/* su for Linux.  Run a shell with substitute user and group IDs.
   Copyright (C) 1992-2006 Free Software Foundation, Inc.
   Copyright (C) 2012 SUSE Linux Products GmbH, Nuernberg

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software Foundation,
   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */

/* Run a shell with the real and effective UID and GID and groups
   of USER, default `root'.

   The shell run is taken from USER's password entry, /bin/sh if
   none is specified there.  If the account has a password, su
   prompts for a password unless run by a user with real UID 0.

   Does not change the current directory.
   Sets `HOME' and `SHELL' from the password entry for USER, and if
   USER is not root, sets `USER' and `LOGNAME' to USER.
   The subshell is not a login shell.

   If one or more ARGs are given, they are passed as additional
   arguments to the subshell.

   Does not handle /bin/sh or other shells specially
   (setting argv[0] to "-su", passing -c only to certain shells, etc.).
   I don't see the point in doing that, and it's ugly.

   Based on an implemenation by David MacKenzie <djm@gnu.ai.mit.edu>.  */

enum
{
  EXIT_CANNOT_INVOKE = 126,
  EXIT_ENOENT = 127
};

#include <config.h>
#include <stdio.h>
#include <getopt.h>
#include <sys/types.h>
#include <pwd.h>
#include <grp.h>
#include <security/pam_appl.h>
#include <security/pam_misc.h>
#include <signal.h>
#include <sys/wait.h>
#include <syslog.h>
#include <utmp.h>

#include "err.h"

#include <stdbool.h>
#include "c.h"
#include "xalloc.h"
#include "nls.h"
#include "pathnames.h"
#include "env.h"
#include "closestream.h"
#include "strutils.h"
#include "ttyutils.h"

/* name of the pam configuration files. separate configs for su and su -  */
#define PAM_SRVNAME_SU "su"
#define PAM_SRVNAME_SU_L "su-l"

#define PAM_SRVNAME_RUNUSER "runuser"
#define PAM_SRVNAME_RUNUSER_L "runuser-l"

#define _PATH_LOGINDEFS_SU	"/etc/defaults/su"
#define _PATH_LOGINDEFS_RUNUSER "/etc/defaults/runuser"

#define is_pam_failure(_rc)	((_rc) != PAM_SUCCESS)

#include "logindefs.h"
#include "su-common.h"

/* The shell to run if none is given in the user's passwd entry.  */
#define DEFAULT_SHELL "/bin/sh"

/* The user to become if none is specified.  */
#define DEFAULT_USER "root"

#ifndef HAVE_ENVIRON_DECL
extern char **environ;
#endif

static void run_shell (char const *, char const *, char **, size_t)
     __attribute__ ((__noreturn__));

/* If true, pass the `-f' option to the subshell.  */
static bool fast_startup;

/* If true, simulate a login instead of just starting a shell.  */
static bool simulate_login;

/* If true, change some environment vars to indicate the user su'd to.  */
static bool change_environment;

/* If true, then don't call setsid() with a command. */
static int same_session = 0;

/* SU_MODE_{RUNUSER,SU} */
static int su_mode;

static bool _pam_session_opened;
static bool _pam_cred_established;
static sig_atomic_t volatile caught_signal = false;
static pam_handle_t *pamh = NULL;

static int restricted = 1;	/* zero for root user */


static struct passwd *
current_getpwuid(void)
{
  uid_t ruid;

  /* GNU Hurd implementation has an extension where a process can exist in a
   * non-conforming environment, and thus be outside the realms of POSIX
   * process identifiers; on this platform, getuid() fails with a status of
   * (uid_t)(-1) and sets errno if a program is run from a non-conforming
   * environment.
   *
   * http://austingroupbugs.net/view.php?id=511
   */
  errno = 0;
  ruid = getuid ();

  return errno == 0 ? getpwuid (ruid) : NULL;
}

/* Log the fact that someone has run su to the user given by PW;
   if SUCCESSFUL is true, they gave the correct password, etc.  */

static void
log_syslog(struct passwd const *pw, bool successful)
{
  const char *new_user, *old_user, *tty;

  new_user = pw->pw_name;
  /* The utmp entry (via getlogin) is probably the best way to identify
     the user, especially if someone su's from a su-shell.  */
  old_user = getlogin ();
  if (!old_user)
    {
      /* getlogin can fail -- usually due to lack of utmp entry.
	 Resort to getpwuid.  */
      struct passwd *pwd = current_getpwuid();
      old_user = pwd ? pwd->pw_name : "";
    }

  if (get_terminal_name(NULL, &tty, NULL) == 0 && tty)
    tty = "none";

  openlog (program_invocation_short_name, 0 , LOG_AUTH);
  syslog (LOG_NOTICE, "%s(to %s) %s on %s",
	  successful ? "" :
	  su_mode == RUNUSER_MODE ? "FAILED RUNUSER " : "FAILED SU ",
	  new_user, old_user, tty);
  closelog ();
}


/*
 * Log failed login attempts in _PATH_BTMP if that exists.
 */
static void log_btmp(struct passwd const *pw)
{
	struct utmp ut;
	struct timeval tv;
	const char *tty_name, *tty_num;

	memset(&ut, 0, sizeof(ut));

	strncpy(ut.ut_user,
		pw && pw->pw_name ? pw->pw_name : "(unknown)",
		sizeof(ut.ut_user));

	get_terminal_name(NULL, &tty_name, &tty_num);
	if (tty_num)
		xstrncpy(ut.ut_id, tty_num, sizeof(ut.ut_id));
	if (tty_name)
		xstrncpy(ut.ut_line, tty_name, sizeof(ut.ut_line));

#if defined(_HAVE_UT_TV)	/* in <utmpbits.h> included by <utmp.h> */
	gettimeofday(&tv, NULL);
	ut.ut_tv.tv_sec = tv.tv_sec;
	ut.ut_tv.tv_usec = tv.tv_usec;
#else
	{
		time_t t;
		time(&t);
		ut.ut_time = t;	/* ut_time is not always a time_t */
	}
#endif
	ut.ut_type = LOGIN_PROCESS;	/* XXX doesn't matter */
	ut.ut_pid = getpid();

	updwtmp(_PATH_BTMP, &ut);
}

static struct pam_conv conv =
{
  misc_conv,
  NULL
};

static void
cleanup_pam (int retcode)
{
  int saved_errno = errno;

  if (_pam_session_opened)
    pam_close_session (pamh, 0);

  if (_pam_cred_established)
    pam_setcred (pamh, PAM_DELETE_CRED | PAM_SILENT);

  pam_end(pamh, retcode);

  errno = saved_errno;
}

/* Signal handler for parent process.  */
static void
su_catch_sig (int sig __attribute__((__unused__)))
{
  caught_signal = true;
}

/* Export env variables declared by PAM modules.  */
static void
export_pamenv (void)
{
  char **env;

  /* This is a copy but don't care to free as we exec later anyways.  */
  env = pam_getenvlist (pamh);
  while (env && *env)
    {
      if (putenv (*env) != 0)
	err (EXIT_FAILURE, NULL);
      env++;
    }
}

static void
create_watching_parent (void)
{
  pid_t child;
  sigset_t ourset;
  int status = 0;
  int retval;

  retval = pam_open_session (pamh, 0);
  if (is_pam_failure(retval))
    {
      cleanup_pam (retval);
      errx (EXIT_FAILURE, _("cannot open session: %s"),
	     pam_strerror (pamh, retval));
    }
  else
    _pam_session_opened = 1;

  child = fork ();
  if (child == (pid_t) -1)
    {
      cleanup_pam (PAM_ABORT);
      err (EXIT_FAILURE, _("cannot create child process"));
    }

  /* the child proceeds to run the shell */
  if (child == 0)
    return;

  /* In the parent watch the child.  */

  /* su without pam support does not have a helper that keeps
     sitting on any directory so let's go to /.  */
  if (chdir ("/") != 0)
    warn (_("cannot change directory to %s"), "/");

  sigfillset (&ourset);
  if (sigprocmask (SIG_BLOCK, &ourset, NULL))
    {
      warn (_("cannot block signals"));
      caught_signal = true;
    }
  if (!caught_signal)
    {
      struct sigaction action;
      action.sa_handler = su_catch_sig;
      sigemptyset (&action.sa_mask);
      action.sa_flags = 0;
      sigemptyset (&ourset);
    if (!same_session)
      {
        if (sigaddset(&ourset, SIGINT) || sigaddset(&ourset, SIGQUIT))
          {
            warn (_("cannot set signal handler"));
            caught_signal = true;
          }
      }
    if (!caught_signal && (sigaddset(&ourset, SIGTERM)
                    || sigaddset(&ourset, SIGALRM)
                    || sigaction(SIGTERM, &action, NULL)
                    || sigprocmask(SIG_UNBLOCK, &ourset, NULL))) {
	  warn (_("cannot set signal handler"));
	  caught_signal = true;
	}
    if (!caught_signal && !same_session && (sigaction(SIGINT, &action, NULL)
                                     || sigaction(SIGQUIT, &action, NULL)))
      {
        warn (_("cannot set signal handler"));
        caught_signal = true;
      }
    }
  if (!caught_signal)
    {
      pid_t pid;
      for (;;)
	{
	  pid = waitpid (child, &status, WUNTRACED);

	  if (pid != (pid_t)-1 && WIFSTOPPED (status))
	    {
	      kill (getpid (), SIGSTOP);
	      /* once we get here, we must have resumed */
	      kill (pid, SIGCONT);
	    }
	  else
	    break;
	}
      if (pid != (pid_t)-1)
	if (WIFSIGNALED (status))
	  {
	    status = WTERMSIG (status) + 128;
	    if (WCOREDUMP (status))
	      fprintf (stderr, _("%s (core dumped)\n"),
                 strsignal (WTERMSIG (status)));
	  }
	else
	  status = WEXITSTATUS (status);
      else
	status = 1;
    }
  else
    status = 1;

  if (caught_signal)
    {
      fprintf (stderr, _("\nSession terminated, killing shell..."));
      kill (child, SIGTERM);
    }

  cleanup_pam (PAM_SUCCESS);

  if (caught_signal)
    {
      sleep (2);
      kill (child, SIGKILL);
      fprintf (stderr, _(" ...killed.\n"));
    }
  exit (status);
}

static void
authenticate (const struct passwd *pw)
{
  const struct passwd *lpw = NULL;
  const char *cp, *srvname = NULL;
  int retval;

  switch (su_mode) {
  case SU_MODE:
    srvname = simulate_login ? PAM_SRVNAME_SU_L : PAM_SRVNAME_SU;
    break;
  case RUNUSER_MODE:
    srvname = simulate_login ? PAM_SRVNAME_RUNUSER_L : PAM_SRVNAME_RUNUSER;
    break;
  }

  retval = pam_start (srvname, pw->pw_name, &conv, &pamh);
  if (is_pam_failure(retval))
    goto done;

  if (isatty (0) && (cp = ttyname (0)) != NULL)
    {
      const char *tty;

      if (strncmp (cp, "/dev/", 5) == 0)
	tty = cp + 5;
      else
	tty = cp;
      retval = pam_set_item (pamh, PAM_TTY, tty);
      if (is_pam_failure(retval))
	goto done;
    }

  lpw = current_getpwuid ();
  if (lpw && lpw->pw_name)
    {
      retval = pam_set_item (pamh, PAM_RUSER, (const void *) lpw->pw_name);
      if (is_pam_failure(retval))
	goto done;
    }

  if (su_mode == RUNUSER_MODE)
    {
      /*
       * This is the only difference between runuser(1) and su(1). The command
       * runuser(1) does not required authentication, because user is root.
       */
      if (restricted)
	errx(EXIT_FAILURE, _("may not be used by non-root users"));
      return;
    }

  retval = pam_authenticate (pamh, 0);
  if (is_pam_failure(retval))
    goto done;

  retval = pam_acct_mgmt (pamh, 0);
  if (retval == PAM_NEW_AUTHTOK_REQD)
    {
      /* Password has expired.  Offer option to change it.  */
      retval = pam_chauthtok (pamh, PAM_CHANGE_EXPIRED_AUTHTOK);
    }

done:

  if (lpw && lpw->pw_name)
     pw = lpw;

  log_syslog(pw, !is_pam_failure(retval));

  if (is_pam_failure(retval))
    {
      const char *msg;

      log_btmp(pw);

      msg  = pam_strerror(pamh, retval);
      pam_end(pamh, retval);
      sleep (getlogindefs_num ("FAIL_DELAY", 1));
      errx (EXIT_FAILURE, "%s", msg?msg:_("incorrect password"));
    }
}

static void
set_path(const struct passwd* pw)
{
  int r;
  if (pw->pw_uid)
    r = logindefs_setenv("PATH", "ENV_PATH", _PATH_DEFPATH);

  else if ((r = logindefs_setenv("PATH", "ENV_ROOTPATH", NULL)) != 0)
    r = logindefs_setenv("PATH", "ENV_SUPATH", _PATH_DEFPATH_ROOT);

  if (r != 0)
    err (EXIT_FAILURE,  _("failed to set PATH"));
}

/* Update `environ' for the new shell based on PW, with SHELL being
   the value for the SHELL environment variable.  */

static void
modify_environment (const struct passwd *pw, const char *shell)
{
  if (simulate_login)
    {
      /* Leave TERM unchanged.  Set HOME, SHELL, USER, LOGNAME, PATH.
         Unset all other environment variables.  */
      char const *term = getenv ("TERM");
      if (term)
	term = xstrdup (term);
      environ = xmalloc ((6 + !!term) * sizeof (char *));
      environ[0] = NULL;
      if (term)
	xsetenv ("TERM", term, 1);
      xsetenv ("HOME", pw->pw_dir, 1);
      if (shell)
	xsetenv ("SHELL", shell, 1);
      xsetenv ("USER", pw->pw_name, 1);
      xsetenv ("LOGNAME", pw->pw_name, 1);
      set_path(pw);
    }
  else
    {
      /* Set HOME, SHELL, and if not becoming a super-user,
	 USER and LOGNAME.  */
      if (change_environment)
        {
          xsetenv ("HOME", pw->pw_dir, 1);
	  if (shell)
            xsetenv ("SHELL", shell, 1);
	  if (getlogindefs_bool ("ALWAYS_SET_PATH", 0))
	    set_path(pw);

          if (pw->pw_uid)
            {
              xsetenv ("USER", pw->pw_name, 1);
              xsetenv ("LOGNAME", pw->pw_name, 1);
            }
        }
    }

  export_pamenv ();
}

/* Become the user and group(s) specified by PW.  */

static void
init_groups (const struct passwd *pw, gid_t *groups, int num_groups)
{
  int retval;

  errno = 0;

  if (num_groups)
    retval = setgroups (num_groups, groups);
  else
    retval = initgroups (pw->pw_name, pw->pw_gid);

  if (retval == -1)
    {
      cleanup_pam (PAM_ABORT);
      err (EXIT_FAILURE, _("cannot set groups"));
    }
  endgrent ();

  retval = pam_setcred (pamh, PAM_ESTABLISH_CRED);
  if (is_pam_failure(retval))
    errx (EXIT_FAILURE, "%s", pam_strerror (pamh, retval));
  else
    _pam_cred_established = 1;
}

static void
change_identity (const struct passwd *pw)
{
  if (setgid (pw->pw_gid))
    err (EXIT_FAILURE,  _("cannot set group id"));
  if (setuid (pw->pw_uid))
    err (EXIT_FAILURE,  _("cannot set user id"));
}

/* Run SHELL, or DEFAULT_SHELL if SHELL is empty.
   If COMMAND is nonzero, pass it to the shell with the -c option.
   Pass ADDITIONAL_ARGS to the shell as more arguments; there
   are N_ADDITIONAL_ARGS extra arguments.  */

static void
run_shell (char const *shell, char const *command, char **additional_args,
	   size_t n_additional_args)
{
  size_t n_args = 1 + fast_startup + 2 * !!command + n_additional_args + 1;
  char const **args = xcalloc (n_args, sizeof *args);
  size_t argno = 1;

  if (simulate_login)
    {
      char *arg0;
      char *shell_basename;

      shell_basename = basename (shell);
      arg0 = xmalloc (strlen (shell_basename) + 2);
      arg0[0] = '-';
      strcpy (arg0 + 1, shell_basename);
      args[0] = arg0;
    }
  else
    args[0] = basename (shell);
  if (fast_startup)
    args[argno++] = "-f";
  if (command)
    {
      args[argno++] = "-c";
      args[argno++] = command;
    }
  memcpy (args + argno, additional_args, n_additional_args * sizeof *args);
  args[argno + n_additional_args] = NULL;
  execv (shell, (char **) args);

  {
    int exit_status = (errno == ENOENT ? EXIT_ENOENT : EXIT_CANNOT_INVOKE);
    warn ("%s", shell);
    exit (exit_status);
  }
}

/* Return true if SHELL is a restricted shell (one not returned by
   getusershell), else false, meaning it is a standard shell.  */

static bool
restricted_shell (const char *shell)
{
  char *line;

  setusershell ();
  while ((line = getusershell ()) != NULL)
    {
      if (*line != '#' && !strcmp (line, shell))
	{
	  endusershell ();
	  return false;
	}
    }
  endusershell ();
  return true;
}

static void __attribute__((__noreturn__))
usage (int status)
{
  if (su_mode == RUNUSER_MODE) {
    fputs(USAGE_HEADER, stdout);
    printf (_(" %s [options] -u <USER> COMMAND\n"), program_invocation_short_name);
    printf (_(" %s [options] [-] [USER [arg]...]\n"), program_invocation_short_name);
    fputs (_("\n"
    "Run COMMAND with the effective <user> id and group id. If -u not\n"
    "given, fallback to su(1) compatible semantic and shell is executed.\n"
    "The options -l, -c, -f, -s are mutually exclusive to -u.\n"), stdout);

    fputs(USAGE_OPTIONS, stdout);

    fputs (_(
    " -u, --user <user>               username\n"), stdout);

  } else {
    fputs(USAGE_HEADER, stdout);
    printf (_(" %s [options] [-] [USER [arg]...]\n"), program_invocation_short_name);
    fputs (_("\n"
    "Change the effective user id and group id to that of USER.\n"
    "A mere - implies -l.   If USER not given, assume root.\n"), stdout);

    fputs(USAGE_OPTIONS, stdout);
  }

  fputs (_(" -m, -p, --preserve-environment  do not reset environment variables\n"), stdout);
  fputs (_(" -g, --group <group>             specify the primary group\n"), stdout);
  fputs (_(" -G, --supp-group <group>        specify a supplemental group\n\n"), stdout);

  fputs (_(" -, -l, --login                  make the shell a login shell\n"), stdout);
  fputs (_(" -c, --command <command>         pass a single command to the shell with -c\n"), stdout);
  fputs (_(" --session-command <command>     pass a single command to the shell with -c\n"), stdout);
  fputs (_("                                 and do not create a new session\n"), stdout);
  fputs (_(" -f, --fast                      pass -f to the shell (for csh or tcsh)\n"), stdout);
  fputs (_(" -s, --shell <shell>             run shell if /etc/shells allows it\n"), stdout);

  fputs(USAGE_SEPARATOR, stdout);
  fputs(USAGE_HELP, stdout);
  fputs(USAGE_VERSION, stdout);
  printf(USAGE_MAN_TAIL(su_mode == SU_MODE ? "su(1)" : "runuser(1)"));
  exit (status);
}

static
void load_config(void)
{
  switch (su_mode) {
  case SU_MODE:
    logindefs_load_file(_PATH_LOGINDEFS_SU);
    break;
  case RUNUSER_MODE:
    logindefs_load_file(_PATH_LOGINDEFS_RUNUSER);
    break;
  }

  logindefs_load_file(_PATH_LOGINDEFS);
}

/*
 * Returns 1 if the current user is not root
 */
static int
evaluate_uid(void)
{
  uid_t ruid = getuid();
  uid_t euid = geteuid();

  /* if we're really root and aren't running setuid */
  return (uid_t) 0 == ruid && ruid == euid ? 0 : 1;
}

int
su_main (int argc, char **argv, int mode)
{
  int optc;
  const char *new_user = DEFAULT_USER, *runuser_user = NULL;
  char *command = NULL;
  int request_same_session = 0;
  char *shell = NULL;
  struct passwd *pw;
  struct passwd pw_copy;
  struct group *gr;
  gid_t groups[NGROUPS_MAX];
  int num_supp_groups = 0;
  int use_gid = 0;

  static const struct option longopts[] = {
    {"command", required_argument, NULL, 'c'},
    {"session-command", required_argument, NULL, 'C'},
    {"fast", no_argument, NULL, 'f'},
    {"login", no_argument, NULL, 'l'},
    {"preserve-environment", no_argument, NULL, 'p'},
    {"shell", required_argument, NULL, 's'},
    {"group", required_argument, NULL, 'g'},
    {"supp-group", required_argument, NULL, 'G'},
    {"user", required_argument, NULL, 'u'},		/* runuser only */
    {"help", no_argument, 0, 'h'},
    {"version", no_argument, 0, 'V'},
    {NULL, 0, NULL, 0}
  };

  setlocale (LC_ALL, "");
  bindtextdomain (PACKAGE, LOCALEDIR);
  textdomain (PACKAGE);
  atexit(close_stdout);

  su_mode = mode;
  fast_startup = false;
  simulate_login = false;
  change_environment = true;

  while ((optc = getopt_long (argc, argv, "+c:fg:G:lmps:u:hV", longopts, NULL)) != -1)
    {
      switch (optc)
	{
	case 'c':
	  command = optarg;
	  break;

        case 'C':
          command = optarg;
          request_same_session = 1;
          break;

	case 'f':
	  fast_startup = true;
	  break;

	case 'g':
	  gr = getgrnam(optarg);
	  if (!gr)
	    errx(EXIT_FAILURE, _("group %s does not exist"), optarg);
	  use_gid = 1;
	  groups[0] = gr->gr_gid;
	  break;

	case 'G':
	  num_supp_groups++;
	  if (num_supp_groups >= NGROUPS_MAX)
	     errx(EXIT_FAILURE,
		  _("can't specify more than %d supplemental groups"),
		  NGROUPS_MAX - 1);
	  gr = getgrnam(optarg);
	  if (!gr)
	    errx(EXIT_FAILURE, _("group %s does not exist"), optarg);
	  groups[num_supp_groups] = gr->gr_gid;
	  break;

	case 'l':
	  simulate_login = true;
	  break;

	case 'm':
	case 'p':
	  change_environment = false;
	  break;

	case 's':
	  shell = optarg;
	  break;

	case 'u':
	  if (su_mode != RUNUSER_MODE)
	    usage (EXIT_FAILURE);
	  runuser_user = optarg;
	  break;

	case 'h':
	  usage(0);

	case 'V':
	  printf(UTIL_LINUX_VERSION);
	  exit(EXIT_SUCCESS);

	default:
	  usage (EXIT_FAILURE);
	}
    }

  restricted = evaluate_uid ();

  if (optind < argc && !strcmp (argv[optind], "-"))
    {
      simulate_login = true;
      ++optind;
    }

  switch (su_mode) {
  case RUNUSER_MODE:
    if (runuser_user) {
      /* runuser -u <user> <command> */
      new_user = runuser_user;
      if (shell || fast_startup || command || simulate_login) {
        errx(EXIT_FAILURE,
	   _("options --{shell,fast,command,session-command,login} and "
	     "--user are mutually exclusive."));
      }
      if (optind == argc)
        errx(EXIT_FAILURE, _("COMMAND not specified."));

      break;
    }
    /* fallthrough if -u <user> is not specified, then follow
     * traditional su(1) behavior
     */
  case SU_MODE:
    if (optind < argc)
      new_user = argv[optind++];
    break;
  }

  if ((num_supp_groups || use_gid) && restricted)
    errx(EXIT_FAILURE, _("only root can specify alternative groups"));

  logindefs_load_defaults = load_config;

  pw = getpwnam (new_user);
  if (! (pw && pw->pw_name && pw->pw_name[0] && pw->pw_dir && pw->pw_dir[0]
	 && pw->pw_passwd))
    errx (EXIT_FAILURE, _("user %s does not exist"), new_user);

  /* Make a copy of the password information and point pw at the local
     copy instead.  Otherwise, some systems (e.g. Linux) would clobber
     the static data through the getlogin call from log_su.
     Also, make sure pw->pw_shell is a nonempty string.
     It may be NULL when NEW_USER is a username that is retrieved via NIS (YP),
     but that doesn't have a default shell listed.  */
  pw_copy = *pw;
  pw = &pw_copy;
  pw->pw_name = xstrdup (pw->pw_name);
  pw->pw_passwd = xstrdup (pw->pw_passwd);
  pw->pw_dir = xstrdup (pw->pw_dir);
  pw->pw_shell = xstrdup (pw->pw_shell && pw->pw_shell[0]
			  ? pw->pw_shell
			  : DEFAULT_SHELL);
  endpwent ();

  if (num_supp_groups && !use_gid)
  {
    pw->pw_gid = groups[1];
    memmove (groups, groups + 1, sizeof(gid_t) * num_supp_groups);
  }
  else if (use_gid)
  {
    pw->pw_gid = groups[0];
    num_supp_groups++;
  }

  authenticate (pw);

  if (request_same_session || !command || !pw->pw_uid)
    same_session = 1;

  /* initialize shell variable only if "-u <user>" not specified */
  if (runuser_user) {
    shell = NULL;
  } else {
    if (!shell && !change_environment)
      shell = getenv ("SHELL");
    if (shell && getuid () != 0 && restricted_shell (pw->pw_shell))
      {
	/* The user being su'd to has a nonstandard shell, and so is
	   probably a uucp account or has restricted access.  Don't
	   compromise the account by allowing access with a standard
	   shell.  */
	warnx (_("using restricted shell %s"), pw->pw_shell);
	shell = NULL;
      }
    shell = xstrdup (shell ? shell : pw->pw_shell);
  }

  init_groups (pw, groups, num_supp_groups);

  create_watching_parent ();
  /* Now we're in the child.  */

  change_identity (pw);
  if (!same_session)
    setsid ();

  /* Set environment after pam_open_session, which may put KRB5CCNAME
     into the pam_env, etc.  */

  modify_environment (pw, shell);

  if (simulate_login && chdir (pw->pw_dir) != 0)
    warn (_("warning: cannot change directory to %s"), pw->pw_dir);

  if (shell)
    run_shell (shell, command, argv + optind, max (0, argc - optind));
  else {
    execvp(argv[optind], &argv[optind]);
    err(EXIT_FAILURE, _("executing %s failed"), argv[optind]);
  }
}

// vim: sw=2 cinoptions=>4,n-2,{2,^-2,\:2,=2,g0,h2,p5,t0,+2,(0,u0,w1,m1