Users and permissions: accounts, login, enforced file permissions, setuid, su, passwd, doas
The kernel now checks owner/group/other on open, create, unlink, rename, exec and path search; honours set-user-id and set-group-id; restricts kill, mount, reboot and mknod; keeps supplementary groups; and implements chown. devtmpfs nodes carry owners and modes that chmod/chown keep. Terminals run login, the image has root and schos, X starts as the ordinary user. Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao
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Nexoniarz
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34 changed files with 1342 additions and 120 deletions
221
cmd/su/main.c
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221
cmd/su/main.c
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// SPDX-License-Identifier: GPL-2.0-or-later
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//
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// su [-|-l] [-p] [-c COMMAND] [USER]
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//
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// Run a shell, or one command, as another user -- root by default. Setuid root,
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// and a program of its own rather than a toybox applet: toybox is one binary
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// under every name, and a setuid copy of it can be asked for any of its other
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// applets by naming them, all as root.
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//
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// The rules are the ones su has always had, kept short:
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//
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// - root may become anyone, with no password;
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// - anyone else is asked for the target's password, and gets three tries;
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// - becoming root takes membership in `wheel` first, whatever the password
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// is -- an account with no password is otherwise a door for every user.
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//
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// -, -l a login shell: a clean environment, the target's home, argv[0] "-sh"
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// -p keep the environment (HOME, SHELL, USER and LOGNAME are still set
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// unless the target is root)
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// -c CMD run CMD with the shell's -c rather than an interactive shell
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#define _GNU_SOURCE
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#include <crypt.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <grp.h>
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#include <pwd.h>
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#include <shadow.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/types.h>
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#include <termios.h>
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#include <unistd.h>
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static int
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in_wheel(uid_t uid, gid_t gid)
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{
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struct group *g = getgrnam("wheel");
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struct passwd *pw = getpwuid(uid);
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if (!g)
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return 0;
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if (gid == g->gr_gid)
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return 1;
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for (char **m = g->gr_mem; pw && m && *m; m++)
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if (strcmp(*m, pw->pw_name) == 0)
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return 1;
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return 0;
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}
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// A line from the terminal with echo off. The password is read from the
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// controlling terminal, not stdin, so a script cannot feed it.
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static int
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read_password(const char *prompt, char *buf, size_t n)
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{
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int fd = open("/dev/tty", O_RDWR | O_NOCTTY);
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struct termios t, old;
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size_t i = 0;
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char c;
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if (fd < 0)
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return -1;
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if (tcgetattr(fd, &old) == 0) {
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t = old;
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t.c_lflag &= ~(ECHO | ECHONL);
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tcsetattr(fd, TCSAFLUSH, &t);
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}
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if (write(fd, prompt, strlen(prompt)) < 0) {}
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while (read(fd, &c, 1) == 1 && c != '\n' && c != '\r')
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if (i + 1 < n)
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buf[i++] = c;
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buf[i] = 0;
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tcsetattr(fd, TCSAFLUSH, &old);
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if (write(fd, "\n", 1) < 0) {}
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close(fd);
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return 0;
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}
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// Does `pw` accept this password? An empty hash accepts anything, a locked one
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// ("*", "!...") nothing.
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static int
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password_ok(const struct passwd *pw, const char *pass)
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{
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const char *hash = pw->pw_passwd;
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struct spwd *sp;
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char *enc;
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if (hash[0] == 'x' && hash[1] == 0) {
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sp = getspnam(pw->pw_name);
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if (!sp)
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return 0;
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hash = sp->sp_pwdp;
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}
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if (hash[0] == 0)
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return 1;
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if (hash[0] == '*' || hash[0] == '!')
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return 0;
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enc = crypt(pass, hash);
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return enc && strcmp(enc, hash) == 0;
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}
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int
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main(int argc, char **argv)
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{
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const char *target = "root", *cmd = 0;
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int login = 0, keep = 0, i;
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struct passwd *pw, *me;
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uid_t uid = getuid();
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char pass[256];
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for (i = 1; i < argc; i++) {
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if (!strcmp(argv[i], "-") || !strcmp(argv[i], "-l"))
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login = 1;
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else if (!strcmp(argv[i], "-p"))
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keep = 1;
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else if (!strcmp(argv[i], "-c") && i + 1 < argc)
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cmd = argv[++i];
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else if (argv[i][0] == '-' && argv[i][1]) {
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fprintf(stderr, "usage: su [-|-l] [-p] [-c COMMAND] [USER]\n");
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return 2;
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} else
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target = argv[i];
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}
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if (geteuid() != 0) {
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fprintf(stderr, "su: must be setuid root\n");
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return 1;
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}
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// getpwnam and getpwuid return the same static buffer, so the second call
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// overwrites the first answer. Keep our own copy of the one that matters.
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if (!(pw = getpwnam(target))) {
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fprintf(stderr, "su: unknown user %s\n", target);
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return 1;
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}
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{
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static struct passwd copy;
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copy = *pw;
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copy.pw_name = strdup(pw->pw_name);
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copy.pw_passwd = strdup(pw->pw_passwd);
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copy.pw_dir = strdup(pw->pw_dir);
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copy.pw_shell = strdup(pw->pw_shell);
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pw = ©
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}
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me = getpwuid(uid);
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if (uid != 0) {
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int tries, ok = 0;
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if (pw->pw_uid == 0 && !in_wheel(uid, getgid())) {
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fprintf(stderr, "su: %s is not in the wheel group\n",
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me ? me->pw_name : "you");
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return 1;
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}
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for (tries = 0; tries < 3 && !ok; tries++) {
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if (pw->pw_passwd[0] == 0 ||
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(pw->pw_passwd[0] == 'x' && getspnam(pw->pw_name) &&
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getspnam(pw->pw_name)->sp_pwdp[0] == 0)) {
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ok = 1;
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break;
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}
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if (read_password("Password: ", pass, sizeof pass) < 0) {
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fprintf(stderr, "su: no terminal\n");
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return 1;
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}
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ok = password_ok(pw, pass);
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memset(pass, 0, sizeof pass);
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if (!ok) {
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sleep(2);
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fprintf(stderr, "su: incorrect password\n");
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}
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}
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if (!ok)
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return 1;
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}
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if (initgroups(pw->pw_name, pw->pw_gid) < 0 ||
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setgid(pw->pw_gid) < 0 || setuid(pw->pw_uid) < 0) {
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fprintf(stderr, "su: cannot become %s: %s\n", pw->pw_name, strerror(errno));
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return 1;
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}
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{
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const char *shell = pw->pw_shell && *pw->pw_shell ? pw->pw_shell : "/bin/sh";
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char *sh_argv[5];
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int n = 0;
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char arg0[256];
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if (login) {
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clearenv();
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setenv("PATH", "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", 1);
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if (getenv("TERM") == 0)
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setenv("TERM", "linux", 1);
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}
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if (login || !keep || pw->pw_uid != 0) {
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setenv("HOME", pw->pw_dir, 1);
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setenv("SHELL", shell, 1);
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setenv("USER", pw->pw_name, 1);
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setenv("LOGNAME", pw->pw_name, 1);
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}
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if (login && chdir(pw->pw_dir) < 0)
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chdir("/");
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if (login) {
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const char *base = strrchr(shell, '/');
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snprintf(arg0, sizeof arg0, "-%s", base ? base + 1 : shell);
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sh_argv[n++] = arg0;
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} else {
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const char *base = strrchr(shell, '/');
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sh_argv[n++] = (char *)(base ? base + 1 : shell);
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}
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if (cmd) {
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sh_argv[n++] = "-c";
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sh_argv[n++] = (char *)cmd;
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}
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sh_argv[n] = 0;
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execv(shell, sh_argv);
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fprintf(stderr, "su: %s: %s\n", shell, strerror(errno));
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}
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return 1;
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}
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