shell/cttyhack.c

/* vi: set sw=4 ts=4: */
/*
 * Copyright (c) 2007 Denys Vlasenko <vda.linux@googlemail.com>
 *
 * Licensed under GPLv2, see file LICENSE in this source tree.
 */
#include "libbb.h"

//applet:IF_CTTYHACK(APPLET(cttyhack, _BB_DIR_BIN, _BB_SUID_DROP))

//kbuild:lib-$(CONFIG_CTTYHACK) += cttyhack.o

//config:config CTTYHACK
//config:	bool "cttyhack"
//config:	default y
//config:	help
//config:	  One common problem reported on the mailing list is "can't access tty;
//config:	  job control turned off" error message which typically appears when
//config:	  one tries to use shell with stdin/stdout opened to /dev/console.
//config:	  This device is special - it cannot be a controlling tty.
//config:
//config:	  Proper solution is to use correct device instead of /dev/console.
//config:
//config:	  cttyhack provides "quick and dirty" solution to this problem.
//config:	  It analyzes stdin with various ioctls, trying to determine whether
//config:	  it is a /dev/ttyN or /dev/ttySN (virtual terminal or serial line).
//config:	  If it detects one, it closes stdin/out/err and reopens that device.
//config:	  Then it executes given program. Opening the device will make
//config:	  that device a controlling tty. This may require cttyhack
//config:	  to be a session leader.
//config:
//config:	  Example for /etc/inittab (for busybox init):
//config:
//config:	  ::respawn:/bin/cttyhack /bin/sh
//config:
//config:	  Starting an interactive shell from boot shell script:
//config:
//config:	  setsid cttyhack sh
//config:
//config:	  Giving controlling tty to shell running with PID 1:
//config:
//config:	  # exec cttyhack sh
//config:
//config:	  Without cttyhack, you need to know exact tty name,
//config:	  and do something like this:
//config:
//config:	  # exec setsid sh -c 'exec sh </dev/tty1 >/dev/tty1 2>&1'
//config:

//usage:#define cttyhack_trivial_usage
//usage:       "PROG ARGS"
//usage:#define cttyhack_full_usage "\n\n"
//usage:       "Give PROG a controlling tty if possible."
//usage:     "\nExample for /etc/inittab (for busybox init):"
//usage:     "\n	::respawn:/bin/cttyhack /bin/sh"
//usage:     "\nGiving controlling tty to shell running with PID 1:"
//usage:     "\n	$ exec cttyhack sh"
//usage:     "\nStarting interactive shell from boot shell script:"
//usage:     "\n	setsid cttyhack sh"

#if !defined(__linux__) && !defined(TIOCGSERIAL) && !ENABLE_WERROR
# warning cttyhack will not be able to detect a controlling tty on this system
#endif

/* From <linux/vt.h> */
struct vt_stat {
	unsigned short v_active;        /* active vt */
	unsigned short v_signal;        /* signal to send */
	unsigned short v_state;         /* vt bitmask */
};
enum { VT_GETSTATE = 0x5603 }; /* get global vt state info */

/* From <linux/serial.h> */
struct serial_struct {
	int	type;
	int	line;
	unsigned int	port;
	int	irq;
	int	flags;
	int	xmit_fifo_size;
	int	custom_divisor;
	int	baud_base;
	unsigned short	close_delay;
	char	io_type;
	char	reserved_char[1];
	int	hub6;
	unsigned short	closing_wait;   /* time to wait before closing */
	unsigned short	closing_wait2;  /* no longer used... */
	unsigned char	*iomem_base;
	unsigned short	iomem_reg_shift;
	unsigned int	port_high;
	unsigned long	iomap_base;	/* cookie passed into ioremap */
	int	reserved[1];
};

int cttyhack_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int cttyhack_main(int argc UNUSED_PARAM, char **argv)
{
	int fd;
	char console[sizeof(int)*3 + 16];
	union {
		struct vt_stat vt;
		struct serial_struct sr;
		char paranoia[sizeof(struct serial_struct) * 3];
	} u;

	if (!*++argv) {
		bb_show_usage();
	}

	strcpy(console, "/dev/tty");
	fd = open(console, O_RDWR);
	if (fd >= 0) {
		/* We already have ctty, nothing to do */
		close(fd);
	} else {
		/* We don't have ctty (or don't have "/dev/tty" node...) */
		if (0) {}
#ifdef TIOCGSERIAL
		else if (ioctl(0, TIOCGSERIAL, &u.sr) == 0) {
			/* this is a serial console */
			sprintf(console + 8, "S%d", u.sr.line);
		}
#endif
#ifdef __linux__
		else if (ioctl(0, VT_GETSTATE, &u.vt) == 0) {
			/* this is linux virtual tty */
			sprintf(console + 8, "S%d" + 1, u.vt.v_active);
		}
#endif
		if (console[8]) {
			fd = xopen(console, O_RDWR);
			//bb_error_msg("switching to '%s'", console);
			dup2(fd, 0);
			dup2(fd, 1);
			dup2(fd, 2);
			while (fd > 2)
				close(fd--);
			/* Some other session may have it as ctty,
			 * steal it from them:
			 */
			ioctl(0, TIOCSCTTY, 1);
		}
	}

	BB_EXECVP_or_die(argv);
}