Makefile

ptygoop: ptygoop.c Makefile
	gcc -o $@ $< `pkg-config --cflags --libs glib-2.0` -Wall -Wextra -Werror

ptygoop.c

#define _GNU_SOURCE

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <termios.h>
#include <unistd.h>

#include <glib.h>

static void
set_controlling_tty_to_stdin (void *user_data G_GNUC_UNUSED)
{
  setsid ();
  ioctl (0, TIOCSCTTY, 0);
  /* If this ioctl isn't supported, the other approach is to (re-)open() the
   * PTY slave device (without O_NOCTTY)
   */
}

int
main (int argc G_GNUC_UNUSED,
      char **argv G_GNUC_UNUSED)
{
  g_autoptr(GError) error = NULL;

  /* Open the master & slave sides of a PTY pair */
  int pt_master = posix_openpt (O_RDWR | O_NOCTTY);
  if (pt_master < 0)
    perror ("posix_openpt");
  if (unlockpt (pt_master) < 0)
    perror ("unlockpt");
  char *pt_slave_name = ptsname (pt_master);
  int pt_slave = open (pt_slave_name, O_RDWR);
  if (pt_slave < 0)
    perror ("open pt_slave_name");

  /* Spawn a privileged worker process, setting its stdin to the slave side of
   * the PTY.
   */
  char *sleep_argv[] = { "/usr/bin/pkexec", "dbus-monitor", "--system", NULL };
  GPid child_pid = -1;
  gboolean ret = g_spawn_async_with_fds (NULL,
                                         sleep_argv,
                                         NULL,
                                         G_SPAWN_DO_NOT_REAP_CHILD,
                                         set_controlling_tty_to_stdin,
                                         NULL,
                                         &child_pid,
                                         pt_slave,
                                         -1,
                                         -1,
                                         &error);
  g_assert_no_error (error);
  g_assert_true (ret);

  /* Give the user time to approve the polkit request */
  g_message ("spawned %d; please approve the polkit request quickly!", child_pid);
  g_usleep (30 * G_USEC_PER_SEC);

  /* signal (child_pid, SIGINT) would fail with EPERM because the target is
   * running as a different uid (and we are not root), but we can send ^C over
   * the TTY and the kernel will do what has to be done.
   */
  g_message ("Sending ^C to %d via PTY", child_pid);
  char ctrl_c = '\x03';
  if (write (pt_master, &ctrl_c, 1) != 1)
    perror ("write");

  int wstatus;
  if (waitpid (child_pid, &wstatus, 0) < 0)
    perror ("waitpid");
  else if (WIFEXITED (wstatus))
    g_message ("exited %d", WEXITSTATUS (wstatus));
  else if (WIFSIGNALED (wstatus) && WTERMSIG (wstatus) != SIGINT)
    g_message ("signalled %d (expected %d)", WTERMSIG (wstatus), SIGINT);
  else if (WIFSIGNALED (wstatus))
    g_message ("signalled %d, as hoped", WTERMSIG (wstatus));
  else
    g_message ("weird hex but ok %x", wstatus);

  return 0;
}