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busybox inittab

更新記錄

item note
20150917 第一版

目錄

inittab 表格 格式如下

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- Format for each entry: <id>:<runlevels>:<action>:<process>
<id>: busybox是用來指定ttyx，目前無用到
<runlevels>: busybox是不支援此功能，目前無用到 
<action>: Valid actions include: sysinit, respawn, askfirst, wait, once,restart, ctrlaltdel, and shutdown.

實際板端範例如下 (/etc/inittab)

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# Example of how to put a getty on a serial line (for a terminal)
::respawn:/sbin/getty -L ttyS000 115200 vt100 -n root -I "Auto login as root ..."

# Stuff to do when restarting the init process
::restart:/sbin/init

# Stuff to do before rebooting
::ctrlaltdel:/sbin/reboot
::shutdown:/bin/umount -a -r
::shutdown:/sbin/swapoff -a

以init為第1個process,其它的都是由init長出來的

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# ps
  PID USER       VSZ STAT COMMAND
    1 root      3176 S    init
    2 root         0 SW   [kthreadd]
    3 root         0 SW   [ksoftirqd/0]
    4 root         0 SW   [kworker/0:0]
    5 root         0 SW   [kworker/u:0]
    6 root         0 SW   [migration/0]
    7 root         0 SW   [migration/1]
    8 root         0 SW   [kworker/1:0]
    9 root         0 SW   [ksoftirqd/1]
   10 root         0 SW<  [khelper]
   11 root         0 SW   [kdevtmpfs]
   12 root         0 SW   [kworker/u:1]
  173 root         0 SW   [sync_supers]

手動reboot程序如下

  • 行號2是由run_shutdown_and_kill_processes呼叫run_actions(SHUTDOWN)
  • 行號3-5是由run_shutdown_and_kill_processes打印出來的
  • run_actions 跑的是init_action_list內容，此內容由/etc/inittab建立
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# reboot
# umount: devtmpfs busy - remounted read-only
The system is going down NOW!
Sent SIGTERM to all processes
Sent SIGKILL toRestarting system.

busybox reboot程序

reboot(SIGTERM)訊號由halt_reboot_pwoff處里

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int init_main(int argc UNUSED_PARAM, char **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);
  • halt_reboot_pwoff 呼叫 run_shutdown_and_kill_processes打印出reboot訊息
  • halt_reboot_pwoff 最後呼叫 pause_and_low_level_reboot(RB_AUTOBOOT) //RB_AUTOBOOT = 0x01234567,
  • pause_and_low_level_reboot:
    執行reboot(magic); 
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/* The SIGUSR[12]/SIGTERM handler */
static void halt_reboot_pwoff(int sig) NORETURN;
{
	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 */

run_shutdown_and_kill_processes抓印出訊息

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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 */
}

kernel reboot訊息

  • kernel使用LINUX_REBOOT_CMD_RESTART對照由busybox傳來的(RB_AUTOBOOT = 0x01234567) reboot 訊息
  • 此時kernel會執行 kernel_restart(NULL);
  • 先執行 kernel_restart_prepare此時會call每個device的shutdown (如spi shutdown)
  • 最後machine_restart(NULL)，設定cpu register 軟啟動

SYSCALL_DEFINE4

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include/linux/reboot.h:#define  LINUX_REBOOT_CMD_RESTART        0x01234567

kernel/sys.c

/*
 * 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;

        mutex_lock(&reboot_mutex);
        switch (cmd) {
        case LINUX_REBOOT_CMD_RESTART:
                kernel_restart(NULL);
                break;

kernel_restart_prepare

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void kernel_restart_prepare(char *cmd)
{
        blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, cmd);
        system_state = SYSTEM_RESTART;
        usermodehelper_disable();
        device_shutdown();
        syscore_shutdown();
}

/**
 *      kernel_restart - reboot the system
 *      @cmd: pointer to buffer containing command to execute for restart
 *              or %NULL
 *
 *      Shutdown everything and perform a clean reboot.
 *      This is not safe to call in interrupt context.
 */
void kernel_restart(char *cmd)
{
        kernel_restart_prepare(cmd);
        if (!cmd)
                printk(KERN_EMERG "Restarting system.\n");
        else
                printk(KERN_EMERG "Restarting system with command '%s'.\n", cmd);
        kmsg_dump(KMSG_DUMP_RESTART);
        machine_restart(cmd);
}
EXPORT_SYMBOL_GPL(kernel_restart);

其它說明

/etc/inittab

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# cat /etc/inittab 
# /etc/inittab init(8) configuration for BusyBox
#
# Copyright (C) 1999-2004 by Erik Andersen <andersen@codepoet.org>
#
#
# Note, BusyBox init doesn't support runlevels.  The runlevels field is
# completely ignored by BusyBox init. If you want runlevels, use sysvinit.
#
#
# Format for each entry: <id>:<runlevels>:<action>:<process>
#
# <id>: WARNING: This field has a non-traditional meaning for BusyBox init!
#
#       The id field is used by BusyBox init to specify the controlling tty for
#       the specified process to run on.  The contents of this field are
#       appended to "/dev/" and used as-is.  There is no need for this field to
#       be unique, although if it isn't you may have strange results.  If this
#       field is left blank, it is completely ignored.  Also note that if
#       BusyBox detects that a serial console is in use, then all entries
#       containing non-empty id fields will _not_ be run.  BusyBox init does
#       nothing with utmp.  We don't need no stinkin' utmp.
#
# <runlevels>: The runlevels field is completely ignored.
#
# <action>: Valid actions include: sysinit, respawn, askfirst, wait, once,
#                                  restart, ctrlaltdel, and shutdown.
#
#       Note: askfirst acts just like respawn, but before running the specified
#       process it displays the line "Please press Enter to activate this
#       console." and then waits for the user to press enter before starting
#       the specified process.
#
#       Note: unrecognised actions (like initdefault) will cause init to emit
#       an error message, and then go along with its business.
#
# <process>: Specifies the process to be executed and it's command line.
#
# Note: BusyBox init works just fine without an inittab. If no inittab is
# found, it has the following default behavior:
#         ::sysinit:/etc/init.d/rcS
#         ::askfirst:/bin/sh
#         ::ctrlaltdel:/sbin/reboot
#         ::shutdown:/sbin/swapoff -a
#         ::shutdown:/bin/umount -a -r
#         ::restart:/sbin/init
#
# if it detects that /dev/console is _not_ a serial console, it will
# also run:
#         tty2::askfirst:/bin/sh
#         tty3::askfirst:/bin/sh
#         tty4::askfirst:/bin/sh
#
# Boot-time system configuration/initialization script.
# This is run first except when booting in single-user mode.

#
::sysinit:/etc/init.d/rcS

# /bin/sh invocations on selected ttys
#
# Note below that we prefix the shell commands with a "-" to indicate to the
# shell that it is supposed to be a login shell.  Normally this is handled by
# login, but since we are bypassing login in this case, BusyBox lets you do
# this yourself...
#
# Start an "askfirst" shell on the console (whatever that may be)
#::askfirst:-/bin/sh
# Start an "askfirst" shell on /dev/tty2-4
# tty2::askfirst:-/bin/sh
# tty3::askfirst:-/bin/sh
# tty4::askfirst:-/bin/sh

# /sbin/getty invocations for selected ttys
# tty4::respawn:/sbin/getty 38400 tty5
# tty5::respawn:/sbin/getty 38400 tty6

參考來源