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Add the ptm device to pty(4).  By opening /dev/ptm and using the PTMGET
ioctl(2), an unprivileged process may allocate a pty and have its owner
and mode set appropriately.  This means that programs such as xterm and
screen no longer need to be setuid.  Programs using the openpty()
function require zero changes and will "just work".

Designed by beck@ and deraadt@; changes by beck@ with cleanup (and
a rewrite of the vnode bits) by art@ and tweaks/bugfixes by me.
Tested by many.

/*	$OpenBSD: pty.c,v 1.11 2004/02/10 01:31:20 millert Exp $	*/
/*-
 * Copyright (c) 1990, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#if defined(LIBC_SCCS) && !defined(lint)
/* from: static char sccsid[] = "@(#)pty.c	8.1 (Berkeley) 6/4/93"; */
static const char rcsid[] = "$Id: pty.c,v 1.11 2004/02/10 01:31:20 millert Exp $";
#endif /* LIBC_SCCS and not lint */

#include <sys/cdefs.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <termios.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <grp.h>
#include <sys/tty.h>

#include "util.h"

#define TTY_LETTERS "pqrstuvwxyzPQRST"

int
openpty(amaster, aslave, name, termp, winp)
	int *amaster, *aslave;
	char *name;
	struct termios *termp;
	struct winsize *winp;
{
	char line[] = "/dev/ptyXX";
	register const char *cp1, *cp2;
	register int master, slave, ttygid;
	struct group *gr;
	struct ptmget ptm;
	int fd;

	/* Try to use /dev/ptm and the PTMGET ioctl to get a properly set up
	 * and owned pty/tty pair. If this fails, (because we might not have
	 * the ptm device, etc.) fall back to using the traditional method
	 * of walking through the pty entries in /dev for the moment, until
	 * there is less chance of people being seriously boned by running
	 * kernels without /dev/ptm in them.
	 */

	fd = open(PATH_PTMDEV, O_RDWR, 0);
	if (fd == -1)
		goto walkit;
	if ((ioctl(fd, PTMGET, &ptm) == -1)) {
		close(fd); 
		goto walkit;
	}
	close(fd);
	master = ptm.cfd;
	slave = ptm.sfd;
	if (name) {
		/*
		 * Manual page says "at least 16 characters".
		 */
		strlcpy(name, ptm.sn, 16);
	}
	*amaster = master;
	*aslave = slave;
	if (termp)
		(void) tcsetattr(slave, TCSAFLUSH, termp);
	if (winp)
		(void) ioctl(slave, TIOCSWINSZ, (char *)winp);
	return (0);
 walkit:
	if ((gr = getgrnam("tty")) != NULL)
		ttygid = gr->gr_gid;
	else
		ttygid = -1;

	for (cp1 = TTY_LETTERS; *cp1; cp1++) {
		line[8] = *cp1;
		for (cp2 = "0123456789abcdef"; *cp2; cp2++) {
			line[9] = *cp2;
			line[5] = 'p';
			if ((master = open(line, O_RDWR, 0)) == -1) {
				if (errno == ENOENT)
					return (-1);	/* out of ptys */
			} else {
				line[5] = 't';
				(void) chown(line, getuid(), ttygid);
				(void) chmod(line, S_IRUSR|S_IWUSR|S_IWGRP);
				(void) revoke(line);
				if ((slave = open(line, O_RDWR, 0)) != -1) {
					*amaster = master;
					*aslave = slave;
					if (name) {
						/*
						 * Manual page says "at least
						 * 16 characters".
						 */
						strlcpy(name, line, 16);
					}
					if (termp)
						(void) tcsetattr(slave,
						    TCSAFLUSH, termp);
					if (winp)
						(void) ioctl(slave, TIOCSWINSZ,
						    (char *)winp);
					return (0);
				}
				(void) close(master);
			}
		}
	}
	errno = ENOENT;	/* out of ptys */
	return (-1);
}

pid_t
forkpty(amaster, name, termp, winp)
	int *amaster;
	char *name;
	struct termios *termp;
	struct winsize *winp;
{
	int master, slave;
	pid_t pid;

	if (openpty(&master, &slave, name, termp, winp) == -1)
		return (-1);
	switch (pid = fork()) {
	case -1:
		return (-1);
	case 0:
		/*
		 * child
		 */
		(void) close(master);
		login_tty(slave);
		return (0);
	}
	/*
	 * parent
	 */
	*amaster = master;
	(void) close(slave);
	return (pid);
}