su: cleanup setenv related code

[thirdparty/util-linux.git] / login-utils / su-common.c

/*
 * su(1) 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
 * Copyright (C) 2016 Karel Zak <kzak@redhat.com>
 *
 * 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.
 *
 *
 * Based on an implementation by David MacKenzie <djm@gnu.ai.mit.edu>.
 */
#include <stdio.h>
#include <getopt.h>
#include <sys/types.h>
#include <pwd.h>
#include <grp.h>
#include <security/pam_appl.h>
#ifdef HAVE_SECURITY_PAM_MISC_H
# include <security/pam_misc.h>
#elif defined(HAVE_SECURITY_OPENPAM_H)
# include <security/openpam.h>
#endif
#include <signal.h>
#include <sys/wait.h>
#include <syslog.h>
#include <utmpx.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"
#include "pwdutils.h"

#include "logindefs.h"
#include "su-common.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/default/su"
#define _PATH_LOGINDEFS_RUNUSER "/etc/default/runuser"

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

/* 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

enum {
        EXIT_CANNOT_INVOKE = 126,
        EXIT_ENOENT = 127
};

/*
 * su/runuser control struct
 */
struct su_context {
        pam_handle_t    *pamh;                  /* PAM handler */
        struct pam_conv conv;                   /* PAM conversation */

        struct passwd   *pwd;                   /* new user info */
        char            *pwdbuf;                /* pwd strings */

        const char      *tty_name;              /* tty_path without /dev prefix */
        const char      *tty_number;            /* end of the tty_path */

        char            *new_user;              /* wanted user */
        char            *old_user;              /* orginal user */

        unsigned int runuser :1,                /* flase=su, true=runuser */
                     runuser_uopt :1,           /* runuser -u specified */
                     isterm :1,                 /* is stdin terminal? */
                     fast_startup :1,           /* pass the `-f' option to the subshell. */
                     simulate_login :1,         /* simulate a login instead of just starting a shell. */
                     change_environment :1,     /* change some environment vars to indicate the user su'd to.*/
                     same_session :1,           /* don't call setsid() with a command. */
                     suppress_pam_info:1,       /* don't print PAM info messages (Last login, etc.). */
                     pam_has_session :1,        /* PAM session opened */
                     pam_has_cred :1,           /* PAM cred established */
                     restricted :1;             /* false for root user */
};


static sig_atomic_t volatile caught_signal = false;

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

static void init_tty(struct su_context *su)
{
        su->isterm = isatty(STDIN_FILENO) ? 1 : 0;
        if (su->isterm)
                get_terminal_name(NULL, &su->tty_name, &su->tty_number);
}

/* 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 su_context *su, bool successful)
{
        openlog(program_invocation_short_name, 0, LOG_AUTH);
        syslog(LOG_NOTICE, "%s(to %s) %s on %s",
               successful ? "" :
               su->runuser ? "FAILED RUNUSER " : "FAILED SU ",
               su->new_user, su->old_user ? : "",
               su->tty_name ? : "none");
        closelog();
}

/*
 * Log failed login attempts in _PATH_BTMP if that exists.
 */
static void log_btmp(struct su_context *su)
{
        struct utmpx ut;
        struct timeval tv;

        memset(&ut, 0, sizeof(ut));
        strncpy(ut.ut_user,
                su->pwd && su->pwd->pw_name ? su->pwd->pw_name : "(unknown)",
                sizeof(ut.ut_user));

        if (su->tty_number)
                xstrncpy(ut.ut_id, su->tty_number, sizeof(ut.ut_id));
        if (su->tty_name)
                xstrncpy(ut.ut_line, su->tty_name, sizeof(ut.ut_line));

        gettimeofday(&tv, NULL);
        ut.ut_tv.tv_sec = tv.tv_sec;
        ut.ut_tv.tv_usec = tv.tv_usec;
        ut.ut_type = LOGIN_PROCESS;     /* XXX doesn't matter */
        ut.ut_pid = getpid();

        updwtmpx(_PATH_BTMP, &ut);
}

static int supam_conv(  int num_msg,
                        const struct pam_message **msg,
                        struct pam_response **resp,
                        void *data)
{
        struct su_context *su = (struct su_context *) data;

        if (su->suppress_pam_info
            && num_msg == 1
            && msg && msg[0]->msg_style == PAM_TEXT_INFO)
                return PAM_SUCCESS;

#ifdef HAVE_SECURITY_PAM_MISC_H
        return misc_conv(num_msg, msg, resp, data);
#elif defined(HAVE_SECURITY_OPENPAM_H)
        return openpam_ttyconv(num_msg, msg, resp, data);
#endif
}

static void supam_cleanup(struct su_context *su, int retcode)
{
        const int saved_errno = errno;

        if (su->pam_has_session)
                pam_close_session(su->pamh, 0);
        if (su->pam_has_cred)
                pam_setcred(su->pamh, PAM_DELETE_CRED | PAM_SILENT);
        pam_end(su->pamh, retcode);
        errno = saved_errno;
}


static void supam_export_environment(struct su_context *su)
{
        /* This is a copy but don't care to free as we exec later anyways.  */
        char **env = pam_getenvlist(su->pamh);

        while (env && *env) {
                if (putenv(*env) != 0)
                        err(EXIT_FAILURE, _("failed to modify environment"));
                env++;
        }
}

static void supam_authenticate(struct su_context *su)
{
        const char *srvname = NULL;
        int retval;

        srvname = su->runuser ?
                   (su->simulate_login ? PAM_SRVNAME_RUNUSER_L : PAM_SRVNAME_RUNUSER) :
                   (su->simulate_login ? PAM_SRVNAME_SU_L : PAM_SRVNAME_SU);

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

        if (su->tty_name) {
                retval = pam_set_item(su->pamh, PAM_TTY, su->tty_name);
                if (is_pam_failure(retval))
                        goto done;
        }
        if (su->old_user) {
                retval = pam_set_item(su->pamh, PAM_RUSER, (const void *) su->old_user);
                if (is_pam_failure(retval))
                        goto done;
        }
        if (su->runuser) {
                /*
                 * This is the only difference between runuser(1) and su(1). The command
                 * runuser(1) does not required authentication, because user is root.
                 */
                if (su->restricted)
                        errx(EXIT_FAILURE, _("may not be used by non-root users"));
                return;
        }

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

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

 done:

        log_syslog(su, !is_pam_failure(retval));

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

                log_btmp(su);

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

static void
create_watching_parent(struct su_context *su)
{
        pid_t child;
        sigset_t ourset;
        struct sigaction oldact[3];
        int status = 0;
        int retval;

        retval = pam_open_session(su->pamh, 0);
        if (is_pam_failure(retval)) {
                supam_cleanup(su, retval);
                errx(EXIT_FAILURE, _("cannot open session: %s"),
                     pam_strerror(su->pamh, retval));
        } else
                su->pam_has_session = 1;

        memset(oldact, 0, sizeof(oldact));

        child = fork();
        if (child == (pid_t) - 1) {
                supam_cleanup(su, 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 (!su->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,
                                                    &oldact[0])
                                       || sigprocmask(SIG_UNBLOCK, &ourset,
                                                      NULL))) {
                        warn(_("cannot set signal handler"));
                        caught_signal = true;
                }
                if (!caught_signal && !su->same_session
                    && (sigaction(SIGINT, &action, &oldact[1])
                        || sigaction(SIGQUIT, &action, &oldact[2]))) {
                        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)) {
                                fprintf(stderr, "%s%s\n",
                                        strsignal(WTERMSIG(status)),
                                        WCOREDUMP(status) ? _(" (core dumped)")
                                        : "");
                                status = WTERMSIG(status) + 128;
                        } else
                                status = WEXITSTATUS(status);
                } else if (caught_signal)
                        status = caught_signal + 128;
                else
                        status = 1;
        } else
                status = 1;

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

        supam_cleanup(su, PAM_SUCCESS);

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

                /* Let's terminate itself with the received signal.
                 *
                 * It seems that shells use WIFSIGNALED() rather than our exit status
                 * value to detect situations when is necessary to cleanup (reset)
                 * terminal settings (kzak -- Jun 2013).
                 */
                switch (caught_signal) {
                case SIGTERM:
                        sigaction(SIGTERM, &oldact[0], NULL);
                        break;
                case SIGINT:
                        sigaction(SIGINT, &oldact[1], NULL);
                        break;
                case SIGQUIT:
                        sigaction(SIGQUIT, &oldact[2], NULL);
                        break;
                default:
                        /* just in case that signal stuff initialization failed and
                         * caught_signal = true */
                        caught_signal = SIGKILL;
                        break;
                }
                kill(getpid(), caught_signal);
        }
        exit(status);
}


static void setenv_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 the %s environment variable"), "PATH");
}

static void modify_environment(struct su_context *su, const char *shell)
{
        const struct passwd *pw = su->pwd;

        /* Leave TERM unchanged.  Set HOME, SHELL, USER, LOGNAME, PATH.
         * Unset all other environment variables.
         */
        if (su->simulate_login) {
                char *term = getenv("TERM");
                if (term)
                        term = xstrdup(term);

                environ = xmalloc((6 + ! !term) * sizeof(char *));
                environ[0] = NULL;
                if (term) {
                        xsetenv("TERM", term, 1);
                        free(term);
                }

                xsetenv("HOME", pw->pw_dir, 1);
                if (shell)
                        xsetenv("SHELL", shell, 1);
                xsetenv("USER", pw->pw_name, 1);
                xsetenv("LOGNAME", pw->pw_name, 1);
                setenv_path(pw);

        /* Set HOME, SHELL, and (if not becoming a superuser) USER and LOGNAME.
         */
        } else if (su->change_environment) {
                xsetenv("HOME", pw->pw_dir, 1);
                if (shell)
                        xsetenv("SHELL", shell, 1);

                if (getlogindefs_bool("ALWAYS_SET_PATH", 0))
                        setenv_path(pw);

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

        supam_export_environment(su);
}

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

static void
init_groups(struct su_context *su, gid_t * groups, size_t num_groups)
{
        int retval;

        errno = 0;

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

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

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

static void
change_identity (const struct passwd * const 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, 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(
                struct su_context *su,
                char const *shell, char const *command, char **additional_args,
                size_t n_additional_args)
{
        size_t n_args = 1 + su->fast_startup + 2 * ! !command + n_additional_args + 1;
        char const **args = xcalloc(n_args, sizeof *args);
        size_t argno = 1;
        int rc;

        if (su->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 (su->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);

        rc = errno == ENOENT ? EXIT_ENOENT : EXIT_CANNOT_INVOKE;
        err(rc, _("failed to execute %s"), shell);
}

/* Return true if SHELL is a restricted shell (one not returned by
 * getusershell), else false, meaning it is a standard shell.
 */
static bool is_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 usage_common(void)
{
        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"), stdout);
        fputs(USAGE_SEPARATOR, 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"
                "                                   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);
        printf(USAGE_HELP_OPTIONS(33));

}

static void __attribute__ ((__noreturn__)) usage_runuser(void)
{
        fputs(USAGE_HEADER, stdout);
        fprintf(stdout,
                _(" %1$s [options] -u <user> [[--] <command>]\n"
                  " %1$s [options] [-] [<user> [<argument>...]]\n"),
                program_invocation_short_name);

        fputs(USAGE_SEPARATOR, stdout);
        fputs(_("Run <command> with the effective user ID and group ID of <user>.  If -u is\n"
               "not given, fall back to su(1)-compatible semantics and execute standard shell.\n"
               "The options -c, -f, -l, and -s are mutually exclusive with -u.\n"), stdout);

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

        fprintf(stdout, USAGE_MAN_TAIL("runuser(1)"));
        exit(EXIT_SUCCESS);
}

static void __attribute__ ((__noreturn__)) usage_su(void)
{
        fputs(USAGE_HEADER, stdout);
        fprintf(stdout,
                _(" %s [options] [-] [<user> [<argument>...]]\n"),
                program_invocation_short_name);

        fputs(USAGE_SEPARATOR, stdout);
        fputs(_("Change the effective user ID and group ID to that of <user>.\n"
                "A mere - implies -l.  If <user> is not given, root is assumed.\n"), stdout);

        fputs(USAGE_OPTIONS, stdout);
        usage_common();

        fprintf(stdout, USAGE_MAN_TAIL("su(1)"));
        exit(EXIT_SUCCESS);
}

static void usage(int mode)
{
        if (mode == SU_MODE)
                usage_su();
        else
                usage_runuser();
}

static void load_config(void *data)
{
        struct su_context *su = (struct su_context *) data;

        logindefs_load_file(su->runuser ? _PATH_LOGINDEFS_RUNUSER : _PATH_LOGINDEFS_SU);
        logindefs_load_file(_PATH_LOGINDEFS);
}

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

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

static gid_t
add_supp_group(const char *name, gid_t ** groups, size_t * ngroups)
{
        struct group *gr;

        if (*ngroups >= NGROUPS_MAX)
                errx(EXIT_FAILURE,
                     P_
                     ("specifying more than %d supplemental group is not possible",
                      "specifying more than %d supplemental groups is not possible",
                      NGROUPS_MAX - 1), NGROUPS_MAX - 1);

        gr = getgrnam(name);
        if (!gr)
                errx(EXIT_FAILURE, _("group %s does not exist"), name);

        *groups = xrealloc(*groups, sizeof(gid_t) * (*ngroups + 1));
        (*groups)[*ngroups] = gr->gr_gid;
        (*ngroups)++;

        return gr->gr_gid;
}

int
su_main(int argc, char **argv, int mode)
{
        struct su_context _su = {
                .conv                   = { supam_conv, NULL },
                .runuser                = (mode == RUNUSER_MODE ? 1 : 0),
                .change_environment     = 1,
                .new_user               = DEFAULT_USER
        }, *su = &_su;

        int optc;
        char *command = NULL;
        int request_same_session = 0;
        char *shell = NULL;

        gid_t *groups = NULL;
        size_t ngroups = 0;
        bool use_supp = false;
        bool use_gid = false;
        gid_t 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->conv.appdata_ptr = (void *) su;

        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':
                        su->fast_startup = true;
                        break;

                case 'g':
                        use_gid = true;
                        gid = add_supp_group(optarg, &groups, &ngroups);
                        break;

                case 'G':
                        use_supp = true;
                        add_supp_group(optarg, &groups, &ngroups);
                        break;

                case 'l':
                        su->simulate_login = true;
                        break;

                case 'm':
                case 'p':
                        su->change_environment = false;
                        break;

                case 's':
                        shell = optarg;
                        break;

                case 'u':
                        if (!su->runuser)
                                errtryhelp(EXIT_FAILURE);
                        su->runuser_uopt = 1;
                        su->new_user = optarg;
                        break;

                case 'h':
                        usage(mode);

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

                default:
                        errtryhelp(EXIT_FAILURE);
                }
        }

        su->restricted = evaluate_uid();

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

        if (su->simulate_login && !su->change_environment) {
                warnx(_
                      ("ignoring --preserve-environment, it's mutually exclusive with --login"));
                su->change_environment = true;
        }

        switch (mode) {
        case RUNUSER_MODE:
                /* runuser -u <user> <command> */
                if (su->runuser_uopt) {
                        if (shell || su->fast_startup || command || su->simulate_login)
                                errx(EXIT_FAILURE,
                                     _("options --{shell,fast,command,session-command,login} and "
                                      "--user are mutually exclusive"));
                        if (optind == argc)
                                errx(EXIT_FAILURE, _("no command was specified"));
                        break;
                }
                /* fallthrough if -u <user> is not specified, then follow
                 * traditional su(1) behavior
                 */
        case SU_MODE:
                if (optind < argc)
                        su->new_user = argv[optind++];
                break;
        }

        if ((use_supp || use_gid) && su->restricted)
                errx(EXIT_FAILURE,
                     _("only root can specify alternative groups"));

        logindefs_set_loader(load_config, (void *) su);
        init_tty(su);

        su->pwd = xgetpwnam(su->new_user, &su->pwdbuf);
        if (!su->pwd
            || !su->pwd->pw_passwd
            || !su->pwd->pw_name || !*su->pwd->pw_name
            || !su->pwd->pw_dir  || !*su->pwd->pw_dir)
                errx(EXIT_FAILURE, _("user %s does not exist"), su->new_user);

        su->new_user = su->pwd->pw_name;
        su->old_user = xgetlogin();

        if (!su->pwd->pw_shell || !*su->pwd->pw_shell)
                su->pwd->pw_shell = DEFAULT_SHELL;

        if (use_supp && !use_gid)
                su->pwd->pw_gid = groups[0];
        else if (use_gid)
                su->pwd->pw_gid = gid;

        supam_authenticate(su);

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

        /* initialize shell variable only if "-u <user>" not specified */
        if (su->runuser_uopt) {
                shell = NULL;
        } else {
                if (!shell && !su->change_environment)
                        shell = getenv("SHELL");

                if (shell
                    && getuid() != 0
                    && is_restricted_shell(su->pwd->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"), su->pwd->pw_shell);
                        shell = NULL;
                }
                shell = xstrdup(shell ? shell : su->pwd->pw_shell);
        }

        init_groups(su, groups, ngroups);

        if (!su->simulate_login || command)
                su->suppress_pam_info = 1;      /* don't print PAM info messages */

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

        change_identity(su->pwd);
        if (!su->same_session)
                setsid();

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

        modify_environment(su, shell);

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

        if (shell)
                run_shell(su, shell, command, argv + optind, max(0, argc - optind));

        execvp(argv[optind], &argv[optind]);
        err(EXIT_FAILURE, _("failed to execute %s"), argv[optind]);
}