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    Responds to SIGUSR1 by halting the system, SIGUSR2 by powering off,
27    and SIGTERM or SIGINT reboot.
28*/
29
30#define FOR_oneit
31#include "toys.h"
32#include <sys/reboot.h>
33
34GLOBALS(
35  char *console;
36)
37
38// The minimum amount of work necessary to get ctrl-c and such to work is:
39//
40// - Fork a child (PID 1 is special: can't exit, has various signals blocked).
41// - Do a setsid() (so we have our own session).
42// - In the child, attach stdio to /dev/tty0 (/dev/console is special)
43// - Exec the rest of the command line.
44//
45// PID 1 then reaps zombies until the child process it spawned exits, at which
46// point it calls sync() and reboot().  I could stick a kill -1 in there.
47
48// Perform actions in response to signals. (Only root can send us signals.)
49static void oneit_signaled(int signal)
50{
51  int action = RB_AUTOBOOT;
52
53  toys.signal = signal;
54  if (signal == SIGUSR1) action = RB_HALT_SYSTEM;
55  if (signal == SIGUSR2) action = RB_POWER_OFF;
56
57  // PID 1 can't call reboot() because it kills the task that calls it,
58  // which causes the kernel to panic before the actual reboot happens.
59  sync();
60  if (!vfork()) reboot(action);
61}
62
63void oneit_main(void)
64{
65  int i, pid, pipes[] = {SIGUSR1, SIGUSR2, SIGTERM, SIGINT};
66
67  // Setup signal handlers for signals of interest
68  for (i = 0; i<ARRAY_LEN(pipes); i++) xsignal(pipes[i], oneit_signaled);
69
70  if (toys.optflags & FLAG_3) {
71    // Ensure next available filehandle is #3
72    while (open("/", 0) < 3);
73    close(3);
74    close(4);
75    if (pipe(pipes)) perror_exit("pipe");
76    fcntl(4, F_SETFD, FD_CLOEXEC);
77  }
78
79  while (!toys.signal) {
80
81    // Create a new child process.
82    pid = vfork();
83    if (pid) {
84
85      // pid 1 reaps zombies until it gets its child, then halts system.
86      // We ignore the return value of write (what would we do with it?)
87      // but save it in a variable we never read to make fortify shut up.
88      // (Real problem is if pid2 never reads, write() fills pipe and blocks.)
89      while (pid != wait(&i)) if (toys.optflags & FLAG_3) i = write(4, &pid, 4);
90      if (toys.optflags & FLAG_n) continue;
91
92      oneit_signaled((toys.optflags & FLAG_p) ? SIGUSR2 : SIGTERM);
93    } else {
94      // Redirect stdio to /dev/tty0, with new session ID, so ctrl-c works.
95      setsid();
96      for (i=0; i<3; i++) {
97        close(i);
98        // Remember, O_CLOEXEC is backwards for xopen()
99        xopen(TT.console ? TT.console : "/dev/tty0", O_RDWR|O_CLOEXEC);
100      }
101
102      // Can't xexec() here, we vforked so we don't want to error_exit().
103      toy_exec(toys.optargs);
104      execvp(*toys.optargs, toys.optargs);
105      perror_msg("%s not in PATH=%s", *toys.optargs, getenv("PATH"));
106
107      break;
108    }
109  }
110
111  // Give reboot() time to kick in, or avoid rapid spinning if exec failed
112  sleep(5);
113  _exit(127);
114}
115