; selinux_set_callback ( SELINUX_CB_AUDIT , cb ) ; selinux_initialize ( ) ; /* These directories were necessarily created before initial policy load * and therefore need their security context restored to the proper value. * This must happen before /dev is populated by ueventd. */ restorecon ( "/dev" ) ; restorecon ( "/dev/socket" ) ; restorecon ( "/dev/__properties__" ) ; restorecon_recursive ( "/sys" ) ; } is_charger = ! strcmp ( bootmode , "charger" ) ; INFO ( "property init\n" ) ; property_load_boot_defaults ( ) ; INFO ( "reading config file\n" ) ; init_parse_config_file ( "/init.rc" ) ; action_for_each_trigger ( "early-init" , action_add_queue_tail ) ; queue_builtin_action ( wait_for_coldboot_done_action , "wait_for_coldboot_done" ) ; queue_builtin_action ( mix_hwrng_into_linux_rng_action , "mix_hwrng_into_linux_rng" ) ; queue_builtin_action ( keychord_init_action , "keychord_init" ) ; queue_builtin_action ( console_init_action , "console_init" ) ; /* execute all the boot actions to get us started */ action_for_each_trigger ( "init" , action_add_queue_tail ) ; /* Repeat mix_hwrng_into_linux_rng in case /dev/hw_random or /dev/random * wasn't ready immediately after wait_for_coldboot_done */ queue_builtin_action ( mix_hwrng_into_linux_rng_action , "mix_hwrng_into_linux_rng" ) ; queue_builtin_action ( property_service_init_action , "property_service_init" ) ; queue_builtin_action ( signal_init_action , "signal_init" ) ; /* Don't mount filesystems or start core system services if in charger mode. */ if ( is_charger ) { action_for_each_trigger ( "charger" , action_add_queue_tail ) ; } else { action_for_each_trigger ( "late-init" , action_add_queue_tail ) ; } /* run all property triggers based on current state of the properties */ queue_builtin_action ( queue_property_triggers_action , "queue_property_triggers" ) ; #if BOOTCHART queue_builtin_action ( bootchart_init_action , "bootchart_init" ) ; #endif for ( ; ; ) { int nr , i , timeout = - 1 ; execute_one_command ( ) ; restart_processes ( ) ; if ( ! property_set_fd_init && get_property_set_fd ( ) > 0 ) { ufds [ fd_count ] . fd = get_property_set_fd ( ) ; ufds [ fd_count ] . events = POLLIN ; ufds [ fd_count ] . revents = 0 ; fd_count ++ ; property_set_fd_init = 1 ; } if ( ! signal_fd_init && get_signal_fd ( ) > 0 ) { ufds [ fd_count ] . fd = get_signal_fd ( ) ; ufds [ fd_count ] . events = POLLIN ; ufds [ fd_count ] . revents = 0 ; fd_count ++ ; signal_fd_init = 1 ; } if ( ! keychord_fd_init && get_keychord_fd ( ) > 0 ) { ufds [ fd_count ] . fd = get_keychord_fd ( ) ; ufds [ fd_count ] . events = POLLIN ; ufds [ fd_count ] . revents = 0 ; fd_count ++ ; keychord_fd_init = 1 ; } if ( process_needs_restart ) { timeout = ( process_needs_restart - gettime ( ) ) * 1000 ; if ( timeout < 0 ) timeout = 0 ; } if ( ! action_queue_empty ( ) || cur_action ) timeout = 0 ; #if BOOTCHART if ( bootchart_count > 0 ) { if ( timeout < 0 || timeout > BOOTCHART_POLLING_MS ) timeout = BOOTCHART_POLLING_MS ; if ( bootchart_step ( ) < 0 || -- bootchart_count == 0 ) { bootchart_finish ( ) ; bootchart_count = 0 ; } } #endif nr = poll ( ufds , fd_count , timeout ) ; if ( nr <= 0 ) continue ; for ( i = 0 ; i < fd_count ; i ++ ) { if ( ufds [ i ] . revents & POLLIN ) { if ( ufds [ i ] . fd == get_property_set_fd ( ) ) handle_property_set_fd ( ) ; else if ( ufds [ i ] . fd == get_keychord_fd ( ) ) handle_keychord ( ) ; else if ( ufds [ i ] . fd == get_signal_fd ( ) ) handle_signal ( ) ; } } } return 0 ; }

init进程主要做的事情都在main函数中，前两个语句：

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if ( ! strcmp ( basename ( argv [ 0 ] ) , "ueventd" ) ) return ueventd_main ( argc , argv ) ; if ( ! strcmp ( basename ( argv [ 0 ] ) , "watchdogd" ) ) return watchdogd_main ( argc , argv ) ;

libc库函数basename去除诸如：

/sbin/ueventd

前面的斜杠和前缀，获得字符串”ueventd”，如果argv数组包含该参数，就执行ueventd_main函数。ueventd也是一个可执行程序，在system\core\init\Android.mk中：

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# Make a symlink from /sbin/ueventd and /sbin/watchdogd to /init SYMLINKS : = \ $ ( TARGET_ROOT_OUT ) / sbin / ueventd \ $ ( TARGET_ROOT_OUT ) / sbin / watchdogd

这段代码编译后生成三个可执行文件：/init、/sbin/ueventd、/sbin/watchdogd。SYMLINKS关键字确定ueventd和watchdogd可执行程序文件作为init的软链接(也称符号链接)存在，查看运行环境：

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root @ soniq_v600 : / # ls -l sbin/ - rwxr - xr - x compass radio 309844 2015 - 12 - 30 14 : 16 adbd - rwxr - xr - x compass radio

Android系统启动概述

启动init进程

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