/* 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.
 */
//config:config CTTYHACK
//config:       bool "cttyhack (2.4 kb)"
//config:       default y
//config:       help
//config:       One common problem reported on the mailing list is the "can't
//config:       access tty; job control turned off" error message, which typically
//config:       appears when one tries to use a shell with stdin/stdout on
//config:       /dev/console.
//config:       This device is special - it cannot be a controlling tty.
//config:
//config:       The proper solution is to use the correct device instead of
//config:       /dev/console.
//config:
//config:       cttyhack provides a "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:       On Linux it also checks sysfs for a pointer to the active console.
//config:       If cttyhack is able to find the real console device, it closes
//config:       stdin/out/err and reopens that device.
//config:       Then it executes the 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:
//config:       Starting getty on a controlling tty from a shell script:
//config:
//config:       # getty 115200 $(cttyhack)

//applet:IF_CTTYHACK(APPLET_NOEXEC(cttyhack, cttyhack, BB_DIR_BIN, BB_SUID_DROP, cttyhack))

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

//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"

#include "libbb.h"

#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;

        strcpy(console, "/dev/tty");
        fd = open(console, O_RDWR);
        if (fd < 0) {
                /* We don't have ctty (or don't have "/dev/tty" node...) */
                do {
#ifdef __linux__
                        /* Note that this method does not use _stdin_.
                         * Thus, "cttyhack </dev/something" can't be used.
                         * However, this method is more reliable than
                         * TIOCGSERIAL check, which assumes that all
                         * serial lines follow /dev/ttySn convention -
                         * which is not always the case.
                         * Therefore, we use this method first:
                         */
                        int s = open_read_close("/sys/class/tty/console/active",
                                console + 5, sizeof(console) - 5);
                        if (s > 0) {
                                char *last;
                                /* Found active console via sysfs (Linux 2.6.38+).
                                 * It looks like "[tty0 ]ttyS0\n" so zap the newline:
                                 */
                                console[4 + s] = '\0';
                                /* If there are multiple consoles,
                                 * take the last one:
                                 */
                                last = strrchr(console + 5, ' ');
                                if (last)
                                        overlapping_strcpy(console + 5, last + 1);
                                break;
                        }

                        if (ioctl(0, VT_GETSTATE, &u.vt) == 0) {
                                /* this is linux virtual tty */
                                sprintf(console + 8, "S%u" + 1, (int)u.vt.v_active);
                                break;
                        }
#endif
#ifdef TIOCGSERIAL
                        if (ioctl(0, TIOCGSERIAL, &u.sr) == 0) {
                                /* this is a serial console; assuming it is named /dev/ttySn */
                                sprintf(console + 8, "S%u", (int)u.sr.line);
                                break;
                        }
#endif
                        /* nope, could not find it */
                        console[0] = '\0';
                } while (0);
        }

        argv++;
        if (!argv[0]) {
                if (!console[0])
                        return EXIT_FAILURE;
                puts(console);
                return EXIT_SUCCESS;
        }

        if (fd < 0) {
                fd = open_or_warn(console, O_RDWR);
                if (fd < 0)
                        goto ret;
        }
        //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,
         * try to steal it from them:
         */
        ioctl(0, TIOCSCTTY, 1);
 ret:
        BB_EXECVP_or_die(argv);
}