基于Linux与Busybox的Reboot命令流程分析_技术

一：Busyobx层的分析

这段时间,在忙到一个项目时,需要在busybox中用到reboot命令,开始在busybox中的shell中输入reboot命令,始终如下的信息,然后就停止在那里了,无法重启...为了彻底的弄明白这个问题,我在网络上找了很久,终于有个人写的一个reboot流程分析,我就借花献佛.在这里重新分析下busybox是如何运行这个命令,同时又是如何调用到Linux内核中的mach_reset中的arch_reset，当针对不同的ARM芯片时，作为Linux内核开发和驱动开发的朋友，对于这个流程还是一定要了解的。要不，出现问题，又如何找出问题呢。忘记了reboot的打印信息了，如下：

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The system is going down NOW !!

Sending SIGTERM to all processes.

Sending SIGKILL to all processes.

Please stand by while rebooTIng the system.

RestarTIng system.

通过分析busybox1.20.0的代码可以看出在init.c中有这样一行的代码，如下：

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int init_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;

int init_main(int argc UNUSED_PARAM, char **argv)

{

staTIc const int magic[] = {

RB_HALT_SYSTEM,

RB_POWER_OFF,

RB_AUTOBOOT

};

staTIc const smallint signals[] = { SIGUSR1, SIGUSR2, SIGTERM };

......

/* struct sysinfo is linux-specific */

#ifdef __linux__

/* Make sure there is enough memory to do something useful. */

if (ENABLE_SWAPONOFF) { //是否配置了swapoff命令

struct sysinfo info;

if (sysinfo(&info) == 0

&& (info.mem_unit ? info.mem_unit : 1) * (long long)info.totalram < 1024*1024

) {

message(L_CONSOLE, "Low memory, forcing swapon");

/* swapon -a requires /proc typically */

new_init_action(SYSINIT, "mount -t proc proc /proc", "");

/* Try to turn on swap */

new_init_action(SYSINIT, "swapon -a", "");

run_actions(SYSINIT); /* wait and removing */

}

#endif

......

/* Make the command line just say "init" - thats all, nothing else */

strncpy(argv[0], "init", strlen(argv[0]));

/* Wipe argv[1]-argv[N] so they don't clutter the ps listing */

while (*++argv)

memset(*argv, 0, strlen(*argv));

/* Set up signal handlers */

if (!DEBUG_INIT) {

struct sigaction sa;

bb_signals(0

+ (1 << SIGUSR1) /* halt */

+ (1 << SIGTERM) /* reboot */

+ (1 << SIGUSR2) /* poweroff */

, halt_reboot_pwoff);//看到这个halt_reboot_pwoff

signal(SIGQUIT, restart_handler); /* re-exec another init */ //看到这个restart_handler函数，这是我们需要分析的

/* Stop handler must allow only SIGCONT inside itself */

memset(&sa, 0, sizeof(sa));

sigfillset(&sa.sa_mask);

sigdelset(&sa.sa_mask, SIGCONT);

sa.sa_handler = stop_handler;

/* NB: sa_flags doesn't have SA_RESTART.

* It must be able to interrupt wait().

sigaction_set(SIGTSTP, &sa); /* pause */

/* Does not work as intended, at least in 2.6.20.

* SIGSTOP is simply ignored by init:

sigaction_set(SIGSTOP, &sa); /* pause */

/* SIGINT (Ctrl-Alt-Del) must interrupt wait(),

* setting handler without SA_RESTART flag.

bb_signals_recursive_norestart((1 << SIGINT), record_signo);

}

......

}

单独拿出halt_reboot_pwoff和restart_handler这个函数来看看

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/* The SIGUSR[12]/SIGTERM handler */

static void halt_reboot_pwoff(int sig) NORETURN;

static void halt_reboot_pwoff(int sig)

{

const char *m;

unsigned rb;

/* We may call run() and it unmasks signals,

* including the one masked inside this signal handler.

* Testcase which would start multiple reboot scripts:

* while true; do reboot; done

* Preventing it:

reset_sighandlers_and_unblock_sigs();

run_shutdown_and_kill_processes();

m = "halt";

rb = RB_HALT_SYSTEM;

if (sig == SIGTERM) {

m = "reboot";

rb = RB_AUTOBOOT;

} else if (sig == SIGUSR2) {

m = "poweroff";

rb = RB_POWER_OFF;

}

message(L_CONSOLE, "Requesting system %s", m);

pause_and_low_level_reboot(rb);

/* not reached */

}

restart_handler函数如下：

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/* Handler for QUIT - exec "restart" action,

* else (no such action defined) do nothing */

static void restart_handler(int sig UNUSED_PARAM)

{

struct init_action *a;

for (a = init_action_list; a; a = a->next) {

if (!(a->action_type & RESTART))

continue;

/* Starting from here, we won't return.

* Thus don't need to worry about preserving errno

* and such.

reset_sighandlers_and_unblock_sigs();

run_shutdown_and_kill_processes();

#ifdef RB_ENABLE_CAD

/* Allow Ctrl-Alt-Del to reboot the system.

* This is how kernel sets it up for init, we follow suit.

reboot(RB_ENABLE_CAD); /* misnomer */

#endif

if (open_stdio_to_tty(a->terminal)) {

dbg_message(L_CONSOLE, "Trying to re-exec %s", a->command);

/* Theoretically should be safe.

* But in practice, kernel bugs may leave

* unkillable processes, and wait() may block forever.

* Oh well. Hoping "new" init won't be too surprised

* by having children it didn't create.

//while (wait(NULL) > 0)

// continue;

init_exec(a->command);

}

/* Open or exec failed */

pause_and_low_level_reboot(RB_HALT_SYSTEM);

/* not reached */

}

通过分析，我们看到他们都会有调用这两个函数：reset_sighandlers_and_unblock_sigs();以及 run_shutdown_and_kill_processes();，我们重点关注如下这个函数：

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static void run_shutdown_and_kill_processes(void)

{

/* Run everything to be run at "shutdown". This is done _prior_

* to killing everything, in case people wish to use scripts to

* shut things down gracefully... */

run_actions(SHUTDOWN);

message(L_CONSOLE | L_LOG, "The system is going down NOW!");

/* Send signals to every process _except_ pid 1 */

kill(-1, SIGTERM);

message(L_CONSOLE | L_LOG, "Sent SIG%s to all processes", "TERM");

sync();

sleep(1);

kill(-1, SIGKILL);

message(L_CONSOLE, "Sent SIG%s to all processes", "KILL");

sync();

/*sleep(1); - callers take care about making a pause */

}

嘿嘿，终于看到了上面的打印信息：The system is going down NOW !! 以及Sending SIGTERM to all processes. 同时在上面的halt_reboot_pwoff和restart_handler中都会调用这样一个函数，如下：

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static void pause_and_low_level_reboot(unsigned magic) NORETURN;

static void pause_and_low_level_reboot(unsigned magic)

{

pid_t pid;

/* Allow time for last message to reach serial console, etc */

sleep(1);

/* We have to fork here, since the kernel calls do_exit(EXIT_SUCCESS)

* in linux/kernel/sys.c, which can cause the machine to panic when

* the init process exits... */

pid = vfork();

if (pid == 0) { /* child */

reboot(magic);

_exit(EXIT_SUCCESS);

}

while (1)

sleep(1);

}

看到了吗？有一个reboot(magic)函数，对于vfork函数，请参考fork函数。这里不多说了.... 我们现在来看看reboot.h文件，如下：

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* Definitions related to the reboot() system call,

* shared between init.c and halt.c.

#include

#ifndef RB_HALT_SYSTEM

# if defined(__linux__)

# define RB_HALT_SYSTEM 0xcdef0123

# define RB_ENABLE_CAD 0x89abcdef

# define RB_DISABLE_CAD 0

# define RB_POWER_OFF 0x4321fedc

# define RB_AUTOBOOT 0x01234567

# elif defined(RB_HALT)

# define RB_HALT_SYSTEM RB_HALT

# endif

#endif

/* Stop system and switch power off if possible. */

#ifndef RB_POWER_OFF

# if defined(RB_POWERDOWN)

# define RB_POWER_OFF RB_POWERDOWN

# elif defined(__linux__)

# define RB_POWER_OFF 0x4321fedc

# else

# warning "poweroff unsupported, using halt as fallback"

# define RB_POWER_OFF RB_HALT_SYSTEM

# endif

#endif

而在linux的内核中的定义如下：

busybox和linux内核中的REBOOT的定义值是一样的。看到了没有了。这个很重要的哦，否则busybox是无法调用linux内核的reboot函数。

二：Linux内核层的分析

Linux内核是如何衔接busybox的reboot函数的呢，如下代码：

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* Reboot system call: for obvious reasons only root may call it,

* and even root needs to set up some magic numbers in the registers

* so that some mistake won't make this reboot the whole machine.

* You can also set the meaning of the ctrl-alt-del-key here.

* reboot doesn't sync: do that yourself before calling this.

SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,

void __user *, arg)

{

char buffer[256];

int ret = 0;

/* We only trust the superuser with rebooting the system. */

if (!capable(CAP_SYS_BOOT))

return -EPERM;

/* For safety, we require "magic" arguments. */

if (magic1 != LINUX_REBOOT_MAGIC1 ||

(magic2 != LINUX_REBOOT_MAGIC2 &&

magic2 != LINUX_REBOOT_MAGIC2A &&

magic2 != LINUX_REBOOT_MAGIC2B &&

magic2 != LINUX_REBOOT_MAGIC2C))

return -EINVAL;

/* Instead of trying to make the power_off code look like

* halt when pm_power_off is not set do it the easy way.

if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off)

cmd = LINUX_REBOOT_CMD_HALT;

lock_kernel();

switch (cmd) {

case LINUX_REBOOT_CMD_RESTART:

kernel_restart(NULL); //这个就是重新启动Linx的命令

break;

case LINUX_REBOOT_CMD_CAD_ON:

C_A_D = 1;

break;

case LINUX_REBOOT_CMD_CAD_OFF:

C_A_D = 0;

break;

case LINUX_REBOOT_CMD_HALT:

kernel_halt();

unlock_kernel();

do_exit(0);

panic("cannot halt");

case LINUX_REBOOT_CMD_POWER_OFF:

kernel_power_off();

unlock_kernel();

do_exit(0);

break;

case LINUX_REBOOT_CMD_RESTART2:

if (strncpy_from_user(&buffer[0], arg, sizeof(buffer) - 1) < 0) {

unlock_kernel();

return -EFAULT;

}

buffer[sizeof(buffer) - 1] = '\0';

kernel_restart(buffer);

break;

#ifdef CONFIG_KEXEC

case LINUX_REBOOT_CMD_KEXEC:

ret = kernel_kexec();

break;

#endif

#ifdef CONFIG_HIBERNATION

case LINUX_REBOOT_CMD_SW_SUSPEND:

ret = hibernate();

break;

#endif

default:

ret = -EINVAL;

break;

}

unlock_kernel();

return ret;

}

继续跟踪kernel_restart()函数，如下：

最终会调用一个machine_restart(cmd)函数，这个是跟具体的芯片有很大的关系的，我们进一步的分析如下：

看到了吗，最终是调用arch_reset来复位整个系统的。同时我们也看到了S3C2440的reset的函数如下：

在arm_pm_restart = s3c24xx_pm_restart()函数，最终也是调用arm_machine_restart(mod, cmd)来实现的。而在arm_machine_restart()函数中，最终也是调用arch_reset()函数来实现，而这个函数是在哪里呢。在S3C2440没有看到arch_reset函数的实现，因此从S3C2410中找到了如下的代码，请继续看下面的代码：

终于看到了arch_reset函数，最终是采用S3C2410或者S3C2440的WatchDog来实现reboot的命令的。大家可以想想，busybox的poweroff命令，是如何实现通过Linux系统关闭整个系统的电源呢，其实很简单，只需要实现下面的函数中的pm_power_off的回调函数即可。