1 /* vi: set sw=4 ts=4: */
2 /*
3 * Mini init implementation for busybox
4 *
5 * Copyright (C) 1995, 1996 by Bruce Perens <bruce@pixar.com>.
6 * Copyright (C) 1999-2004 by Erik Andersen <andersen@codepoet.org>
7 * Adjusted by so many folks, it's impossible to keep track.
8 *
9 * Licensed under GPLv2 or later, see file LICENSE in this source tree.
10 */
11
12 //config:config INIT
13 //config: bool "init"
14 //config: default y
15 //config: select FEATURE_SYSLOG
16 //config: help
17 //config: init is the first program run when the system boots.
18 //config:
19 //config:config LINUXRC
20 //config: bool "Support running init from within an initrd (not initramfs)"
21 //config: default y
22 //config: select FEATURE_SYSLOG
23 //config: help
24 //config: Legacy support for running init under the old-style initrd. Allows
25 //config: the name linuxrc to act as init, and it doesn't assume init is PID 1.
26 //config:
27 //config: This does not apply to initramfs, which runs /init as PID 1 and
28 //config: requires no special support.
29 //config:
30 //config:config FEATURE_USE_INITTAB
31 //config: bool "Support reading an inittab file"
32 //config: default y
33 //config: depends on INIT || LINUXRC
34 //config: help
35 //config: Allow init to read an inittab file when the system boot.
36 //config:
37 //config:config FEATURE_KILL_REMOVED
38 //config: bool "Support killing processes that have been removed from inittab"
39 //config: default n
40 //config: depends on FEATURE_USE_INITTAB
41 //config: help
42 //config: When respawn entries are removed from inittab and a SIGHUP is
43 //config: sent to init, this option will make init kill the processes
44 //config: that have been removed.
45 //config:
46 //config:config FEATURE_KILL_DELAY
47 //config: int "How long to wait between TERM and KILL (0 - send TERM only)" if FEATURE_KILL_REMOVED
48 //config: range 0 1024
49 //config: default 0
50 //config: depends on FEATURE_KILL_REMOVED
51 //config: help
52 //config: With nonzero setting, init sends TERM, forks, child waits N
53 //config: seconds, sends KILL and exits. Setting it too high is unwise
54 //config: (child will hang around for too long and could actually kill
55 //config: the wrong process!)
56 //config:
57 //config:config FEATURE_INIT_SCTTY
58 //config: bool "Run commands with leading dash with controlling tty"
59 //config: default y
60 //config: depends on INIT || LINUXRC
61 //config: help
62 //config: If this option is enabled, init will try to give a controlling
63 //config: tty to any command which has leading hyphen (often it's "-/bin/sh").
64 //config: More precisely, init will do "ioctl(STDIN_FILENO, TIOCSCTTY, 0)".
65 //config: If device attached to STDIN_FILENO can be a ctty but is not yet
66 //config: a ctty for other session, it will become this process' ctty.
67 //config: This is not the traditional init behavour, but is often what you want
68 //config: in an embedded system where the console is only accessed during
69 //config: development or for maintenance.
70 //config: NB: using cttyhack applet may work better.
71 //config:
72 //config:config FEATURE_INIT_SYSLOG
73 //config: bool "Enable init to write to syslog"
74 //config: default y
75 //config: depends on INIT || LINUXRC
76 //config:
77 //config:config FEATURE_EXTRA_QUIET
78 //config: bool "Be _extra_ quiet on boot"
79 //config: default y
80 //config: depends on INIT || LINUXRC
81 //config: help
82 //config: Prevent init from logging some messages to the console during boot.
83 //config:
84 //config:config FEATURE_INIT_COREDUMPS
85 //config: bool "Support dumping core for child processes (debugging only)"
86 //config: default n # not Y because this is a debug option
87 //config: depends on INIT || LINUXRC
88 //config: help
89 //config: If this option is enabled and the file /.init_enable_core
90 //config: exists, then init will call setrlimit() to allow unlimited
91 //config: core file sizes. If this option is disabled, processes
92 //config: will not generate any core files.
93 //config:
94 //config:config INIT_TERMINAL_TYPE
95 //config: string "Initial terminal type"
96 //config: default "linux"
97 //config: depends on INIT || LINUXRC
98 //config: help
99 //config: This is the initial value set by init for the TERM environment
100 //config: variable. This variable is used by programs which make use of
101 //config: extended terminal capabilities.
102 //config:
103 //config: Note that on Linux, init attempts to detect serial terminal and
104 //config: sets TERM to "vt102" if one is found.
105 //config:
106 //config:config FEATURE_INIT_MODIFY_CMDLINE
107 //config: bool "Modify the command-line to \"init\""
108 //config: default y
109 //config: depends on INIT || LINUXRC
110 //config: help
111 //config: When launched as PID 1 and after parsing its arguments, init
112 //config: wipes all the arguments but argv[0] and rewrites argv[0] to
113 //config: contain only "init", so that its command-line appears solely as
114 //config: "init" in tools such as ps.
115 //config: If this option is set to Y, init will keep its original behavior,
116 //config: otherwise, all the arguments including argv[0] will be preserved,
117 //config: be they parsed or ignored by init.
118 //config: The original command-line used to launch init can then be
119 //config: retrieved in /proc/1/cmdline on Linux, for example.
120
121 //applet:IF_INIT(APPLET(init, BB_DIR_SBIN, BB_SUID_DROP))
122 //applet:IF_LINUXRC(APPLET_ODDNAME(linuxrc, init, BB_DIR_ROOT, BB_SUID_DROP, linuxrc))
123
124 //kbuild:lib-$(CONFIG_INIT) += init.o
125 //kbuild:lib-$(CONFIG_LINUXRC) += init.o
126
127 #define DEBUG_SEGV_HANDLER 0
128
129 #include "libbb.h"
130 #include <syslog.h>
131 #include <sys/resource.h>
132 #ifdef __linux__
133 # include <linux/vt.h>
134 # include <sys/sysinfo.h>
135 #endif
136 #include "reboot.h" /* reboot() constants */
137
138 #if DEBUG_SEGV_HANDLER
139 # undef _GNU_SOURCE
140 # define _GNU_SOURCE 1
141 # undef __USE_GNU
142 # define __USE_GNU 1
143 # include <execinfo.h>
144 # include <sys/ucontext.h>
145 #endif
146
147 /* Used only for sanitizing purposes in set_sane_term() below. On systems where
148 * the baud rate is stored in a separate field, we can safely disable them. */
149 #ifndef CBAUD
150 # define CBAUD 0
151 # define CBAUDEX 0
152 #endif
153
154 /* Was a CONFIG_xxx option. A lot of people were building
155 * not fully functional init by switching it on! */
156 #define DEBUG_INIT 0
157
158 #define CONSOLE_NAME_SIZE 32
159
160 /* Default sysinit script. */
161 #ifndef INIT_SCRIPT
162 # define INIT_SCRIPT "/etc/init.d/rcS"
163 #endif
164
165 /* Each type of actions can appear many times. They will be
166 * handled in order. RESTART is an exception, only 1st is used.
167 */
168 /* Start these actions first and wait for completion */
169 #define SYSINIT 0x01
170 /* Start these after SYSINIT and wait for completion */
171 #define WAIT 0x02
172 /* Start these after WAIT and *dont* wait for completion */
173 #define ONCE 0x04
174 /*
175 * NB: while SYSINIT/WAIT/ONCE are being processed,
176 * SIGHUP ("reread /etc/inittab") will be processed only after
177 * each group of actions. If new inittab adds, say, a SYSINIT action,
178 * it will not be run, since init is already "past SYSINIT stage".
179 */
180 /* Start these after ONCE are started, restart on exit */
181 #define RESPAWN 0x08
182 /* Like RESPAWN, but wait for <Enter> to be pressed on tty */
183 #define ASKFIRST 0x10
184 /*
185 * Start these on SIGINT, and wait for completion.
186 * Then go back to respawning RESPAWN and ASKFIRST actions.
187 * NB: kernel sends SIGINT to us if Ctrl-Alt-Del was pressed.
188 */
189 #define CTRLALTDEL 0x20
190 /*
191 * Start these before killing all processes in preparation for
192 * running RESTART actions or doing low-level halt/reboot/poweroff
193 * (initiated by SIGUSR1/SIGTERM/SIGUSR2).
194 * Wait for completion before proceeding.
195 */
196 #define SHUTDOWN 0x40
197 /*
198 * exec() on SIGQUIT. SHUTDOWN actions are started and waited for,
199 * then all processes are killed, then init exec's 1st RESTART action,
200 * replacing itself by it. If no RESTART action specified,
201 * SIGQUIT has no effect.
202 */
203 #define RESTART 0x80
204
205
206 /* A linked list of init_actions, to be read from inittab */
207 struct init_action {
208 struct init_action *next;
209 pid_t pid;
210 uint8_t action_type;
211 char terminal[CONSOLE_NAME_SIZE];
212 char command[1];
213 };
214
215 static struct init_action *init_action_list = NULL;
216
217 static const char *log_console = VC_5;
218
219 enum {
220 L_LOG = 0x1,
221 L_CONSOLE = 0x2,
222 };
223
224 /* Print a message to the specified device.
225 * "where" may be bitwise-or'd from L_LOG | L_CONSOLE
226 * NB: careful, we can be called after vfork!
227 */
228 #define dbg_message(...) do { if (DEBUG_INIT) message(__VA_ARGS__); } while (0)
229 static void message(int where, const char *fmt, ...)
230 __attribute__ ((format(printf, 2, 3)));
message(int where,const char * fmt,...)231 static void message(int where, const char *fmt, ...)
232 {
233 va_list arguments;
234 unsigned l;
235 char msg[128];
236
237 msg[0] = '\r';
238 va_start(arguments, fmt);
239 l = 1 + vsnprintf(msg + 1, sizeof(msg) - 2, fmt, arguments);
240 if (l > sizeof(msg) - 2)
241 l = sizeof(msg) - 2;
242 va_end(arguments);
243
244 #if ENABLE_FEATURE_INIT_SYSLOG
245 msg[l] = '\0';
246 if (where & L_LOG) {
247 /* Log the message to syslogd */
248 openlog(applet_name, 0, LOG_DAEMON);
249 /* don't print "\r" */
250 syslog(LOG_INFO, "%s", msg + 1);
251 closelog();
252 }
253 msg[l++] = '\n';
254 msg[l] = '\0';
255 #else
256 {
257 static int log_fd = -1;
258
259 msg[l++] = '\n';
260 msg[l] = '\0';
261 /* Take full control of the log tty, and never close it.
262 * It's mine, all mine! Muhahahaha! */
263 if (log_fd < 0) {
264 if (!log_console) {
265 log_fd = STDERR_FILENO;
266 } else {
267 log_fd = device_open(log_console, O_WRONLY | O_NONBLOCK | O_NOCTTY);
268 if (log_fd < 0) {
269 bb_error_msg("can't log to %s", log_console);
270 where = L_CONSOLE;
271 } else {
272 close_on_exec_on(log_fd);
273 }
274 }
275 }
276 if (where & L_LOG) {
277 full_write(log_fd, msg, l);
278 if (log_fd == STDERR_FILENO)
279 return; /* don't print dup messages */
280 }
281 }
282 #endif
283
284 if (where & L_CONSOLE) {
285 /* Send console messages to console so people will see them. */
286 full_write(STDERR_FILENO, msg, l);
287 }
288 }
289
console_init(void)290 static void console_init(void)
291 {
292 #ifdef VT_OPENQRY
293 int vtno;
294 #endif
295 char *s;
296
297 s = getenv("CONSOLE");
298 if (!s)
299 s = getenv("console");
300 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
301 /* BSD people say their kernels do not open fd 0,1,2; they need this: */
302 if (!s)
303 s = (char*)"/dev/console";
304 #endif
305 if (s) {
306 int fd = open(s, O_RDWR | O_NONBLOCK | O_NOCTTY);
307 if (fd >= 0) {
308 dup2(fd, STDIN_FILENO);
309 dup2(fd, STDOUT_FILENO);
310 xmove_fd(fd, STDERR_FILENO);
311 }
312 dbg_message(L_LOG, "console='%s'", s);
313 } else {
314 /* Make sure fd 0,1,2 are not closed
315 * (so that they won't be used by future opens) */
316 bb_sanitize_stdio();
317 // Users report problems
318 // /* Make sure init can't be blocked by writing to stderr */
319 // fcntl(STDERR_FILENO, F_SETFL, fcntl(STDERR_FILENO, F_GETFL) | O_NONBLOCK);
320 }
321
322 s = getenv("TERM");
323 #ifdef VT_OPENQRY
324 if (ioctl(STDIN_FILENO, VT_OPENQRY, &vtno) != 0) {
325 /* Not a linux terminal, probably serial console.
326 * Force the TERM setting to vt102
327 * if TERM is set to linux (the default) */
328 if (!s || strcmp(s, "linux") == 0)
329 putenv((char*)"TERM=vt102");
330 if (!ENABLE_FEATURE_INIT_SYSLOG)
331 log_console = NULL;
332 } else
333 #endif
334 if (!s)
335 putenv((char*)"TERM=" CONFIG_INIT_TERMINAL_TYPE);
336 }
337
338 /* Set terminal settings to reasonable defaults.
339 * NB: careful, we can be called after vfork! */
set_sane_term(void)340 static void set_sane_term(void)
341 {
342 struct termios tty;
343
344 tcgetattr(STDIN_FILENO, &tty);
345
346 /* set control chars */
347 tty.c_cc[VINTR] = 3; /* C-c */
348 tty.c_cc[VQUIT] = 28; /* C-\ */
349 tty.c_cc[VERASE] = 127; /* C-? */
350 tty.c_cc[VKILL] = 21; /* C-u */
351 tty.c_cc[VEOF] = 4; /* C-d */
352 tty.c_cc[VSTART] = 17; /* C-q */
353 tty.c_cc[VSTOP] = 19; /* C-s */
354 tty.c_cc[VSUSP] = 26; /* C-z */
355
356 #ifdef __linux__
357 /* use line discipline 0 */
358 tty.c_line = 0;
359 #endif
360
361 /* Make it be sane */
362 #ifndef CRTSCTS
363 # define CRTSCTS 0
364 #endif
365 /* added CRTSCTS to fix Debian bug 528560 */
366 tty.c_cflag &= CBAUD | CBAUDEX | CSIZE | CSTOPB | PARENB | PARODD | CRTSCTS;
367 tty.c_cflag |= CREAD | HUPCL | CLOCAL;
368
369 /* input modes */
370 tty.c_iflag = ICRNL | IXON | IXOFF;
371
372 /* output modes */
373 tty.c_oflag = OPOST | ONLCR;
374
375 /* local modes */
376 tty.c_lflag = ISIG | ICANON | ECHO | ECHOE | ECHOK | ECHOCTL | ECHOKE | IEXTEN;
377
378 tcsetattr_stdin_TCSANOW(&tty);
379 }
380
381 /* Open the new terminal device.
382 * NB: careful, we can be called after vfork! */
open_stdio_to_tty(const char * tty_name)383 static int open_stdio_to_tty(const char* tty_name)
384 {
385 /* empty tty_name means "use init's tty", else... */
386 if (tty_name[0]) {
387 int fd;
388
389 close(STDIN_FILENO);
390 /* fd can be only < 0 or 0: */
391 fd = device_open(tty_name, O_RDWR);
392 if (fd) {
393 message(L_LOG | L_CONSOLE, "can't open %s: %s",
394 tty_name, strerror(errno));
395 return 0; /* failure */
396 }
397 dup2(STDIN_FILENO, STDOUT_FILENO);
398 dup2(STDIN_FILENO, STDERR_FILENO);
399 }
400 set_sane_term();
401 return 1; /* success */
402 }
403
reset_sighandlers_and_unblock_sigs(void)404 static void reset_sighandlers_and_unblock_sigs(void)
405 {
406 bb_signals(0
407 + (1 << SIGUSR1)
408 + (1 << SIGUSR2)
409 + (1 << SIGTERM)
410 + (1 << SIGQUIT)
411 + (1 << SIGINT)
412 + (1 << SIGHUP)
413 + (1 << SIGTSTP)
414 + (1 << SIGSTOP)
415 , SIG_DFL);
416 sigprocmask_allsigs(SIG_UNBLOCK);
417 }
418
419 /* Wrapper around exec:
420 * Takes string.
421 * If chars like '>' detected, execs '[-]/bin/sh -c "exec ......."'.
422 * Otherwise splits words on whitespace, deals with leading dash,
423 * and uses plain exec().
424 * NB: careful, we can be called after vfork!
425 */
init_exec(const char * command)426 static void init_exec(const char *command)
427 {
428 /* +8 allows to write VLA sizes below more efficiently: */
429 unsigned command_size = strlen(command) + 8;
430 /* strlen(command) + strlen("exec ")+1: */
431 char buf[command_size];
432 /* strlen(command) / 2 + 4: */
433 char *cmd[command_size / 2];
434 int dash;
435
436 dash = (command[0] == '-' /* maybe? && command[1] == '/' */);
437 command += dash;
438
439 /* See if any special /bin/sh requiring characters are present */
440 if (strpbrk(command, "~`!$^&*()=|\\{}[];\"'<>?") != NULL) {
441 sprintf(buf, "exec %s", command); /* excluding "-" */
442 /* NB: LIBBB_DEFAULT_LOGIN_SHELL define has leading dash */
443 cmd[0] = (char*)(LIBBB_DEFAULT_LOGIN_SHELL + !dash);
444 cmd[1] = (char*)"-c";
445 cmd[2] = buf;
446 cmd[3] = NULL;
447 command = LIBBB_DEFAULT_LOGIN_SHELL + 1;
448 } else {
449 /* Convert command (char*) into cmd (char**, one word per string) */
450 char *word, *next;
451 int i = 0;
452 next = strcpy(buf, command - dash); /* command including "-" */
453 command = next + dash;
454 while ((word = strsep(&next, " \t")) != NULL) {
455 if (*word != '\0') { /* not two spaces/tabs together? */
456 cmd[i] = word;
457 i++;
458 }
459 }
460 cmd[i] = NULL;
461 }
462 /* If we saw leading "-", it is interactive shell.
463 * Try harder to give it a controlling tty.
464 */
465 if (ENABLE_FEATURE_INIT_SCTTY && dash) {
466 /* _Attempt_ to make stdin a controlling tty. */
467 ioctl(STDIN_FILENO, TIOCSCTTY, 0 /*only try, don't steal*/);
468 }
469 /* Here command never contains the dash, cmd[0] might */
470 BB_EXECVP(command, cmd);
471 message(L_LOG | L_CONSOLE, "can't run '%s': %s", command, strerror(errno));
472 /* returns if execvp fails */
473 }
474
475 /* Used only by run_actions */
run(const struct init_action * a)476 static pid_t run(const struct init_action *a)
477 {
478 pid_t pid;
479
480 /* Careful: don't be affected by a signal in vforked child */
481 sigprocmask_allsigs(SIG_BLOCK);
482 if (BB_MMU && (a->action_type & ASKFIRST))
483 pid = fork();
484 else
485 pid = vfork();
486 if (pid < 0)
487 message(L_LOG | L_CONSOLE, "can't fork");
488 if (pid) {
489 sigprocmask_allsigs(SIG_UNBLOCK);
490 return pid; /* Parent or error */
491 }
492
493 /* Child */
494
495 /* Reset signal handlers that were set by the parent process */
496 reset_sighandlers_and_unblock_sigs();
497
498 /* Create a new session and make ourself the process group leader */
499 setsid();
500
501 /* Open the new terminal device */
502 if (!open_stdio_to_tty(a->terminal))
503 _exit(EXIT_FAILURE);
504
505 /* NB: on NOMMU we can't wait for input in child, so
506 * "askfirst" will work the same as "respawn". */
507 if (BB_MMU && (a->action_type & ASKFIRST)) {
508 static const char press_enter[] ALIGN1 =
509 #ifdef CUSTOMIZED_BANNER
510 #include CUSTOMIZED_BANNER
511 #endif
512 "\nPlease press Enter to activate this console. ";
513 char c;
514 /*
515 * Save memory by not exec-ing anything large (like a shell)
516 * before the user wants it. This is critical if swap is not
517 * enabled and the system has low memory. Generally this will
518 * be run on the second virtual console, and the first will
519 * be allowed to start a shell or whatever an init script
520 * specifies.
521 */
522 dbg_message(L_LOG, "waiting for enter to start '%s'"
523 "(pid %d, tty '%s')\n",
524 a->command, getpid(), a->terminal);
525 full_write(STDOUT_FILENO, press_enter, sizeof(press_enter) - 1);
526 while (safe_read(STDIN_FILENO, &c, 1) == 1 && c != '\n')
527 continue;
528 }
529
530 /*
531 * When a file named /.init_enable_core exists, setrlimit is called
532 * before processes are spawned to set core file size as unlimited.
533 * This is for debugging only. Don't use this is production, unless
534 * you want core dumps lying about....
535 */
536 if (ENABLE_FEATURE_INIT_COREDUMPS) {
537 if (access("/.init_enable_core", F_OK) == 0) {
538 struct rlimit limit;
539 limit.rlim_cur = RLIM_INFINITY;
540 limit.rlim_max = RLIM_INFINITY;
541 setrlimit(RLIMIT_CORE, &limit);
542 }
543 }
544
545 /* Log the process name and args */
546 message(L_LOG, "starting pid %d, tty '%s': '%s'",
547 getpid(), a->terminal, a->command);
548
549 /* Now run it. The new program will take over this PID,
550 * so nothing further in init.c should be run. */
551 init_exec(a->command);
552 /* We're still here? Some error happened. */
553 _exit(-1);
554 }
555
mark_terminated(pid_t pid)556 static struct init_action *mark_terminated(pid_t pid)
557 {
558 struct init_action *a;
559
560 if (pid > 0) {
561 update_utmp_DEAD_PROCESS(pid);
562 for (a = init_action_list; a; a = a->next) {
563 if (a->pid == pid) {
564 a->pid = 0;
565 return a;
566 }
567 }
568 }
569 return NULL;
570 }
571
waitfor(pid_t pid)572 static void waitfor(pid_t pid)
573 {
574 /* waitfor(run(x)): protect against failed fork inside run() */
575 if (pid <= 0)
576 return;
577
578 /* Wait for any child (prevent zombies from exiting orphaned processes)
579 * but exit the loop only when specified one has exited. */
580 while (1) {
581 pid_t wpid = wait(NULL);
582 mark_terminated(wpid);
583 /* Unsafe. SIGTSTP handler might have wait'ed it already */
584 /*if (wpid == pid) break;*/
585 /* More reliable: */
586 if (kill(pid, 0))
587 break;
588 }
589 }
590
591 /* Run all commands of a particular type */
run_actions(int action_type)592 static void run_actions(int action_type)
593 {
594 struct init_action *a;
595
596 for (a = init_action_list; a; a = a->next) {
597 if (!(a->action_type & action_type))
598 continue;
599
600 if (a->action_type & (SYSINIT | WAIT | ONCE | CTRLALTDEL | SHUTDOWN)) {
601 pid_t pid = run(a);
602 if (a->action_type & (SYSINIT | WAIT | CTRLALTDEL | SHUTDOWN))
603 waitfor(pid);
604 }
605 if (a->action_type & (RESPAWN | ASKFIRST)) {
606 /* Only run stuff with pid == 0. If pid != 0,
607 * it is already running
608 */
609 if (a->pid == 0)
610 a->pid = run(a);
611 }
612 }
613 }
614
new_init_action(uint8_t action_type,const char * command,const char * cons)615 static void new_init_action(uint8_t action_type, const char *command, const char *cons)
616 {
617 struct init_action *a, **nextp;
618
619 /* Scenario:
620 * old inittab:
621 * ::shutdown:umount -a -r
622 * ::shutdown:swapoff -a
623 * new inittab:
624 * ::shutdown:swapoff -a
625 * ::shutdown:umount -a -r
626 * On reload, we must ensure entries end up in correct order.
627 * To achieve that, if we find a matching entry, we move it
628 * to the end.
629 */
630 nextp = &init_action_list;
631 while ((a = *nextp) != NULL) {
632 /* Don't enter action if it's already in the list.
633 * This prevents losing running RESPAWNs.
634 */
635 if (strcmp(a->command, command) == 0
636 && strcmp(a->terminal, cons) == 0
637 ) {
638 /* Remove from list */
639 *nextp = a->next;
640 /* Find the end of the list */
641 while (*nextp != NULL)
642 nextp = &(*nextp)->next;
643 a->next = NULL;
644 goto append;
645 }
646 nextp = &a->next;
647 }
648
649 a = xzalloc(sizeof(*a) + strlen(command));
650
651 /* Append to the end of the list */
652 append:
653 *nextp = a;
654 a->action_type = action_type;
655 strcpy(a->command, command);
656 safe_strncpy(a->terminal, cons, sizeof(a->terminal));
657 dbg_message(L_LOG | L_CONSOLE, "command='%s' action=%x tty='%s'\n",
658 a->command, a->action_type, a->terminal);
659 }
660
661 /* NOTE that if CONFIG_FEATURE_USE_INITTAB is NOT defined,
662 * then parse_inittab() simply adds in some default
663 * actions (i.e., runs INIT_SCRIPT and then starts a pair
664 * of "askfirst" shells). If CONFIG_FEATURE_USE_INITTAB
665 * _is_ defined, but /etc/inittab is missing, this
666 * results in the same set of default behaviors.
667 */
parse_inittab(void)668 static void parse_inittab(void)
669 {
670 #if ENABLE_FEATURE_USE_INITTAB
671 char *token[4];
672 parser_t *parser = config_open2("/etc/inittab", fopen_for_read);
673
674 if (parser == NULL)
675 #endif
676 {
677 /* No inittab file - set up some default behavior */
678 /* Sysinit */
679 new_init_action(SYSINIT, INIT_SCRIPT, "");
680 /* Askfirst shell on tty1-4 */
681 new_init_action(ASKFIRST, bb_default_login_shell, "");
682 //TODO: VC_1 instead of ""? "" is console -> ctty problems -> angry users
683 new_init_action(ASKFIRST, bb_default_login_shell, VC_2);
684 new_init_action(ASKFIRST, bb_default_login_shell, VC_3);
685 new_init_action(ASKFIRST, bb_default_login_shell, VC_4);
686 /* Reboot on Ctrl-Alt-Del */
687 new_init_action(CTRLALTDEL, "reboot", "");
688 /* Umount all filesystems on halt/reboot */
689 new_init_action(SHUTDOWN, "umount -a -r", "");
690 /* Swapoff on halt/reboot */
691 new_init_action(SHUTDOWN, "swapoff -a", "");
692 /* Restart init when a QUIT is received */
693 new_init_action(RESTART, "init", "");
694 return;
695 }
696
697 #if ENABLE_FEATURE_USE_INITTAB
698 /* optional_tty:ignored_runlevel:action:command
699 * Delims are not to be collapsed and need exactly 4 tokens
700 */
701 while (config_read(parser, token, 4, 0, "#:",
702 PARSE_NORMAL & ~(PARSE_TRIM | PARSE_COLLAPSE))) {
703 /* order must correspond to SYSINIT..RESTART constants */
704 static const char actions[] ALIGN1 =
705 "sysinit\0""wait\0""once\0""respawn\0""askfirst\0"
706 "ctrlaltdel\0""shutdown\0""restart\0";
707 int action;
708 char *tty = token[0];
709
710 if (!token[3]) /* less than 4 tokens */
711 goto bad_entry;
712 action = index_in_strings(actions, token[2]);
713 if (action < 0 || !token[3][0]) /* token[3]: command */
714 goto bad_entry;
715 /* turn .*TTY -> /dev/TTY */
716 if (tty[0]) {
717 tty = concat_path_file("/dev/", skip_dev_pfx(tty));
718 }
719 new_init_action(1 << action, token[3], tty);
720 if (tty[0])
721 free(tty);
722 continue;
723 bad_entry:
724 message(L_LOG | L_CONSOLE, "Bad inittab entry at line %d",
725 parser->lineno);
726 }
727 config_close(parser);
728 #endif
729 }
730
731 static void pause_and_low_level_reboot(unsigned magic) NORETURN;
pause_and_low_level_reboot(unsigned magic)732 static void pause_and_low_level_reboot(unsigned magic)
733 {
734 pid_t pid;
735
736 /* Allow time for last message to reach serial console, etc */
737 sleep(1);
738
739 /* We have to fork here, since the kernel calls do_exit(EXIT_SUCCESS)
740 * in linux/kernel/sys.c, which can cause the machine to panic when
741 * the init process exits... */
742 pid = vfork();
743 if (pid == 0) { /* child */
744 reboot(magic);
745 _exit(EXIT_SUCCESS);
746 }
747 while (1)
748 sleep(1);
749 }
750
run_shutdown_and_kill_processes(void)751 static void run_shutdown_and_kill_processes(void)
752 {
753 /* Run everything to be run at "shutdown". This is done _prior_
754 * to killing everything, in case people wish to use scripts to
755 * shut things down gracefully... */
756 run_actions(SHUTDOWN);
757
758 message(L_CONSOLE | L_LOG, "The system is going down NOW!");
759
760 /* Send signals to every process _except_ pid 1 */
761 kill(-1, SIGTERM);
762 message(L_CONSOLE | L_LOG, "Sent SIG%s to all processes", "TERM");
763 sync();
764 sleep(1);
765
766 kill(-1, SIGKILL);
767 message(L_CONSOLE, "Sent SIG%s to all processes", "KILL");
768 sync();
769 /*sleep(1); - callers take care about making a pause */
770 }
771
772 /* Signal handling by init:
773 *
774 * For process with PID==1, on entry kernel sets all signals to SIG_DFL
775 * and unmasks all signals. However, for process with PID==1,
776 * default action (SIG_DFL) on any signal is to ignore it,
777 * even for special signals SIGKILL and SIGCONT.
778 * Also, any signal can be caught or blocked.
779 * (but SIGSTOP is still handled specially, at least in 2.6.20)
780 *
781 * We install two kinds of handlers, "immediate" and "delayed".
782 *
783 * Immediate handlers execute at any time, even while, say, sysinit
784 * is running.
785 *
786 * Delayed handlers just set a flag variable. The variable is checked
787 * in the main loop and acted upon.
788 *
789 * halt/poweroff/reboot and restart have immediate handlers.
790 * They only traverse linked list of struct action's, never modify it,
791 * this should be safe to do even in signal handler. Also they
792 * never return.
793 *
794 * SIGSTOP and SIGTSTP have immediate handlers. They just wait
795 * for SIGCONT to happen.
796 *
797 * SIGHUP has a delayed handler, because modifying linked list
798 * of struct action's from a signal handler while it is manipulated
799 * by the program may be disastrous.
800 *
801 * Ctrl-Alt-Del has a delayed handler. Not a must, but allowing
802 * it to happen even somewhere inside "sysinit" would be a bit awkward.
803 *
804 * There is a tiny probability that SIGHUP and Ctrl-Alt-Del will collide
805 * and only one will be remembered and acted upon.
806 */
807
808 /* The SIGPWR/SIGUSR[12]/SIGTERM handler */
809 static void halt_reboot_pwoff(int sig) NORETURN;
halt_reboot_pwoff(int sig)810 static void halt_reboot_pwoff(int sig)
811 {
812 const char *m;
813 unsigned rb;
814
815 /* We may call run() and it unmasks signals,
816 * including the one masked inside this signal handler.
817 * Testcase which would start multiple reboot scripts:
818 * while true; do reboot; done
819 * Preventing it:
820 */
821 reset_sighandlers_and_unblock_sigs();
822
823 run_shutdown_and_kill_processes();
824
825 m = "halt";
826 rb = RB_HALT_SYSTEM;
827 if (sig == SIGTERM) {
828 m = "reboot";
829 rb = RB_AUTOBOOT;
830 } else if (sig == SIGUSR2) {
831 m = "poweroff";
832 rb = RB_POWER_OFF;
833 }
834 message(L_CONSOLE, "Requesting system %s", m);
835 pause_and_low_level_reboot(rb);
836 /* not reached */
837 }
838
839 /* Handler for QUIT - exec "restart" action,
840 * else (no such action defined) do nothing */
exec_restart_action(void)841 static void exec_restart_action(void)
842 {
843 struct init_action *a;
844
845 for (a = init_action_list; a; a = a->next) {
846 if (!(a->action_type & RESTART))
847 continue;
848
849 /* Starting from here, we won't return.
850 * Thus don't need to worry about preserving errno
851 * and such.
852 */
853
854 reset_sighandlers_and_unblock_sigs();
855
856 run_shutdown_and_kill_processes();
857
858 #ifdef RB_ENABLE_CAD
859 /* Allow Ctrl-Alt-Del to reboot the system.
860 * This is how kernel sets it up for init, we follow suit.
861 */
862 reboot(RB_ENABLE_CAD); /* misnomer */
863 #endif
864
865 if (open_stdio_to_tty(a->terminal)) {
866 dbg_message(L_CONSOLE, "Trying to re-exec %s", a->command);
867 /* Theoretically should be safe.
868 * But in practice, kernel bugs may leave
869 * unkillable processes, and wait() may block forever.
870 * Oh well. Hoping "new" init won't be too surprised
871 * by having children it didn't create.
872 */
873 //while (wait(NULL) > 0)
874 // continue;
875 init_exec(a->command);
876 }
877 /* Open or exec failed */
878 pause_and_low_level_reboot(RB_HALT_SYSTEM);
879 /* not reached */
880 }
881 }
882
883 /* The SIGSTOP/SIGTSTP handler
884 * NB: inside it, all signals except SIGCONT are masked
885 * via appropriate setup in sigaction().
886 */
stop_handler(int sig UNUSED_PARAM)887 static void stop_handler(int sig UNUSED_PARAM)
888 {
889 smallint saved_bb_got_signal;
890 int saved_errno;
891
892 saved_bb_got_signal = bb_got_signal;
893 saved_errno = errno;
894 signal(SIGCONT, record_signo);
895
896 while (1) {
897 pid_t wpid;
898
899 if (bb_got_signal == SIGCONT)
900 break;
901 /* NB: this can accidentally wait() for a process
902 * which we waitfor() elsewhere! waitfor() must have
903 * code which is resilient against this.
904 */
905 wpid = wait_any_nohang(NULL);
906 mark_terminated(wpid);
907 sleep(1);
908 }
909
910 signal(SIGCONT, SIG_DFL);
911 errno = saved_errno;
912 bb_got_signal = saved_bb_got_signal;
913 }
914
915 #if ENABLE_FEATURE_USE_INITTAB
reload_inittab(void)916 static void reload_inittab(void)
917 {
918 struct init_action *a, **nextp;
919
920 message(L_LOG, "reloading /etc/inittab");
921
922 /* Disable old entries */
923 for (a = init_action_list; a; a = a->next)
924 a->action_type = 0;
925
926 /* Append new entries, or modify existing entries
927 * (incl. setting a->action_type) if cmd and device name
928 * match new ones. End result: only entries with
929 * a->action_type == 0 are stale.
930 */
931 parse_inittab();
932
933 #if ENABLE_FEATURE_KILL_REMOVED
934 /* Kill stale entries */
935 /* Be nice and send SIGTERM first */
936 for (a = init_action_list; a; a = a->next)
937 if (a->action_type == 0 && a->pid != 0)
938 kill(a->pid, SIGTERM);
939 if (CONFIG_FEATURE_KILL_DELAY) {
940 /* NB: parent will wait in NOMMU case */
941 if ((BB_MMU ? fork() : vfork()) == 0) { /* child */
942 sleep(CONFIG_FEATURE_KILL_DELAY);
943 for (a = init_action_list; a; a = a->next)
944 if (a->action_type == 0 && a->pid != 0)
945 kill(a->pid, SIGKILL);
946 _exit(EXIT_SUCCESS);
947 }
948 }
949 #endif
950
951 /* Remove stale entries and SYSINIT entries.
952 * We never rerun SYSINIT entries anyway,
953 * removing them too saves a few bytes
954 */
955 nextp = &init_action_list;
956 while ((a = *nextp) != NULL) {
957 /*
958 * Why pid == 0 check?
959 * Process can be removed from inittab and added *later*.
960 * If we delete its entry but process still runs,
961 * duplicate is spawned when the entry is re-added.
962 */
963 if ((a->action_type & ~SYSINIT) == 0 && a->pid == 0) {
964 *nextp = a->next;
965 free(a);
966 } else {
967 nextp = &a->next;
968 }
969 }
970
971 /* Not needed: */
972 /* run_actions(RESPAWN | ASKFIRST); */
973 /* - we return to main loop, which does this automagically */
974 }
975 #endif
976
check_delayed_sigs(void)977 static int check_delayed_sigs(void)
978 {
979 int sigs_seen = 0;
980
981 while (1) {
982 smallint sig = bb_got_signal;
983
984 if (!sig)
985 return sigs_seen;
986 bb_got_signal = 0;
987 sigs_seen = 1;
988 #if ENABLE_FEATURE_USE_INITTAB
989 if (sig == SIGHUP)
990 reload_inittab();
991 #endif
992 if (sig == SIGINT)
993 run_actions(CTRLALTDEL);
994 if (sig == SIGQUIT) {
995 exec_restart_action();
996 /* returns only if no restart action defined */
997 }
998 if ((1 << sig) & (0
999 #ifdef SIGPWR
1000 + (1 << SIGPWR)
1001 #endif
1002 + (1 << SIGUSR1)
1003 + (1 << SIGUSR2)
1004 + (1 << SIGTERM)
1005 )) {
1006 halt_reboot_pwoff(sig);
1007 }
1008 }
1009 }
1010
1011 #if DEBUG_SEGV_HANDLER
1012 static
handle_sigsegv(int sig,siginfo_t * info,void * ucontext)1013 void handle_sigsegv(int sig, siginfo_t *info, void *ucontext)
1014 {
1015 long ip;
1016 ucontext_t *uc;
1017
1018 uc = ucontext;
1019 ip = uc->uc_mcontext.gregs[REG_EIP];
1020 fdprintf(2, "signal:%d address:0x%lx ip:0x%lx\n",
1021 sig,
1022 /* this is void*, but using %p would print "(null)"
1023 * even for ptrs which are not exactly 0, but, say, 0x123:
1024 */
1025 (long)info->si_addr,
1026 ip);
1027 {
1028 /* glibc extension */
1029 void *array[50];
1030 int size;
1031 size = backtrace(array, 50);
1032 backtrace_symbols_fd(array, size, 2);
1033 }
1034 for (;;) sleep(9999);
1035 }
1036 #endif
1037
sleep_much(void)1038 static void sleep_much(void)
1039 {
1040 sleep(30 * 24*60*60);
1041 }
1042
1043 int init_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
init_main(int argc UNUSED_PARAM,char ** argv)1044 int init_main(int argc UNUSED_PARAM, char **argv)
1045 {
1046 if (argv[1] && strcmp(argv[1], "-q") == 0) {
1047 return kill(1, SIGHUP);
1048 }
1049
1050 #if DEBUG_SEGV_HANDLER
1051 {
1052 struct sigaction sa;
1053 memset(&sa, 0, sizeof(sa));
1054 sa.sa_sigaction = handle_sigsegv;
1055 sa.sa_flags = SA_SIGINFO;
1056 sigaction(SIGSEGV, &sa, NULL);
1057 sigaction(SIGILL, &sa, NULL);
1058 sigaction(SIGFPE, &sa, NULL);
1059 sigaction(SIGBUS, &sa, NULL);
1060 }
1061 #endif
1062
1063 if (!DEBUG_INIT) {
1064 /* Expect to be invoked as init with PID=1 or be invoked as linuxrc */
1065 if (getpid() != 1
1066 && (!ENABLE_LINUXRC || applet_name[0] != 'l') /* not linuxrc? */
1067 ) {
1068 bb_error_msg_and_die("must be run as PID 1");
1069 }
1070 #ifdef RB_DISABLE_CAD
1071 /* Turn off rebooting via CTL-ALT-DEL - we get a
1072 * SIGINT on CAD so we can shut things down gracefully... */
1073 reboot(RB_DISABLE_CAD); /* misnomer */
1074 #endif
1075 }
1076
1077 /* If, say, xmalloc would ever die, we don't want to oops kernel
1078 * by exiting.
1079 * NB: we set die_func *after* PID 1 check and bb_show_usage.
1080 * Otherwise, for example, "init u" ("please rexec yourself"
1081 * command for sysvinit) will show help text (which isn't too bad),
1082 * *and sleep forever* (which is bad!)
1083 */
1084 die_func = sleep_much;
1085
1086 /* Figure out where the default console should be */
1087 console_init();
1088 set_sane_term();
1089 xchdir("/");
1090 setsid();
1091
1092 /* Make sure environs is set to something sane */
1093 putenv((char *) "HOME=/");
1094 putenv((char *) bb_PATH_root_path);
1095 putenv((char *) "SHELL=/bin/sh");
1096 putenv((char *) "USER=root"); /* needed? why? */
1097
1098 if (argv[1])
1099 xsetenv("RUNLEVEL", argv[1]);
1100
1101 #if !ENABLE_FEATURE_EXTRA_QUIET
1102 /* Hello world */
1103 message(L_CONSOLE | L_LOG, "init started: %s", bb_banner);
1104 #endif
1105
1106 #if 0
1107 /* It's 2013, does anyone really still depend on this? */
1108 /* If you do, consider adding swapon to sysinit actions then! */
1109 /* struct sysinfo is linux-specific */
1110 # ifdef __linux__
1111 /* Make sure there is enough memory to do something useful. */
1112 /*if (ENABLE_SWAPONOFF) - WRONG: we may have non-bbox swapon*/ {
1113 struct sysinfo info;
1114
1115 if (sysinfo(&info) == 0
1116 && (info.mem_unit ? info.mem_unit : 1) * (long long)info.totalram < 1024*1024
1117 ) {
1118 message(L_CONSOLE, "Low memory, forcing swapon");
1119 /* swapon -a requires /proc typically */
1120 new_init_action(SYSINIT, "mount -t proc proc /proc", "");
1121 /* Try to turn on swap */
1122 new_init_action(SYSINIT, "swapon -a", "");
1123 run_actions(SYSINIT); /* wait and removing */
1124 }
1125 }
1126 # endif
1127 #endif
1128
1129 /* Check if we are supposed to be in single user mode */
1130 if (argv[1]
1131 && (strcmp(argv[1], "single") == 0 || strcmp(argv[1], "-s") == 0 || LONE_CHAR(argv[1], '1'))
1132 ) {
1133 /* ??? shouldn't we set RUNLEVEL="b" here? */
1134 /* Start a shell on console */
1135 new_init_action(RESPAWN, bb_default_login_shell, "");
1136 } else {
1137 /* Not in single user mode - see what inittab says */
1138
1139 /* NOTE that if CONFIG_FEATURE_USE_INITTAB is NOT defined,
1140 * then parse_inittab() simply adds in some default
1141 * actions (i.e., INIT_SCRIPT and a pair
1142 * of "askfirst" shells) */
1143 parse_inittab();
1144 }
1145
1146 #if ENABLE_SELINUX
1147 if (getenv("SELINUX_INIT") == NULL) {
1148 int enforce = 0;
1149
1150 putenv((char*)"SELINUX_INIT=YES");
1151 if (selinux_init_load_policy(&enforce) == 0) {
1152 BB_EXECVP(argv[0], argv);
1153 } else if (enforce > 0) {
1154 /* SELinux in enforcing mode but load_policy failed */
1155 message(L_CONSOLE, "can't load SELinux Policy. "
1156 "Machine is in enforcing mode. Halting now.");
1157 return EXIT_FAILURE;
1158 }
1159 }
1160 #endif
1161
1162 if (ENABLE_FEATURE_INIT_MODIFY_CMDLINE) {
1163 /* Make the command line just say "init" - that's all, nothing else */
1164 strncpy(argv[0], "init", strlen(argv[0]));
1165 /* Wipe argv[1]-argv[N] so they don't clutter the ps listing */
1166 while (*++argv)
1167 nuke_str(*argv);
1168 }
1169
1170 /* Set up signal handlers */
1171 if (!DEBUG_INIT) {
1172 struct sigaction sa;
1173
1174 /* Stop handler must allow only SIGCONT inside itself */
1175 memset(&sa, 0, sizeof(sa));
1176 sigfillset(&sa.sa_mask);
1177 sigdelset(&sa.sa_mask, SIGCONT);
1178 sa.sa_handler = stop_handler;
1179 /* NB: sa_flags doesn't have SA_RESTART.
1180 * It must be able to interrupt wait().
1181 */
1182 sigaction_set(SIGTSTP, &sa); /* pause */
1183 /* Does not work as intended, at least in 2.6.20.
1184 * SIGSTOP is simply ignored by init:
1185 */
1186 sigaction_set(SIGSTOP, &sa); /* pause */
1187
1188 /* These signals must interrupt wait(),
1189 * setting handler without SA_RESTART flag.
1190 */
1191 bb_signals_recursive_norestart(0
1192 + (1 << SIGINT) /* Ctrl-Alt-Del */
1193 + (1 << SIGQUIT) /* re-exec another init */
1194 #ifdef SIGPWR
1195 + (1 << SIGPWR) /* halt */
1196 #endif
1197 + (1 << SIGUSR1) /* halt */
1198 + (1 << SIGTERM) /* reboot */
1199 + (1 << SIGUSR2) /* poweroff */
1200 #if ENABLE_FEATURE_USE_INITTAB
1201 + (1 << SIGHUP) /* reread /etc/inittab */
1202 #endif
1203 , record_signo);
1204 }
1205
1206 /* Now run everything that needs to be run */
1207 /* First run the sysinit command */
1208 run_actions(SYSINIT);
1209 check_delayed_sigs();
1210 /* Next run anything that wants to block */
1211 run_actions(WAIT);
1212 check_delayed_sigs();
1213 /* Next run anything to be run only once */
1214 run_actions(ONCE);
1215
1216 /* Now run the looping stuff for the rest of forever.
1217 */
1218 while (1) {
1219 int maybe_WNOHANG;
1220
1221 maybe_WNOHANG = check_delayed_sigs();
1222
1223 /* (Re)run the respawn/askfirst stuff */
1224 run_actions(RESPAWN | ASKFIRST);
1225 maybe_WNOHANG |= check_delayed_sigs();
1226
1227 /* Don't consume all CPU time - sleep a bit */
1228 sleep(1);
1229 maybe_WNOHANG |= check_delayed_sigs();
1230
1231 /* Wait for any child process(es) to exit.
1232 *
1233 * If check_delayed_sigs above reported that a signal
1234 * was caught, wait will be nonblocking. This ensures
1235 * that if SIGHUP has reloaded inittab, respawn and askfirst
1236 * actions will not be delayed until next child death.
1237 */
1238 if (maybe_WNOHANG)
1239 maybe_WNOHANG = WNOHANG;
1240 while (1) {
1241 pid_t wpid;
1242 struct init_action *a;
1243
1244 /* If signals happen _in_ the wait, they interrupt it,
1245 * bb_signals_recursive_norestart set them up that way
1246 */
1247 wpid = waitpid(-1, NULL, maybe_WNOHANG);
1248 if (wpid <= 0)
1249 break;
1250
1251 a = mark_terminated(wpid);
1252 if (a) {
1253 message(L_LOG, "process '%s' (pid %d) exited. "
1254 "Scheduling for restart.",
1255 a->command, wpid);
1256 }
1257 /* See if anyone else is waiting to be reaped */
1258 maybe_WNOHANG = WNOHANG;
1259 }
1260 } /* while (1) */
1261 }
1262
1263 //usage:#define linuxrc_trivial_usage NOUSAGE_STR
1264 //usage:#define linuxrc_full_usage ""
1265
1266 //usage:#define init_trivial_usage
1267 //usage: ""
1268 //usage:#define init_full_usage "\n\n"
1269 //usage: "Init is the first process started during boot. It never exits."
1270 //usage: IF_FEATURE_USE_INITTAB(
1271 //usage: "\n""It (re)spawns children according to /etc/inittab."
1272 //usage: )
1273 //usage: IF_NOT_FEATURE_USE_INITTAB(
1274 //usage: "\n""This version of init doesn't use /etc/inittab,"
1275 //usage: "\n""has fixed set of processed to run."
1276 //usage: )
1277 //usage:
1278 //usage:#define init_notes_usage
1279 //usage: "This version of init is designed to be run only by the kernel.\n"
1280 //usage: "\n"
1281 //usage: "BusyBox init doesn't support multiple runlevels. The runlevels field of\n"
1282 //usage: "the /etc/inittab file is completely ignored by BusyBox init. If you want\n"
1283 //usage: "runlevels, use sysvinit.\n"
1284 //usage: "\n"
1285 //usage: "BusyBox init works just fine without an inittab. If no inittab is found,\n"
1286 //usage: "it has the following default behavior:\n"
1287 //usage: "\n"
1288 //usage: " ::sysinit:/etc/init.d/rcS\n"
1289 //usage: " ::askfirst:/bin/sh\n"
1290 //usage: " ::ctrlaltdel:/sbin/reboot\n"
1291 //usage: " ::shutdown:/sbin/swapoff -a\n"
1292 //usage: " ::shutdown:/bin/umount -a -r\n"
1293 //usage: " ::restart:/sbin/init\n"
1294 //usage: " tty2::askfirst:/bin/sh\n"
1295 //usage: " tty3::askfirst:/bin/sh\n"
1296 //usage: " tty4::askfirst:/bin/sh\n"
1297 //usage: "\n"
1298 //usage: "If you choose to use an /etc/inittab file, the inittab entry format is as follows:\n"
1299 //usage: "\n"
1300 //usage: " <id>:<runlevels>:<action>:<process>\n"
1301 //usage: "\n"
1302 //usage: " <id>:\n"
1303 //usage: "\n"
1304 //usage: " WARNING: This field has a non-traditional meaning for BusyBox init!\n"
1305 //usage: " The id field is used by BusyBox init to specify the controlling tty for\n"
1306 //usage: " the specified process to run on. The contents of this field are\n"
1307 //usage: " appended to \"/dev/\" and used as-is. There is no need for this field to\n"
1308 //usage: " be unique, although if it isn't you may have strange results. If this\n"
1309 //usage: " field is left blank, then the init's stdin/out will be used.\n"
1310 //usage: "\n"
1311 //usage: " <runlevels>:\n"
1312 //usage: "\n"
1313 //usage: " The runlevels field is completely ignored.\n"
1314 //usage: "\n"
1315 //usage: " <action>:\n"
1316 //usage: "\n"
1317 //usage: " Valid actions include: sysinit, respawn, askfirst, wait,\n"
1318 //usage: " once, restart, ctrlaltdel, and shutdown.\n"
1319 //usage: "\n"
1320 //usage: " The available actions can be classified into two groups: actions\n"
1321 //usage: " that are run only once, and actions that are re-run when the specified\n"
1322 //usage: " process exits.\n"
1323 //usage: "\n"
1324 //usage: " Run only-once actions:\n"
1325 //usage: "\n"
1326 //usage: " 'sysinit' is the first item run on boot. init waits until all\n"
1327 //usage: " sysinit actions are completed before continuing. Following the\n"
1328 //usage: " completion of all sysinit actions, all 'wait' actions are run.\n"
1329 //usage: " 'wait' actions, like 'sysinit' actions, cause init to wait until\n"
1330 //usage: " the specified task completes. 'once' actions are asynchronous,\n"
1331 //usage: " therefore, init does not wait for them to complete. 'restart' is\n"
1332 //usage: " the action taken to restart the init process. By default this should\n"
1333 //usage: " simply run /sbin/init, but can be a script which runs pivot_root or it\n"
1334 //usage: " can do all sorts of other interesting things. The 'ctrlaltdel' init\n"
1335 //usage: " actions are run when the system detects that someone on the system\n"
1336 //usage: " console has pressed the CTRL-ALT-DEL key combination. Typically one\n"
1337 //usage: " wants to run 'reboot' at this point to cause the system to reboot.\n"
1338 //usage: " Finally the 'shutdown' action specifies the actions to taken when\n"
1339 //usage: " init is told to reboot. Unmounting filesystems and disabling swap\n"
1340 //usage: " is a very good here.\n"
1341 //usage: "\n"
1342 //usage: " Run repeatedly actions:\n"
1343 //usage: "\n"
1344 //usage: " 'respawn' actions are run after the 'once' actions. When a process\n"
1345 //usage: " started with a 'respawn' action exits, init automatically restarts\n"
1346 //usage: " it. Unlike sysvinit, BusyBox init does not stop processes from\n"
1347 //usage: " respawning out of control. The 'askfirst' actions acts just like\n"
1348 //usage: " respawn, except that before running the specified process it\n"
1349 //usage: " displays the line \"Please press Enter to activate this console.\"\n"
1350 //usage: " and then waits for the user to press enter before starting the\n"
1351 //usage: " specified process.\n"
1352 //usage: "\n"
1353 //usage: " Unrecognized actions (like initdefault) will cause init to emit an\n"
1354 //usage: " error message, and then go along with its business. All actions are\n"
1355 //usage: " run in the order they appear in /etc/inittab.\n"
1356 //usage: "\n"
1357 //usage: " <process>:\n"
1358 //usage: "\n"
1359 //usage: " Specifies the process to be executed and its command line.\n"
1360 //usage: "\n"
1361 //usage: "Example /etc/inittab file:\n"
1362 //usage: "\n"
1363 //usage: " # This is run first except when booting in single-user mode\n"
1364 //usage: " #\n"
1365 //usage: " ::sysinit:/etc/init.d/rcS\n"
1366 //usage: " \n"
1367 //usage: " # /bin/sh invocations on selected ttys\n"
1368 //usage: " #\n"
1369 //usage: " # Start an \"askfirst\" shell on the console (whatever that may be)\n"
1370 //usage: " ::askfirst:-/bin/sh\n"
1371 //usage: " # Start an \"askfirst\" shell on /dev/tty2-4\n"
1372 //usage: " tty2::askfirst:-/bin/sh\n"
1373 //usage: " tty3::askfirst:-/bin/sh\n"
1374 //usage: " tty4::askfirst:-/bin/sh\n"
1375 //usage: " \n"
1376 //usage: " # /sbin/getty invocations for selected ttys\n"
1377 //usage: " #\n"
1378 //usage: " tty4::respawn:/sbin/getty 38400 tty4\n"
1379 //usage: " tty5::respawn:/sbin/getty 38400 tty5\n"
1380 //usage: " \n"
1381 //usage: " \n"
1382 //usage: " # Example of how to put a getty on a serial line (for a terminal)\n"
1383 //usage: " #\n"
1384 //usage: " #::respawn:/sbin/getty -L ttyS0 9600 vt100\n"
1385 //usage: " #::respawn:/sbin/getty -L ttyS1 9600 vt100\n"
1386 //usage: " #\n"
1387 //usage: " # Example how to put a getty on a modem line\n"
1388 //usage: " #::respawn:/sbin/getty 57600 ttyS2\n"
1389 //usage: " \n"
1390 //usage: " # Stuff to do when restarting the init process\n"
1391 //usage: " ::restart:/sbin/init\n"
1392 //usage: " \n"
1393 //usage: " # Stuff to do before rebooting\n"
1394 //usage: " ::ctrlaltdel:/sbin/reboot\n"
1395 //usage: " ::shutdown:/bin/umount -a -r\n"
1396 //usage: " ::shutdown:/sbin/swapoff -a\n"
1397