ssu.c

#include <grp.h>
#include <pwd.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

#ifndef ENV_PATH
#define ENV_PATH "/bin:/sbin:/usr/bin:/usr/sbin"
#endif

static int exec_file(char **argv)
{
    int status;
    pid_t pid;

    switch ((pid = fork())) {
    case -1:
        perror("fork");
        return 1;
    case 0:
        execvp(argv[0], argv);
        fprintf(stderr, "execvp %s: %s\n", argv[0], strerror(errno));
        _exit((errno == ENOENT) + 126);
    default:
        waitpid(pid, &status, 0);
        return WIFSIGNALED(status) ? WTERMSIG(status) + 128 : WEXITSTATUS(status);
    }
}

static void print_usage(const char *name)
{
    fprintf(stderr, "usage: %s [-p] [-u user] -s\n", name);
    fprintf(stderr, "       %s [-p] [-u user] -e file...\n", name);
    fprintf(stderr, "       %s [-p] [-u user] command [args...]\n", name);
}

int main(int argc, char **argv)
{
    const char *term, *name = argv[0], *user = "root";
    unsigned pflag = 0, eflag = 0, sflag = 0;
    extern char **environ;
    struct passwd *pw;
    int opt;

    while ((opt = getopt(argc, argv, "u:hpes")) != -1) {
        switch (opt) {
        case 'p':
            pflag = 1;
            break;
        case 'e':
            eflag = 1;
            break;
        case 's':
            sflag = 2;
            break;
        case 'u':
            user = optarg;
            break;
        case 'h':
            print_usage(name);
            return 0;
        case '?':
            print_usage(name);
            return 1;
        }
    }

    argv += optind - sflag - eflag;
    argc -= optind;

    if ((!sflag && !argc) || (sflag && argc) || (eflag && !argc)) {
        print_usage(name);
        return 2;
    }

    if (getuid() != 0 && geteuid() != 0) {
        fprintf(stderr, "%s: %s\n", name, strerror(EPERM));
        return 1;
    }

    pw = getpwnam(user);

    if (!pw) {
        if (errno) {
            perror("getpwnam");
        }
        else {
            fprintf(stderr, "getpwnam %s: no such user\n", user);
        }

        return 1;
    }

    if (initgroups(pw->pw_name, pw->pw_gid) == -1) {
        perror("initgroups");
        return 1;
    }

    if (setgid(pw->pw_gid) == -1) {
        perror("setgid");
        return 1;
    }

    if (setuid(pw->pw_uid) == -1) {
        perror("setuid");
        return 1;
    }

    if (eflag) {
        argv[0] = getenv("EDITOR");

        if (!argv[0] || argv[0][0] == '\0') {
            argv[0] = "vi";
        }

        return exec_file(argv);
    }

    if (!pflag) {
        term = getenv("TERM");
        environ = NULL;

        if (term && term[0] != '\0') {
            setenv("TERM", term, 0);
        }

        setenv("LOGNAME", pw->pw_name, 0);
        setenv("SHELL", pw->pw_shell, 0);
        setenv("USER", pw->pw_name, 0);
        setenv("HOME", pw->pw_dir, 0);
        setenv("PATH", ENV_PATH, 0);
    }

    if (sflag) {
        argv[1] = NULL;
        argv[0] = getenv("SHELL");

        if (!argv[0] || argv[0][0] == '\0') {
            argv[0] = "/bin/sh";
        }
    }

    return exec_file(argv);
}