1/* oneit.c - tiny init replacement to launch a single child process.
2 *
3 * Copyright 2005, 2007 by Rob Landley <rob@landley.net>.
4
5USE_ONEIT(NEWTOY(oneit, "^<1nc:p3[!pn]", TOYFLAG_SBIN))
6
7config ONEIT
8  bool "oneit"
9  default y
10  help
11    usage: oneit [-p] [-c /dev/tty0] command [...]
12
13    Simple init program that runs a single supplied command line with a
14    controlling tty (so CTRL-C can kill it).
15
16    -c	Which console device to use (/dev/console doesn't do CTRL-C, etc).
17    -p	Power off instead of rebooting when command exits.
18    -r	Restart child when it exits.
19    -3	Write 32 bit PID of each exiting reparented process to fd 3 of child.
20    	(Blocking writes, child must read to avoid eventual deadlock.)
21
22    Spawns a single child process (because PID 1 has signals blocked)
23    in its own session, reaps zombies until the child exits, then
24    reboots the system (or powers off with -p, or restarts the child with -r).
25*/
26
27#define FOR_oneit
28#include "toys.h"
29#include <sys/reboot.h>
30
31GLOBALS(
32  char *console;
33)
34
35// The minimum amount of work necessary to get ctrl-c and such to work is:
36//
37// - Fork a child (PID 1 is special: can't exit, has various signals blocked).
38// - Do a setsid() (so we have our own session).
39// - In the child, attach stdio to /dev/tty0 (/dev/console is special)
40// - Exec the rest of the command line.
41//
42// PID 1 then reaps zombies until the child process it spawned exits, at which
43// point it calls sync() and reboot().  I could stick a kill -1 in there.
44
45// Perform actions in response to signals. (Only root can send us signals.)
46static void oneit_signaled(int signal)
47{
48  int action = RB_AUTOBOOT;
49
50  toys.signal = signal;
51  if (signal == SIGUSR1) action = RB_HALT_SYSTEM;
52  if (signal == SIGUSR2) action = RB_POWER_OFF;
53
54  // PID 1 can't call reboot() because it kills the task that calls it,
55  // which causes the kernel to panic before the actual reboot happens.
56  sync();
57  if (!vfork()) reboot(action);
58}
59
60void oneit_main(void)
61{
62  int i, pid, pipes[] = {SIGUSR1, SIGUSR2, SIGTERM, SIGINT};
63
64  if (FLAG_3) {
65    // Ensure next available filehandle is #3
66    while (open("/", 0) < 3);
67    close(3);
68    close(4);
69    if (pipe(pipes)) perror_exit("pipe");
70    fcntl(4, F_SETFD, FD_CLOEXEC);
71  }
72
73  // Setup signal handlers for signals of interest
74  for (i = 0; i<ARRAY_LEN(pipes); i++) xsignal(pipes[i], oneit_signaled);
75
76  while (!toys.signal) {
77
78    // Create a new child process.
79    pid = vfork();
80    if (pid) {
81
82      // pid 1 reaps zombies until it gets its child, then halts system.
83      // We ignore the return value of write (what would we do with it?)
84      // but save it in a variable we never read to make fortify shut up.
85      // (Real problem is if pid2 never reads, write() fills pipe and blocks.)
86      while (pid != wait(&i)) if (FLAG_3) i = write(4, &pid, 4);
87      if (toys.optflags & FLAG_n) continue;
88
89      oneit_signaled((toys.optflags & FLAG_p) ? SIGUSR2 : SIGTERM);
90    } else {
91      // Redirect stdio to /dev/tty0, with new session ID, so ctrl-c works.
92      setsid();
93      for (i=0; i<3; i++) {
94        close(i);
95        // Remember, O_CLOEXEC is backwards for xopen()
96        xopen(TT.console ? TT.console : "/dev/tty0", O_RDWR|O_CLOEXEC);
97      }
98
99      // Can't xexec() here, we vforked so we don't want to error_exit().
100      toy_exec(toys.optargs);
101      execvp(*toys.optargs, toys.optargs);
102      perror_msg("%s not in PATH=%s", *toys.optargs, getenv("PATH"));
103
104      break;
105    }
106  }
107
108  // Give reboot() time to kick in, or avoid rapid spinning if exec failed
109  sleep(5);
110  _exit(127);
111}
112