Linux根文件系统分析之init和busybox

DIR:Main.c-init_post函数static int noinline init_post(void)
{
free_initmem();
unlock_kernel();
mark_rodata_ro();
system_state = SYSTEM_RUNNING;
numa_default_policy(); if (sys_open((const char __user *) "/dev/console", O_RDWR, ) < )
printk(KERN_WARNING "Warning: unable to open an initial console.\n"); (void) sys_dup();
(void) sys_dup(); if (ramdisk_execute_command) {
run_init_process(ramdisk_execute_command);
printk(KERN_WARNING "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) {
run_init_process(execute_command);
printk(KERN_WARNING "Failed to execute %s. Attempting "
"defaults...\n", execute_command);
}
run_init_process("/sbin/init");
run_init_process("/etc/init");
run_init_process("/bin/init");
run_init_process("/bin/sh"); panic("No init found. Try passing init= option to kernel.");
}

VFS: Mounted root (yaffs filesystem).
Freeing init memory: 140K
Warning: unable to open an initial console.
Failed to execute /linuxrc. Attempting defaults...
Kernel panic - not syncing: No init found. Try passing init= option to kernel.

查看inittab文件得知inittab格式：
Format for each entry:
# <id>:<runlevels>:<action>:<process>#id： The id field is used by BusyBox init to specify the controlling tty for the specified process to run on. 
#runlevels: The runlevels field is completely ignored.
#action: Valid actions include: sysinit, respawn, askfirst, wait, once,
# restart, ctrlaltdel, and shutdown.#process: Specifies the process to be executed and it's command line.

DIR: init.c-parse_inittab函数
/* Reboot on Ctrl-Alt-Del */
new_init_action(CTRLALTDEL, "reboot", "");
/* Umount all filesystems on halt/reboot */
new_init_action(SHUTDOWN, "umount -a -r", "");
/* Swapoff on halt/reboot */
if (ENABLE_SWAPONOFF) new_init_action(SHUTDOWN, "swapoff -a", "");
/* Prepare to restart init when a HUP is received */
new_init_action(RESTART, "init", "");
/* Askfirst shell on tty1-4 */
new_init_action(ASKFIRST, bb_default_login_shell, "");
new_init_action(ASKFIRST, bb_default_login_shell, VC_2);
new_init_action(ASKFIRST, bb_default_login_shell, VC_3);
new_init_action(ASKFIRST, bb_default_login_shell, VC_4);
/* sysinit */
new_init_action(SYSINIT, INIT_SCRIPT, "");

从默认的new_init_action反推出默认的配置文件
/* Reboot on Ctrl-Alt-Del */
new_init_action(CTRLALTDEL, "reboot", ""); ::ctrlaltdel:reboot
/* Umount all filesystems on halt/reboot */
new_init_action(SHUTDOWN, "umount -a -r", ""); ::shutdown:umount -a -r
/* Swapoff on halt/reboot */
if (ENABLE_SWAPONOFF) new_init_action(SHUTDOWN, "swapoff -a", ""); //PC机上当内存不够，就把应用程序移动到硬盘，然后让新的应用程序读入内存。在嵌入式Linux中不长用
/* Prepare to restart init when a HUP is received */
new_init_action(RESTART, "init", ""); ::restart:init
/* Askfirst shell on tty1-4 */
new_init_action(ASKFIRST, bb_default_login_shell, ""); ::askfirst:-/bin/sh
new_init_action(ASKFIRST, bb_default_login_shell, VC_2); tty2::askfirst:-/bin/sh
new_init_action(ASKFIRST, bb_default_login_shell, VC_3); tty3::askfirst:-/bin/sh
new_init_action(ASKFIRST, bb_default_login_shell, VC_4); tty4::askfirst:-/bin/sh
/* sysinit */
new_init_action(SYSINIT, INIT_SCRIPT, ""); ::sysinit:/etc/init.d/rcS

DIR:init.c-new_init_action函数static void new_init_action(int action, const char *command, const char *cons)
{
struct init_action *new_action, *a, *last; if (strcmp(cons, bb_dev_null) == && (action & ASKFIRST))
return; /* Append to the end of the list */
for (a = last = init_action_list; a; a = a->next) {
/* don't enter action if it's already in the list,
* but do overwrite existing actions */
if ((strcmp(a->command, command) == )
&& (strcmp(a->terminal, cons) == )
) {
a->action = action;
return;
}
last = a;
} new_action = xzalloc(sizeof(struct init_action));
if (last) {
last->next = new_action;
} else {
init_action_list = new_action;
}
strcpy(new_action->command, command);
new_action->action = action;
strcpy(new_action->terminal, cons);
messageD(L_LOG | L_CONSOLE, "command='%s' action=%d tty='%s'\n",
new_action->command, new_action->action, new_action->terminal);
}/* Set up a linked list of init_actions, to be read from inittab */
struct init_action {
 struct init_action *next;
 int action;
 pid_t pid;
 char command[INIT_BUFFS_SIZE];
 char terminal[CONSOLE_NAME_SIZE];
};

init_main程序简要结构： init.c->init_main-> parse_inittab run_actions(SYSINIT);
run_actions(WAIT);
run_actions(ONCE); while () {
run_actions(RESPAWN);
run_actions(ASKFIRST);
wpid = wait(NULL);
while (wpid > ) {
a->pid = ;
}
}

DIR:init.c-run_actions函数static void run_actions(int action)
{
struct init_action *a, *tmp; for (a = init_action_list; a; a = tmp) {
tmp = a->next;
if (a->action == action) {
/* a->terminal of "" means "init's console" */
if (a->terminal[] && access(a->terminal, R_OK | W_OK)) {
delete_init_action(a);
} else if (a->action & (SYSINIT | WAIT | CTRLALTDEL | SHUTDOWN | RESTART)) {
waitfor(a, );
delete_init_action(a);
} else if (a->action & ONCE) {
run(a);
delete_init_action(a);
} else if (a->action & (RESPAWN | ASKFIRST)) {
/* Only run stuff with pid==0. If they have
* a pid, that means it is still running */
if (a->pid == ) {
a->pid = run(a);
}
}
}
}
}

SYSINIT、WAIT、ONCE时机的程序运行机制 run_actions(SYSINIT);
waitfor(a, ); //执行应用程序，等待他执行完毕
run(a); //创建process子进程
BB_EXECVP(cmdpath, cmd);
waitpid(runpid, &status, );//等待它结束
delete_init_action(a); //执行完一次，就在init_action_list链表里删除 run_actions(WAIT);
waitfor(a, ); //执行应用程序，等待他执行完毕
run(a); //创建process子进程
BB_EXECVP(cmdpath, cmd);
waitpid(runpid, &status, );//等待它结束
delete_init_action(a); //执行完一次，就在init_action_list链表里删除 run_actions(ONCE);
run(a); //创建process子进程
delete_init_action(a); //执行完一次，就在init_action_list链表里删除
//可见init进程不会等待ONCE子进程执行完毕。

RESPAWN和ASKFIRST运行机制while () {
run_actions(RESPAWN);
if (a->pid == ) {
a->pid = run(a);
} run_actions(ASKFIRST);
if (a->pid == ) {
a->pid = run(a);
}
wpid = wait(NULL); //等待子进程X退出
while (wpid > ) {
a->pid = ; //子进程X退出后，就设置X的pid等于0.然后重新进入死循环，再次执行这个已经退出的子进程X。也就是说哪个子进程退出，再重新执行哪个，而不是全部再执行。
}
}

DIR:init.c-init_main函数 signal(SIGHUP, exec_signal);
signal(SIGQUIT, exec_signal);
signal(SIGUSR1, shutdown_signal);
signal(SIGUSR2, shutdown_signal);
signal(SIGINT, ctrlaltdel_signal);
signal(SIGTERM, shutdown_signal);
signal(SIGCONT, cont_handler);
signal(SIGSTOP, stop_handler);
signal(SIGTSTP, stop_handler);

static void shutdown_system(void)
{
sigset_t block_signals; /* 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); /* first disable all our signals */
sigemptyset(&block_signals);
sigaddset(&block_signals, SIGHUP);
sigaddset(&block_signals, SIGQUIT);
sigaddset(&block_signals, SIGCHLD);
sigaddset(&block_signals, SIGUSR1);
sigaddset(&block_signals, SIGUSR2);
sigaddset(&block_signals, SIGINT);
sigaddset(&block_signals, SIGTERM);
sigaddset(&block_signals, SIGCONT);
sigaddset(&block_signals, SIGSTOP);
sigaddset(&block_signals, SIGTSTP);
sigprocmask(SIG_BLOCK, &block_signals, NULL); message(L_CONSOLE | L_LOG, "The system is going down NOW!"); /* Allow Ctrl-Alt-Del to reboot system. */
init_reboot(RB_ENABLE_CAD); /* Send signals to every process _except_ pid 1 */
message(L_CONSOLE | L_LOG, "Sending SIG%s to all processes", "TERM");
kill(-, SIGTERM);
sync();
sleep(); message(L_CONSOLE | L_LOG, "Sending SIG%s to all processes", "KILL");
kill(-, SIGKILL);
sync();
sleep();
}

Building:
=========The BusyBox build process is similar to the Linux kernel build: make menuconfig # This creates a file called ".config"
make # This creates the "busybox" executable
make install # or make CONFIG_PREFIX=/path/from/root installThe full list of configuration and install options is available by typing: make help