path: root/src/exec_pty.c

/*
 * SPDX-License-Identifier: ISC
 *
 * Copyright (c) 2009-2023 Todd C. Miller <Todd.Miller@sudo.ws>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

/*
 * This is an open source non-commercial project. Dear PVS-Studio, please check it.
 * PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
 */

#include <config.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <sys/ioctl.h>

#if defined(HAVE_STDINT_H)
# include <stdint.h>
#elif defined(HAVE_INTTYPES_H)
# include <inttypes.h>
#endif
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <termios.h>		/* for struct winsize on HP-UX */

#include <sudo.h>
#include <sudo_exec.h>
#include <sudo_plugin.h>
#include <sudo_plugin_int.h>

/* Tail queue of messages to send to the monitor. */
struct monitor_message {
    TAILQ_ENTRY(monitor_message) entries;
    struct command_status cstat;
};
TAILQ_HEAD(monitor_message_list, monitor_message);
static struct monitor_message_list monitor_messages =
    TAILQ_HEAD_INITIALIZER(monitor_messages);
static unsigned int term_raw_flags;
static struct exec_closure pty_ec;

static void sync_ttysize(struct exec_closure *ec);
static void schedule_signal(struct exec_closure *ec, int signo);

/*
 * Allocate a pty if /dev/tty is a tty.
 * Fills in io_fds[SFD_USERTTY], io_fds[SFD_LEADER] and io_fds[SFD_FOLLOWER].
 * Returns the dyamically allocated pty name on success, NULL on failure.
 */
static char *
pty_setup(struct command_details *details)
{
    char *ptyname = NULL;
    debug_decl(pty_setup, SUDO_DEBUG_EXEC);

    io_fds[SFD_USERTTY] = open(_PATH_TTY, O_RDWR);
    if (io_fds[SFD_USERTTY] == -1) {
	sudo_debug_printf(SUDO_DEBUG_INFO, "%s: no %s, not allocating a pty",
	    __func__, _PATH_TTY);
	debug_return_ptr(NULL);
    }

    ptyname = get_pty(&io_fds[SFD_LEADER], &io_fds[SFD_FOLLOWER],
	details->cred.euid);
    if (ptyname == NULL)
	sudo_fatal("%s", U_("unable to allocate pty"));

    /* Add entry to utmp/utmpx? */
    if (ISSET(details->flags, CD_SET_UTMP))
	utmp_login(details->tty, ptyname, io_fds[SFD_FOLLOWER], details->utmp_user);

    /* Update tty name in command details (used by monitor, SELinux, AIX). */
    details->tty = ptyname;

    sudo_debug_printf(SUDO_DEBUG_INFO,
	"%s: %s fd %d, pty leader fd %d, pty follower fd %d",
	__func__, _PATH_TTY, io_fds[SFD_USERTTY], io_fds[SFD_LEADER],
	io_fds[SFD_FOLLOWER]);

    debug_return_str(ptyname);
}

/*
 * Restore user's terminal settings and update utmp, as needed.
 */
static void
pty_cleanup(struct exec_closure *ec, int wstatus)
{
    debug_decl(pty_cleanup, SUDO_DEBUG_EXEC);

    /* Restore terminal settings. */
    if (ec->term_raw) {
	/* Only restore the terminal if sudo is the foreground process. */
	const pid_t tcpgrp = tcgetpgrp(io_fds[SFD_USERTTY]);
	if (tcpgrp == ec->ppgrp) {
	    if (!sudo_term_restore(io_fds[SFD_USERTTY], false))
		sudo_warn("%s", U_("unable to restore terminal settings"));
	    ec->term_raw = false;
	}
    }

    /* Update utmp */
    if (ISSET(ec->details->flags, CD_SET_UTMP) && ec->ptyname != NULL)
	utmp_logout(ec->ptyname, wstatus);

    debug_return;
}

/*
 * Cleanup hook for sudo_fatal()/sudo_fatalx()
 */
static void
pty_cleanup_hook(void)
{
    pty_cleanup(&pty_ec, 0);
}

/*
 * Check whether sudo is running in the foreground.
 * Updates the foreground flag in the closure.
 * Returns 0 if there is no tty, the foreground process group ID
 * on success, or -1 on failure (tty revoked).
 */
static pid_t
check_foreground(struct exec_closure *ec)
{
    int ret = 0;
    debug_decl(check_foreground, SUDO_DEBUG_EXEC);

    if (io_fds[SFD_USERTTY] != -1) {
	if ((ret = tcgetpgrp(io_fds[SFD_USERTTY])) != -1) {
	    ec->foreground = ret == ec->ppgrp;
	}
    }
    debug_return_int(ret);
}

/*
 * Restore the terminal when sudo is resumed in response to SIGCONT.
 */
static bool
resume_terminal(struct exec_closure *ec)
{
    debug_decl(resume_terminal, SUDO_DEBUG_EXEC);

    if (check_foreground(ec) == -1) {
	/* User's tty was revoked. */
	debug_return_bool(false);
    }

    /* Update the pty settings based on the user's terminal. */
    if (!sudo_term_copy(io_fds[SFD_USERTTY], io_fds[SFD_LEADER])) {
	sudo_debug_printf(SUDO_DEBUG_ERROR|SUDO_DEBUG_ERRNO,
	    "%s: unable to copy terminal settings to pty", __func__);
	debug_return_bool(false);
    }
    sync_ttysize(ec);

    sudo_debug_printf(SUDO_DEBUG_INFO, "parent is in %s (%s -> %s)",
	ec->foreground ? "foreground" : "background",
	ec->term_raw ? "raw" : "cooked",
	ec->foreground ? "raw" : "cooked");

    if (ec->foreground) {
	/* Foreground process, set tty to raw mode. */
	ec->term_raw = sudo_term_raw(io_fds[SFD_USERTTY], term_raw_flags);
    } else {
	/* Background process, no access to tty. */
	ec->term_raw = false;
    }

    debug_return_bool(true);
}

/*
 * Suspend sudo if the underlying command is suspended.
 * Returns SIGCONT_FG if the command should be resumed in the
 * foreground or SIGCONT_BG if it is a background process.
 */
static int
suspend_sudo_pty(struct exec_closure *ec, int signo)
{
    char signame[SIG2STR_MAX];
    struct sigaction sa, osa, saved_sigcont;
    int ret = 0;
    debug_decl(suspend_sudo_pty, SUDO_DEBUG_EXEC);

    /*
     * Ignore SIGCONT when we suspend to avoid calling resume_terminal()
     * multiple times.
     */
    memset(&sa, 0, sizeof(sa));
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    sa.sa_handler = SIG_DFL;
    if (sudo_sigaction(SIGCONT, &sa, &saved_sigcont) != 0)
	sudo_warn("%s", U_("unable to set handler for SIGCONT"));

    if (sig2str(signo, signame) == -1)
	(void)snprintf(signame, sizeof(signame), "%d", signo);

    switch (signo) {
    case SIGTTOU:
    case SIGTTIN:
	/*
	 * If sudo is already the foreground process, just resume the command
	 * in the foreground.  If not, we'll suspend sudo and resume later.
	 */
	if (!ec->foreground) {
	    if (check_foreground(ec) == -1) {
		/* User's tty was revoked. */
		break;
	    }
	}
	if (ec->foreground) {
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"%s: command received SIG%s, parent running in the foregound",
		__func__, signame);
	    if (!ec->term_raw) {
		if (sudo_term_raw(io_fds[SFD_USERTTY], term_raw_flags))
		    ec->term_raw = true;
	    }
	    ret = SIGCONT_FG; /* resume command in foreground */
	    break;
	}
	FALLTHROUGH;
    case SIGSTOP:
    case SIGTSTP:
    default:
	/* Flush any remaining output and deschedule I/O events. */
	del_io_events(true);

	/* Restore original tty mode before suspending. */
	if (ec->term_raw) {
	    if (!sudo_term_restore(io_fds[SFD_USERTTY], false))
		sudo_warn("%s", U_("unable to restore terminal settings"));
	    ec->term_raw = false;
	}

	/* Log the suspend event. */
	log_suspend(ec, signo);

	/* Suspend self and continue command when we resume. */
	if (signo != SIGSTOP) {
	    if (sudo_sigaction(signo, &sa, &osa) != 0)
		sudo_warn(U_("unable to set handler for SIG%s"), signame);
	}

	/*
	 * We stop sudo's process group, even if sudo is not the process
	 * group leader.  If we only send the signal to sudo itself,
	 * the shell will not notice if it is not in monitor mode.
	 * This can happen when sudo is run from a shell script, for
	 * example.  In this case we need to signal the shell itself.
	 * If the process group leader is no longer present, we must kill
	 * the command since there will be no one to resume us.
	 */
	sudo_debug_printf(SUDO_DEBUG_INFO, "%s: killpg(%d, SIG%s) [parent]",
	    __func__, (int)ec->ppgrp, signame);
	if ((ec->ppgrp != ec->sudo_pid && kill(ec->ppgrp, 0) == -1) ||
		killpg(ec->ppgrp, signo) == -1) {
	    sudo_debug_printf(SUDO_DEBUG_ERROR,
		"%s: no parent to suspend, terminating command.", __func__);
	    terminate_command(ec->cmnd_pid, true);
	    ec->cmnd_pid = -1;
	}

	if (signo != SIGSTOP) {
	    if (sudo_sigaction(signo, &osa, NULL) != 0)
		sudo_warn(U_("unable to restore handler for SIG%s"), signame);
	}

	/* If we failed to suspend, the command is no longer running. */
	if (ec->cmnd_pid == -1)
	    break;

	/* Log the resume event. */
	log_suspend(ec, SIGCONT);

	/* Update the pty's terminal settings and restore /dev/tty settings. */
	if (!resume_terminal(ec))
	    break;

	/*
	 * We always resume the command in the foreground if sudo itself
	 * is the foreground process (and we were able to set /dev/tty to
	 * raw mode).  This helps work around poorly behaved programs that
	 * catch SIGTTOU/SIGTTIN but suspend themselves with SIGSTOP.  At
	 * worst, sudo will go into the background but upon resume the
	 * command will be runnable.  Otherwise, we can get into a
	 * situation where the command will immediately suspend itself.
	 */
	ret = ec->term_raw ? SIGCONT_FG : SIGCONT_BG;
	break;
    }

    if (sudo_sigaction(SIGCONT, &saved_sigcont, NULL) != 0)
	sudo_warn("%s", U_("unable to restore handler for SIGCONT"));

    debug_return_int(ret);
}

/*
 * SIGTTIN signal handler for read_callback that just sets a flag.
 */
static volatile sig_atomic_t got_sigttin;

static void
sigttin(int signo)
{
    got_sigttin = 1;
}

/*
 * Read an iobuf that is ready.
 */
static void
read_callback(int fd, int what, void *v)
{
    struct io_buffer *iob = v;
    struct sudo_event_base *evbase = sudo_ev_get_base(iob->revent);
    struct sigaction sa, osa;
    int saved_errno;
    ssize_t n;
    debug_decl(read_callback, SUDO_DEBUG_EXEC);

    /*
     * We ignore SIGTTIN by default but we need to handle it when reading
     * from the terminal.  A signal event won't work here because the
     * read() would be restarted, preventing the callback from running.
     */
    memset(&sa, 0, sizeof(sa));
    sigemptyset(&sa.sa_mask);
    sa.sa_handler = sigttin;
    got_sigttin = 0;
    sigaction(SIGTTIN, &sa, &osa);
    n = read(fd, iob->buf + iob->len, sizeof(iob->buf) - iob->len);
    saved_errno = errno;
    sigaction(SIGTTIN, &osa, NULL);
    errno = saved_errno;

    switch (n) {
	case -1:
	    if (got_sigttin) {
		/* Schedule SIGTTIN to be forwarded to the command. */
		schedule_signal(iob->ec, SIGTTIN);
	    }
	    if (errno == EAGAIN || errno == EINTR) {
		/* Not an error, retry later. */
		break;
	    }
	    /* Treat read error as fatal and close the fd. */
	    sudo_debug_printf(SUDO_DEBUG_ERROR,
		"error reading fd %d: %s", fd, strerror(errno));
	    FALLTHROUGH;
	case 0:
	    /* got EOF or pty has gone away */
	    if (n == 0) {
		sudo_debug_printf(SUDO_DEBUG_INFO,
		    "read EOF from fd %d", fd);
	    }
	    safe_close(fd);
	    ev_free_by_fd(evbase, fd);
	    /* If writer already consumed the buffer, close it too. */
	    if (iob->wevent != NULL && iob->off == iob->len) {
		safe_close(sudo_ev_get_fd(iob->wevent));
		ev_free_by_fd(evbase, sudo_ev_get_fd(iob->wevent));
		iob->off = iob->len = 0;
	    }
	    break;
	default:
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"read %zd bytes from fd %d", n, fd);
	    if (!iob->action(iob->buf + iob->len, (unsigned int)n, iob)) {
		terminate_command(iob->ec->cmnd_pid, true);
		iob->ec->cmnd_pid = -1;
	    }
	    iob->len += (unsigned int)n;
	    /* Disable reader if buffer is full. */
	    if (iob->len == sizeof(iob->buf))
		sudo_ev_del(evbase, iob->revent);
	    /* Enable writer now that there is new data in the buffer. */
	    if (iob->wevent != NULL) {
		if (sudo_ev_add(evbase, iob->wevent, NULL, false) == -1)
		    sudo_fatal("%s", U_("unable to add event to queue"));
	    }
	    break;
    }

    debug_return;
}

/*
 * SIGTTOU signal handler for write_callback that just sets a flag.
 */
static volatile sig_atomic_t got_sigttou;

static void
sigttou(int signo)
{
    got_sigttou = 1;
}

/*
 * Write an iobuf that is ready.
 */
static void
write_callback(int fd, int what, void *v)
{
    struct io_buffer *iob = v;
    struct sudo_event_base *evbase = sudo_ev_get_base(iob->wevent);
    struct sigaction sa, osa;
    int saved_errno;
    ssize_t n;
    debug_decl(write_callback, SUDO_DEBUG_EXEC);

    /*
     * We ignore SIGTTOU by default but we need to handle it when writing
     * to the terminal.  A signal event won't work here because the
     * write() would be restarted, preventing the callback from running.
     */
    memset(&sa, 0, sizeof(sa));
    sigemptyset(&sa.sa_mask);
    sa.sa_handler = sigttou;
    got_sigttou = 0;
    sigaction(SIGTTOU, &sa, &osa);
    n = write(fd, iob->buf + iob->off, iob->len - iob->off);
    saved_errno = errno;
    sigaction(SIGTTOU, &osa, NULL);
    errno = saved_errno;

    if (n == -1) {
	switch (errno) {
	case EPIPE:
	case ENXIO:
	case EIO:
	case EBADF:
	    /* other end of pipe closed or pty revoked */
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"unable to write %u bytes to fd %d",
		iob->len - iob->off, fd);
	    /* Close reader if there is one. */
	    if (iob->revent != NULL) {
		safe_close(sudo_ev_get_fd(iob->revent));
		ev_free_by_fd(evbase, sudo_ev_get_fd(iob->revent));
	    }
	    safe_close(fd);
	    ev_free_by_fd(evbase, fd);
	    break;
	case EINTR:
	    if (got_sigttou) {
		/* Schedule SIGTTOU to be forwarded to the command. */
		schedule_signal(iob->ec, SIGTTOU);
	    }
	    FALLTHROUGH;
	case EAGAIN:
	    /* Not an error, retry later. */
	    break;
	default:
	    /* XXX - need a way to distinguish non-exec error. */
	    iob->ec->cstat->type = CMD_ERRNO;
	    iob->ec->cstat->val = errno;
	    sudo_debug_printf(SUDO_DEBUG_ERROR,
		"error writing fd %d: %s", fd, strerror(errno));
	    sudo_ev_loopbreak(evbase);
	    break;
	}
    } else {
	sudo_debug_printf(SUDO_DEBUG_INFO,
	    "wrote %zd of %u bytes to fd %d", n, iob->len - iob->off, fd);
	iob->off += (unsigned int)n;
	/* Disable writer and reset the buffer if fully consumed. */
	if (iob->off == iob->len) {
	    iob->off = iob->len = 0;
	    sudo_ev_del(evbase, iob->wevent);
	    /* Forward the EOF from reader to writer. */
	    if (iob->revent == NULL) {
		safe_close(fd);
		ev_free_by_fd(evbase, fd);
	    }
	}
	/*
	 * Enable reader if buffer is not full but avoid reading /dev/tty
	 * if not in raw mode or the command is no longer running.
	 */
	if (iob->revent != NULL && iob->len != sizeof(iob->buf)) {
	    if (!USERTTY_EVENT(iob->revent) ||
		    (iob->ec->term_raw && iob->ec->cmnd_pid != -1)) {
		if (sudo_ev_add(evbase, iob->revent, NULL, false) == -1)
		    sudo_fatal("%s", U_("unable to add event to queue"));
	    }
	}
    }

    debug_return;
}

/*
 * We already closed the follower so reads from the leader will not block.
 */
static void
pty_finish(struct exec_closure *ec, struct command_status *cstat)
{
    int flags;
    debug_decl(pty_finish, SUDO_DEBUG_EXEC);

    /* Flush any remaining output (the plugin already got it) and free bufs. */
    if (io_fds[SFD_USERTTY] != -1) {
	flags = fcntl(io_fds[SFD_USERTTY], F_GETFL, 0);
	if (flags != -1 && ISSET(flags, O_NONBLOCK)) {
	    CLR(flags, O_NONBLOCK);
	    (void) fcntl(io_fds[SFD_USERTTY], F_SETFL, flags);
	}
    }
    del_io_events(false);
    free_io_bufs();

    /* Restore terminal settings and update utmp. */
    pty_cleanup(ec, cstat->type == CMD_WSTATUS ? cstat->val : 0);

    debug_return;
}

/*
 * Send command status to the monitor (currently just signal forwarding).
 */
static void
send_command_status(struct exec_closure *ec, int type, int val)
{
    struct monitor_message *msg;
    debug_decl(send_command_status, SUDO_DEBUG_EXEC);

    if ((msg = calloc(1, sizeof(*msg))) == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    msg->cstat.type = type;
    msg->cstat.val = val;
    TAILQ_INSERT_TAIL(&monitor_messages, msg, entries);

    if (sudo_ev_add(ec->evbase, ec->fwdchannel_event, NULL, true) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    /* Restart event loop to send the command immediately. */
    sudo_ev_loopcontinue(ec->evbase);

    debug_return;
}

/*
 * Schedule a signal to be forwarded.
 */
static void
schedule_signal(struct exec_closure *ec, int signo)
{
    debug_decl(schedule_signal, SUDO_DEBUG_EXEC);

    if (signo == 0)
	debug_return;

    if (sudo_debug_needed(SUDO_DEBUG_DIAG)) {
	char signame[SIG2STR_MAX];
	if (signo == SIGCONT_FG)
	    strlcpy(signame, "CONT_FG", sizeof(signame));
	else if (signo == SIGCONT_BG)
	    strlcpy(signame, "CONT_BG", sizeof(signame));
	else if (sig2str(signo, signame) == -1)
	    (void)snprintf(signame, sizeof(signame), "%d", signo);
	sudo_debug_printf(SUDO_DEBUG_DIAG, "scheduled SIG%s for command",
	    signame);
    }

    send_command_status(ec, CMD_SIGNO, signo);

    debug_return;
}

/*
 * Free any remaining monitor messages in the queue.
 */
static void
flush_monitor_messages(void)
{
    struct monitor_message *msg;
    debug_decl(flush_monitor_messages, SUDO_DEBUG_EXEC);

    while ((msg = TAILQ_FIRST(&monitor_messages)) != NULL) {
	TAILQ_REMOVE(&monitor_messages, msg, entries);
	free(msg);
    }

    debug_return;
}

static void
backchannel_cb(int fd, int what, void *v)
{
    struct exec_closure *ec = v;
    struct command_status cstat;
    ssize_t nread;
    debug_decl(backchannel_cb, SUDO_DEBUG_EXEC);

    /*
     * Read command status from the monitor.
     * Note that the backchannel is a *blocking* socket.
     */
    nread = recv(fd, &cstat, sizeof(cstat), MSG_WAITALL);
    switch (nread) {
    case -1:
	switch (errno) {
	case EINTR:
	case EAGAIN:
	    /* Nothing ready. */
	    break;
	default:
	    if (ec->cstat->val == CMD_INVALID) {
		ec->cstat->type = CMD_ERRNO;
		ec->cstat->val = errno;
		sudo_debug_printf(SUDO_DEBUG_ERROR,
		    "%s: failed to read command status: %s",
		    __func__, strerror(errno));
		sudo_ev_loopbreak(ec->evbase);
	    }
	    break;
	}
	break;
    case 0:
	/* EOF, monitor exited or was killed. */
	sudo_debug_printf(SUDO_DEBUG_INFO,
	    "EOF on backchannel, monitor dead?");
	if (ec->cstat->type == CMD_INVALID) {
	    /* XXX - need new CMD_ type for monitor errors. */
	    ec->cstat->type = CMD_ERRNO;
	    ec->cstat->val = ECONNRESET;
	}
	sudo_ev_loopexit(ec->evbase);
	break;
    case sizeof(cstat):
	/* Check command status. */
	switch (cstat.type) {
	case CMD_PID:
	    ec->cmnd_pid = cstat.val;
	    sudo_debug_printf(SUDO_DEBUG_INFO, "executed %s, pid %d",
		ec->details->command, (int)ec->cmnd_pid);
	    if (ISSET(ec->details->flags, CD_USE_PTRACE)) {
		/* Try to seize control of the command using ptrace(2). */
		int rc = exec_ptrace_seize(ec->cmnd_pid);
		if (rc == 0) {
		    /* There is another tracer present. */
		    CLR(ec->details->flags, CD_INTERCEPT|CD_LOG_SUBCMDS|CD_USE_PTRACE);
		} else if (rc == -1) {
		    if (ec->cstat->type == CMD_INVALID) {
			ec->cstat->type = CMD_ERRNO;
			ec->cstat->val = errno;
		    }
		    sudo_ev_loopbreak(ec->evbase);
		}
	    }
	    break;
	case CMD_WSTATUS:
	    if (WIFSTOPPED(cstat.val)) {
		int signo;

		/* Suspend parent and tell monitor how to resume on return. */
		sudo_debug_printf(SUDO_DEBUG_INFO,
		    "command stopped, suspending parent");
		signo = suspend_sudo_pty(ec, WSTOPSIG(cstat.val));
		schedule_signal(ec, signo);
		/* Re-enable I/O events */
		add_io_events(ec);
	    } else {
		/* Command exited or was killed, either way we are done. */
		sudo_debug_printf(SUDO_DEBUG_INFO, "command exited or was killed");
		sudo_ev_loopexit(ec->evbase);
		*ec->cstat = cstat;
	    }
	    break;
	case CMD_ERRNO:
	    /* Monitor was unable to execute command or broken pipe. */
	    sudo_debug_printf(SUDO_DEBUG_INFO, "errno from monitor: %s",
		strerror(cstat.val));
	    sudo_ev_loopbreak(ec->evbase);
	    *ec->cstat = cstat;
	    break;
	}
	/* Keep reading command status messages until EAGAIN or EOF. */
	break;
    default:
	/* Short read, should not happen. */
	if (ec->cstat->val == CMD_INVALID) {
	    ec->cstat->type = CMD_ERRNO;
	    ec->cstat->val = EIO;
	    sudo_debug_printf(SUDO_DEBUG_ERROR,
		"%s: failed to read command status: short read", __func__);
	    sudo_ev_loopbreak(ec->evbase);
	}
	break;
    }
    debug_return;
}

/*
 * Handle changes to the monitors's status (SIGCHLD).
 */
static void
handle_sigchld_pty(struct exec_closure *ec)
{
    int n, status;
    pid_t pid;
    debug_decl(handle_sigchld_pty, SUDO_DEBUG_EXEC);

    /* There may be multiple children in intercept mode. */
    for (;;) {
	do {
	    pid = waitpid(-1, &status, __WALL|WUNTRACED|WNOHANG);
	} while (pid == -1 && errno == EINTR);
	switch (pid) {
	case -1:
	    if (errno != ECHILD) {
		sudo_warn(U_("%s: %s"), __func__, "waitpid");
		debug_return;
	    }
	    FALLTHROUGH;
	case 0:
	    /* Nothing left to wait for. */
	    debug_return;
	}

	if (WIFEXITED(status)) {
	    sudo_debug_printf(SUDO_DEBUG_INFO, "%s: process %d exited: %d",
		__func__, (int)pid, WEXITSTATUS(status));
	    if (pid == ec->monitor_pid)
		ec->monitor_pid = -1;
	} else if (WIFSIGNALED(status)) {
	    if (sudo_debug_needed(SUDO_DEBUG_INFO)) {
		char signame[SIG2STR_MAX];
		if (sig2str(WTERMSIG(status), signame) == -1) {
		    (void)snprintf(signame, sizeof(signame), "%d",
			WTERMSIG(status));
		}
		sudo_debug_printf(SUDO_DEBUG_INFO,
		    "%s: process %d killed, SIG%s",
		    __func__, (int)pid, signame);
	    }
	    if (pid == ec->monitor_pid)
		ec->monitor_pid = -1;
	} else if (WIFSTOPPED(status)) {
	    if (pid != ec->monitor_pid) {
		if (ISSET(ec->details->flags, CD_USE_PTRACE))
		    exec_ptrace_stopped(pid, status, ec->intercept);
		continue;
	    }

	    /*
	     * If the monitor dies we get notified via backchannel_cb().
	     * If it was stopped, we should stop too (the command keeps
	     * running in its pty) and continue it when we come back.
	     */
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"monitor stopped, suspending sudo");
	    n = suspend_sudo_pty(ec, WSTOPSIG(status));
	    kill(pid, SIGCONT);
	    schedule_signal(ec, n);
	    /* Re-enable I/O events */
	    add_io_events(ec);
	} else {
	    sudo_debug_printf(SUDO_DEBUG_WARN,
		"%s: unexpected wait status 0x%x for process (%d)",
		__func__, status, (int)pid);
	}
    }
}

/* Signal callback */
static void
signal_cb_pty(int signo, int what, void *v)
{
    struct sudo_ev_siginfo_container *sc = v;
    struct exec_closure *ec = sc->closure;
    debug_decl(signal_cb_pty, SUDO_DEBUG_EXEC);

    if (ec->monitor_pid == -1)
	debug_return;

    if (sudo_debug_needed(SUDO_DEBUG_DIAG)) {
	char signame[SIG2STR_MAX];
	if (sig2str(signo, signame) == -1)
	    (void)snprintf(signame, sizeof(signame), "%d", signo);
	sudo_debug_printf(SUDO_DEBUG_DIAG,
	    "%s: evbase %p, monitor: %d, signo %s(%d), cstat %p", __func__,
	ec->evbase, (int)ec->monitor_pid, signame, signo, ec->cstat);
    }

    switch (signo) {
    case SIGCHLD:
	handle_sigchld_pty(ec);
	break;
    case SIGCONT:
	resume_terminal(ec);
	break;
    case SIGWINCH:
	sync_ttysize(ec);
	break;
    default:
	/*
	 * Do not forward signals sent by the command itself or a member of the
	 * command's process group (but only when either sudo or the command is
	 * the process group leader).  We don't want the command to indirectly
	 * kill itself.  For example, this can happen with some versions of
	 * reboot that call kill(-1, SIGTERM) to kill all other processes.
	 */
	if (USER_SIGNALED(sc->siginfo) && sc->siginfo->si_pid != 0) {
	    pid_t si_pgrp;

	    if (sc->siginfo->si_pid == ec->cmnd_pid)
		debug_return;
	    si_pgrp = getpgid(sc->siginfo->si_pid);
	    if (si_pgrp != -1) {
		if (si_pgrp == ec->cmnd_pid || si_pgrp == ec->sudo_pid)
		    debug_return;
	    }
	}
	/* Schedule signal to be forwarded to the command. */
	schedule_signal(ec, signo);
	break;
    }

    debug_return;
}

/*
 * Forward signals in monitor_messages to the monitor so it can
 * deliver them to the command.
 */
static void
fwdchannel_cb(int sock, int what, void *v)
{
    struct exec_closure *ec = v;
    struct monitor_message *msg;
    ssize_t nsent;
    debug_decl(fwdchannel_cb, SUDO_DEBUG_EXEC);

    while ((msg = TAILQ_FIRST(&monitor_messages)) != NULL) {
	switch (msg->cstat.type) {
	case CMD_SIGNO:
	    if (sudo_debug_needed(SUDO_DEBUG_INFO)) {
		char signame[SIG2STR_MAX];
		if (msg->cstat.val == SIGCONT_FG)
		    strlcpy(signame, "CONT_FG", sizeof(signame));
		else if (msg->cstat.val == SIGCONT_BG)
		    strlcpy(signame, "CONT_BG", sizeof(signame));
		else if (sig2str(msg->cstat.val, signame) == -1) {
		    (void)snprintf(signame, sizeof(signame), "%d",
			msg->cstat.val);
		}
		sudo_debug_printf(SUDO_DEBUG_INFO,
		    "sending SIG%s to monitor over backchannel", signame);
	    }
	    break;
	default:
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"sending cstat type %d, value %d to monitor over backchannel",
		msg->cstat.type, msg->cstat.val);
	    break;
	}
	TAILQ_REMOVE(&monitor_messages, msg, entries);
	nsent = send(sock, &msg->cstat, sizeof(msg->cstat), 0);
	if (nsent != sizeof(msg->cstat)) {
	    if (errno == EPIPE) {
		sudo_debug_printf(SUDO_DEBUG_ERROR,
		    "broken pipe writing to monitor over backchannel");
		/* Other end of socket gone, empty out monitor_messages. */
		free(msg);
		flush_monitor_messages();
		/* XXX - need new CMD_ type for monitor errors. */
		ec->cstat->type = CMD_ERRNO;
		ec->cstat->val = errno;
		sudo_ev_loopbreak(ec->evbase);
	    }
	    break;
	}
	free(msg);
    }
}

/*
 * Fill in the exec closure and setup initial exec events.
 * Allocates events for the signal pipe and backchannel.
 * Forwarded signals on the backchannel are enabled on demand.
 */
static void
fill_exec_closure(struct exec_closure *ec, struct command_status *cstat,
    struct command_details *details, const struct user_details *user_details,
    struct sudo_event_base *evbase, pid_t sudo_pid, pid_t ppgrp, int backchannel)
{
    debug_decl(fill_exec_closure, SUDO_DEBUG_EXEC);

    /* Fill in the non-event part of the closure. */
    ec->sudo_pid = sudo_pid;
    ec->ppgrp = ppgrp;
    ec->cmnd_pid = -1;
    ec->cstat = cstat;
    ec->details = details;
    ec->rows = user_details->ts_rows;
    ec->cols = user_details->ts_cols;

    /* Reset cstat for running the command. */
    cstat->type = CMD_INVALID;
    cstat->val = 0;

    /* Setup event base and events. */
    ec->evbase = evbase;

    /* Event for command status via backchannel. */
    ec->backchannel_event = sudo_ev_alloc(backchannel,
	SUDO_EV_READ|SUDO_EV_PERSIST, backchannel_cb, ec);
    if (ec->backchannel_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->backchannel_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));
    sudo_debug_printf(SUDO_DEBUG_INFO, "backchannel fd %d\n", backchannel);

    /* Events for local signals. */
    ec->sigint_event = sudo_ev_alloc(SIGINT,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigint_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigint_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigquit_event = sudo_ev_alloc(SIGQUIT,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigquit_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigquit_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigtstp_event = sudo_ev_alloc(SIGTSTP,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigtstp_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigtstp_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigterm_event = sudo_ev_alloc(SIGTERM,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigterm_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigterm_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sighup_event = sudo_ev_alloc(SIGHUP,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sighup_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sighup_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigalrm_event = sudo_ev_alloc(SIGALRM,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigalrm_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigalrm_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigusr1_event = sudo_ev_alloc(SIGUSR1,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigusr1_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigusr1_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigusr2_event = sudo_ev_alloc(SIGUSR2,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigusr2_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigusr2_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigchld_event = sudo_ev_alloc(SIGCHLD,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigchld_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigchld_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigcont_event = sudo_ev_alloc(SIGCONT,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigcont_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigcont_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    ec->sigwinch_event = sudo_ev_alloc(SIGWINCH,
	SUDO_EV_SIGINFO, signal_cb_pty, ec);
    if (ec->sigwinch_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
    if (sudo_ev_add(ec->evbase, ec->sigwinch_event, NULL, false) == -1)
	sudo_fatal("%s", U_("unable to add event to queue"));

    /* The signal forwarding event gets added on demand. */
    ec->fwdchannel_event = sudo_ev_alloc(backchannel,
	SUDO_EV_WRITE, fwdchannel_cb, ec);
    if (ec->fwdchannel_event == NULL)
	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));

    /* Set the default event base. */
    sudo_ev_base_setdef(ec->evbase);

    debug_return;
}

/*
 * Execute a command in a pty, potentially with I/O logging, and
 * wait for it to finish.
 * This is a little bit tricky due to how POSIX job control works and
 * we fact that we have two different controlling terminals to deal with.
 */
bool
exec_pty(struct command_details *details,
    const struct user_details *user_details, struct sudo_event_base *evbase,
    struct command_status *cstat)
{
    int io_pipe[3][2] = { { -1, -1 }, { -1, -1 }, { -1, -1 } };
    bool interpose[3] = { false, false, false };
    struct stat sb, tty_sbuf, *tty_sb = NULL;
    int sv[2], intercept_sv[2] = { -1, -1 };
    struct exec_closure *ec = &pty_ec;
    struct plugin_container *plugin;
    int evloop_retries = -1;
    bool cmnd_foreground;
    sigset_t set, oset;
    struct sigaction sa;
    pid_t ppgrp, sudo_pid;
    debug_decl(exec_pty, SUDO_DEBUG_EXEC);

    /*
     * Allocate a pty if sudo is running in a terminal.
     */
    ec->ptyname = pty_setup(details);
    if (ec->ptyname == NULL)
	debug_return_bool(false);

    /* Register cleanup function */
    sudo_fatal_callback_register(pty_cleanup_hook);

    /*
     * We communicate with the monitor over a bi-directional pair of sockets.
     * Parent sends signal info to monitor and monitor sends back wait status.
     */
    if (socketpair(PF_UNIX, SOCK_STREAM, 0, sv) == -1 ||
	    fcntl(sv[0], F_SETFD, FD_CLOEXEC) == -1 ||
	    fcntl(sv[1], F_SETFD, FD_CLOEXEC) == -1)
	sudo_fatal("%s", U_("unable to create sockets"));

    if (ISSET(details->flags, CD_INTERCEPT|CD_LOG_SUBCMDS)) {
	if (!ISSET(details->flags, CD_USE_PTRACE)) {
	    /*
	     * Allocate a socketpair for communicating with sudo_intercept.so.
	     * This must be inherited across exec, hence no FD_CLOEXEC.
	     */
	    if (socketpair(PF_UNIX, SOCK_STREAM, 0, intercept_sv) == -1)
		sudo_fatal("%s", U_("unable to create sockets"));
	}
    }

    /*
     * We don't want to receive SIGTTIN/SIGTTOU.
     * XXX - this affects tcsetattr() and tcsetpgrp() too.
     */
    memset(&sa, 0, sizeof(sa));
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    sa.sa_handler = SIG_IGN;
    if (sudo_sigaction(SIGTTIN, &sa, NULL) != 0)
	sudo_warn(U_("unable to set handler for signal %d"), SIGTTIN);
    if (sudo_sigaction(SIGTTOU, &sa, NULL) != 0)
	sudo_warn(U_("unable to set handler for signal %d"), SIGTTOU);

    /*
     * The policy plugin's session init must be run before we fork
     * or certain pam modules won't be able to track their state.
     */
    if (policy_init_session(details) != true)
	sudo_fatalx("%s", U_("policy plugin failed session initialization"));

    /*
     * Child will run the command in the pty, parent will pass data
     * to and from pty.
     */
    init_ttyblock();
    ppgrp = getpgrp();	/* parent's pgrp, so child can signal us */
    sudo_pid = getpid();

    /* Determine whether any of std{in,out,err} should be logged. */
    TAILQ_FOREACH(plugin, &io_plugins, entries) {
	if (plugin->u.io->log_stdin)
	    interpose[STDIN_FILENO] = true;
	if (plugin->u.io->log_stdout)
	    interpose[STDOUT_FILENO] = true;
	if (plugin->u.io->log_stderr)
	    interpose[STDERR_FILENO] = true;
    }

    /*
     * Setup stdin/stdout/stderr for command, to be duped after forking.
     */
    io_fds[SFD_STDIN] = io_fds[SFD_FOLLOWER];
    io_fds[SFD_STDOUT] = io_fds[SFD_FOLLOWER];
    io_fds[SFD_STDERR] = io_fds[SFD_FOLLOWER];

    if (io_fds[SFD_USERTTY] != -1) {
	/* Read from /dev/tty, write to pty leader */
	if (!ISSET(details->flags, CD_BACKGROUND)) {
	    io_buf_new(io_fds[SFD_USERTTY], io_fds[SFD_LEADER],
		log_ttyin, read_callback, write_callback, ec);
	}

	/* Read from pty leader, write to /dev/tty */
	io_buf_new(io_fds[SFD_LEADER], io_fds[SFD_USERTTY],
	    log_ttyout, read_callback, write_callback, ec);

	/* Are we the foreground process? */
	ec->foreground = tcgetpgrp(io_fds[SFD_USERTTY]) == ppgrp;
	sudo_debug_printf(SUDO_DEBUG_INFO, "sudo is running in the %s",
	    ec->foreground ? "foreground" : "background");
    }

    /*
     * If stdin, stdout or stderr is not the user's tty and logging is
     * enabled, use a pipe to interpose ourselves instead of using the
     * pty fd.  We always use a pipe for stdin when in background mode.
     */
    if (user_details->tty != NULL && stat(user_details->tty, &tty_sbuf) != -1)
	tty_sb = &tty_sbuf;

    if (!fd_matches_tty(STDIN_FILENO, tty_sb, &sb)) {
	if (!interpose[STDIN_FILENO]) {
	    /* Not logging stdin, do not interpose. */
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"stdin not user's tty, not logging");
	    if (S_ISFIFO(sb.st_mode))
		SET(details->flags, CD_EXEC_BG);
	    io_fds[SFD_STDIN] = dup(STDIN_FILENO);
	    if (io_fds[SFD_STDIN] == -1)
		sudo_fatal("dup");
	} else {
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"stdin not user's tty, creating a pipe");
	    SET(details->flags, CD_EXEC_BG);
	    if (pipe2(io_pipe[STDIN_FILENO], O_CLOEXEC) != 0)
		sudo_fatal("%s", U_("unable to create pipe"));
	    io_buf_new(STDIN_FILENO, io_pipe[STDIN_FILENO][1],
		log_stdin, read_callback, write_callback, ec);
	    io_fds[SFD_STDIN] = io_pipe[STDIN_FILENO][0];
	}

	if (ec->foreground && ppgrp != sudo_pid) {
	    /*
	     * If sudo is not the process group leader and stdin is not
	     * a tty we may be running as a background job via a shell
	     * script.  Start the command in the background to avoid
	     * changing the terminal mode from a background process.
	     */
	    SET(details->flags, CD_EXEC_BG);
	}
    } else if (ISSET(details->flags, CD_BACKGROUND)) {
	    /*
	     * Running in background (sudo -b), no access to terminal input.
	     * In non-pty mode, the command runs in an orphaned process
	     * group and reads from the controlling terminal fail with EIO.
	     * We cannot do the same while running in a pty but if we set
	     * stdin to a half-closed pipe, reads from it will get EOF.
	     */
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"terminal input not available, creating empty pipe");
	    SET(details->flags, CD_EXEC_BG);
	    if (pipe2(io_pipe[STDIN_FILENO], O_CLOEXEC) != 0)
		sudo_fatal("%s", U_("unable to create pipe"));
	    io_fds[SFD_STDIN] = io_pipe[STDIN_FILENO][0];
	    close(io_pipe[STDIN_FILENO][1]);
	    io_pipe[STDIN_FILENO][1] = -1;
    }
    if (!fd_matches_tty(STDOUT_FILENO, tty_sb, &sb)) {
	if (!interpose[STDOUT_FILENO]) {
	    /* Not logging stdout, do not interpose. */
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"stdout not user's tty, not logging");
	    if (S_ISFIFO(sb.st_mode)) {
		SET(details->flags, CD_EXEC_BG);
		term_raw_flags = SUDO_TERM_OFLAG;
	    }
	    io_fds[SFD_STDOUT] = dup(STDOUT_FILENO);
	    if (io_fds[SFD_STDOUT] == -1)
		sudo_fatal("dup");
	} else {
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"stdout not user's tty, creating a pipe");
	    SET(details->flags, CD_EXEC_BG);
	    term_raw_flags = SUDO_TERM_OFLAG;
	    if (pipe2(io_pipe[STDOUT_FILENO], O_CLOEXEC) != 0)
		sudo_fatal("%s", U_("unable to create pipe"));
	    io_buf_new(io_pipe[STDOUT_FILENO][0], STDOUT_FILENO,
		log_stdout, read_callback, write_callback, ec);
	    io_fds[SFD_STDOUT] = io_pipe[STDOUT_FILENO][1];
	}
    }
    if (!fd_matches_tty(STDERR_FILENO, tty_sb, &sb)) {
	if (!interpose[STDERR_FILENO]) {
	    /* Not logging stderr, do not interpose. */
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"stderr not user's tty, not logging");
	    io_fds[SFD_STDERR] = dup(STDERR_FILENO);
	    if (io_fds[SFD_STDERR] == -1)
		sudo_fatal("dup");
	} else {
	    sudo_debug_printf(SUDO_DEBUG_INFO,
		"stderr /dev/tty, creating a pipe");
	    if (pipe2(io_pipe[STDERR_FILENO], O_CLOEXEC) != 0)
		sudo_fatal("%s", U_("unable to create pipe"));
	    io_buf_new(io_pipe[STDERR_FILENO][0], STDERR_FILENO,
		log_stderr, read_callback, write_callback, ec);
	    io_fds[SFD_STDERR] = io_pipe[STDERR_FILENO][1];
	}
    }

    /*
     * Copy terminal settings from user tty -> pty.  If sudo is a
     * background process, we'll re-init the pty when foregrounded.
     */
    if (!sudo_term_copy(io_fds[SFD_USERTTY], io_fds[SFD_LEADER])) {
	sudo_debug_printf(SUDO_DEBUG_ERROR|SUDO_DEBUG_ERRNO,
	    "%s: unable to copy terminal settings to pty", __func__);
	ec->foreground = false;
    }
    /* Start in raw mode unless the command will run in the background. */
    cmnd_foreground = ec->foreground && !ISSET(details->flags, CD_EXEC_BG);
    if (cmnd_foreground) {
	if (sudo_term_raw(io_fds[SFD_USERTTY], 0))
	    ec->term_raw = true;
    }

    sudo_debug_printf(SUDO_DEBUG_INFO,
	"%s: follower: %d, stdin: %d, stdout: %d, stderr: %d", __func__,
	io_fds[SFD_FOLLOWER], io_fds[SFD_STDIN], io_fds[SFD_STDOUT],
	io_fds[SFD_STDERR]);

    /*
     * Block signals until we have our handlers setup in the parent so
     * we don't miss SIGCHLD if the command exits immediately.
     */
    sigfillset(&set);
    sigprocmask(SIG_BLOCK, &set, &oset);

    /* Check for early termination or suspend signals before we fork. */
    if (sudo_terminated(cstat)) {
	sigprocmask(SIG_SETMASK, &oset, NULL);
	debug_return_bool(true);
    }

    ec->monitor_pid = sudo_debug_fork();
    switch (ec->monitor_pid) {
    case -1:
	sudo_fatal("%s", U_("unable to fork"));
	break;
    case 0:
	/* child */
	close(sv[0]);
	if (intercept_sv[0] != -1)
	    close(intercept_sv[0]);
	/* Close the other end of the stdin/stdout/stderr pipes and exec. */
	if (io_pipe[STDIN_FILENO][1] != -1)
	    close(io_pipe[STDIN_FILENO][1]);
	if (io_pipe[STDOUT_FILENO][0] != -1)
	    close(io_pipe[STDOUT_FILENO][0]);
	if (io_pipe[STDERR_FILENO][0] != -1)
	    close(io_pipe[STDERR_FILENO][0]);

	/* Only run the cleanup hook in the parent. */
	sudo_fatal_callback_deregister(pty_cleanup_hook);

	/*                      
	 * If stdin/stdout is not the user's tty, start the command in
	 * the background since it might be part of a pipeline that reads
	 * from /dev/tty.  In this case, we rely on the command receiving
	 * SIGTTOU or SIGTTIN when it needs access to the controlling tty.
	 */                                                              
	exec_monitor(details, &oset, cmnd_foreground, sv[1], intercept_sv[1]);
	cstat->type = CMD_ERRNO;
	cstat->val = errno;
	if (send(sv[1], cstat, sizeof(*cstat), 0) == -1) {
            sudo_debug_printf(SUDO_DEBUG_ERROR|SUDO_DEBUG_ERRNO,
                "%s: unable to send status to parent", __func__);
	}
	_exit(EXIT_FAILURE);
	/* NOTREACHED */
    }

    /*
     * We close the pty follower so only the monitor and command have a
     * reference to it.  This ensures that we can don't block reading
     * from the leader when the command and monitor have exited.
     */
    if (io_fds[SFD_FOLLOWER] != -1) {
	close(io_fds[SFD_FOLLOWER]);
	io_fds[SFD_FOLLOWER] = -1;
    }

    /* Tell the monitor to continue now that the follower is closed. */
    cstat->type = CMD_SIGNO;
    cstat->val = 0;
    if (send(sv[0], cstat, sizeof(*cstat), 0) == -1)
	sudo_fatal("%s", U_("unable to send message to monitor process"));

    /* Close the other end of the stdin/stdout/stderr pipes and socketpair. */
    if (io_pipe[STDIN_FILENO][0] != -1)
	close(io_pipe[STDIN_FILENO][0]);
    if (io_pipe[STDOUT_FILENO][1] != -1)
	close(io_pipe[STDOUT_FILENO][1]);
    if (io_pipe[STDERR_FILENO][1] != -1)
	close(io_pipe[STDERR_FILENO][1]);
    close(sv[1]);

    /* No longer need execfd. */
    if (details->execfd != -1) {
	close(details->execfd);
	details->execfd = -1;
    }

    /* Set command timeout if specified. */
    if (ISSET(details->flags, CD_SET_TIMEOUT))
	alarm(details->timeout);

    /*
     * Fill in exec closure, allocate event base, signal events and
     * the backchannel event.
     */
    fill_exec_closure(ec, cstat, details, user_details, evbase,
	sudo_pid, ppgrp, sv[0]);

    /* Create event and closure for intercept mode. */
    if (ISSET(details->flags, CD_INTERCEPT|CD_LOG_SUBCMDS)) {
	ec->intercept = intercept_setup(intercept_sv[0], ec->evbase, details);
	if (ec->intercept == NULL)
	    terminate_command(ec->cmnd_pid, true);
    }

    /* Restore signal mask now that signal handlers are setup. */
    sigprocmask(SIG_SETMASK, &oset, NULL);

    /*
     * I/O logging must be in the C locale for floating point numbers
     * to be logged consistently.
     */
    setlocale(LC_ALL, "C");

    /*
     * In the event loop we pass input from user tty to leader
     * and pass output from leader to stdout and IO plugin.
     * Try to recover on ENXIO, it means the tty was revoked.
     */
    add_io_events(ec);
    do {
	if (sudo_ev_dispatch(ec->evbase) == -1)
	    sudo_warn("%s", U_("error in event loop"));
	if (sudo_ev_got_break(ec->evbase)) {
	    /* error from callback or monitor died */
	    sudo_debug_printf(SUDO_DEBUG_ERROR, "event loop exited prematurely");
	    /* XXX: no good way to know if we should terminate the command. */
	    if (cstat->val == CMD_INVALID && ec->cmnd_pid != -1) {
		/* no status message, kill command */
		terminate_command(ec->cmnd_pid, true);
		ec->cmnd_pid = -1;
		/* TODO: need way to pass an error to the sudo front end */
		cstat->type = CMD_WSTATUS;
		cstat->val = W_EXITCODE(1, SIGKILL);
	    }
	} else if (!sudo_ev_got_exit(ec->evbase)) {
	    switch (errno) {
	    case ENXIO:
	    case EIO:
	    case EBADF:
		/* /dev/tty was revoked, remove tty events and retry (once) */
		if (evloop_retries == -1 && io_fds[SFD_USERTTY] != -1) {
		    ev_free_by_fd(ec->evbase, io_fds[SFD_USERTTY]);
		    evloop_retries = 1;
		}
		break;
	    }
	}
    } while (evloop_retries-- > 0);

    /* De-register cleanup hook, the pty is going away. */
    sudo_fatal_callback_deregister(pty_cleanup_hook);

    /* Flush any remaining output, free I/O bufs and events, do logout. */
    pty_finish(ec, cstat);

    /* Free things up. */
    free_exec_closure(ec);

    debug_return_bool(true);
}

/*
 * Propagate tty size change to pty being used by the command, pass
 * new window size to I/O plugins and deliver SIGWINCH to the command.
 */
static void
sync_ttysize(struct exec_closure *ec)
{
    struct winsize wsize;
    debug_decl(sync_ttysize, SUDO_DEBUG_EXEC);

    if (ioctl(io_fds[SFD_USERTTY], TIOCGWINSZ, &wsize) == 0) {
	if (wsize.ws_row != ec->rows || wsize.ws_col != ec->cols) {
	    sudo_debug_printf(SUDO_DEBUG_INFO, "%s: %d x %d -> %hd x %hd",
		__func__, ec->rows, ec->cols, wsize.ws_row, wsize.ws_col);

	    /* Log window change event. */
	    log_winchange(ec, wsize.ws_row, wsize.ws_col);

	    /* Update pty window size and send command SIGWINCH. */
	    (void)ioctl(io_fds[SFD_LEADER], IOCTL_REQ_CAST TIOCSWINSZ, &wsize);
	    killpg(ec->cmnd_pid, SIGWINCH);

	    /* Update rows/cols. */
	    ec->rows = wsize.ws_row;
	    ec->cols = wsize.ws_col;
	}
    }

    debug_return;
}