cmd/tcpd/safe_finger.c

 /*
  * safe_finger - finger client wrapper that protects against nasty stuff
  * from finger servers. Use this program for automatic reverse finger
  * probes, not the raw finger command.
  *
  * Build with: cc -o safe_finger safe_finger.c
  *
  * The problem: some programs may react to stuff in the first column. Other
  * programs may get upset by thrash anywhere on a line. File systems may
  * fill up as the finger server keeps sending data. Text editors may bomb
  * out on extremely long lines. The finger server may take forever because
  * it is somehow wedged. The code below takes care of all this badness.
  *
  * Author: Wietse Venema, Eindhoven University of Technology, The Netherlands.
  */

#ifndef lint
static char sccsid[] = "@(#) safe_finger.c 1.4 94/12/28 17:42:41";
#endif

/* System libraries */

#include <sys/types.h>
#include <sys/stat.h>
#include <signal.h>
#include <stdio.h>
#include <ctype.h>
#include <pwd.h>

extern void exit();

/* Local stuff */

char    path[] = "PATH=/bin:/usr/bin:/usr/ucb:/usr/bsd:/etc:/usr/etc:/usr/sbin";

#define TIME_LIMIT  60      /* Do not keep listinging forever */
#define INPUT_LENGTH    100000      /* Do not keep listinging forever */
#define LINE_LENGTH 128     /* Editors can choke on long lines */
#define FINGER_PROGRAM  "finger"    /* Most, if not all, UNIX systems */
#define UNPRIV_NAME "nobody"    /* Preferred privilege level */
#define UNPRIV_UGID 32767       /* Default uid and gid */

int     finger_pid;

void    cleanup(sig)
int     sig;
{
    kill(finger_pid, SIGKILL);
    exit(0);
}

int
main(argc, argv)
int     argc;
char  **argv;
{
    int     c;
    int     line_length = 0;
    int     finger_status;
    int     wait_pid;
    int     input_count = 0;
    struct passwd *pwd;

    /*
     * First of all, let's don't run with superuser privileges.
     */
    if (getuid() == 0 || geteuid() == 0) {
    if ((pwd = getpwnam(UNPRIV_NAME)) && pwd->pw_uid > 0) {
        setgid(pwd->pw_gid);
        setuid(pwd->pw_uid);
    } else {
        setgid(UNPRIV_UGID);
        setuid(UNPRIV_UGID);
    }
    }

    /*
     * Redirect our standard input through the raw finger command.
     */
    if (putenv(path)) {
    fprintf(stderr, "%s: putenv: out of memory", argv[0]);
    exit(1);
    }
    argv[0] = FINGER_PROGRAM;
    finger_pid = pipe_stdin(argv);

    /*
     * Don't wait forever (Peter Wemm <peter@gecko.DIALix.oz.au>).
     */
    signal(SIGALRM, cleanup);
    (void) alarm(TIME_LIMIT);

    /*
     * Main filter loop.
     */
    while ((c = getchar()) != EOF) {
    if (input_count++ >= INPUT_LENGTH) {    /* don't listen forever */
        fclose(stdin);
        printf("\n\n Input truncated to %d bytes...\n", input_count - 1);
        break;
    }
    if (c == '\n') {            /* good: end of line */
        putchar(c);
        line_length = 0;
    } else {
        if (line_length >= LINE_LENGTH) {   /* force end of line */
        printf("\\\n");
        line_length = 0;
        }
        if (line_length == 0) {     /* protect left margin */
        putchar(' ');
        line_length++;
        }
        if (isascii(c) && (isprint(c) || isspace(c))) { /* text */
        if (c == '\\') {
            putchar(c);
            line_length++;
        }
        putchar(c);
        line_length++;
        } else {                /* quote all other thash */
        printf("\\%03o", c & 0377);
        line_length += 4;
        }
    }
    }

    /*
     * Wait until the finger child process has terminated and account for its
     * exit status. Which will always be zero on most systems.
     */
    while ((wait_pid = wait(&finger_status)) != -1 && wait_pid != finger_pid)
     /* void */ ;
    return (wait_pid != finger_pid || finger_status != 0);
}

/* perror_exit - report system error text and terminate */

void    perror_exit(text)
char   *text;
{
    perror(text);
    exit(1);
}

/* pipe_stdin - pipe stdin through program (from my ANSI to OLD C converter) */

int     pipe_stdin(argv)
char  **argv;
{
    int     pipefds[2];
    int     pid;
    int     i;
    struct stat st;

    /*
     * The code that sets up the pipe requires that file descriptors 0,1,2
     * are already open. All kinds of mysterious things will happen if that
     * is not the case. The following loops makes sure that descriptors 0,1,2
     * are set up properly.
     */

    for (i = 0; i < 3; i++) {
    if (fstat(i, &st) == -1 && open("/dev/null", 2) != i)
        perror_exit("open /dev/null");
    }

    /*
     * Set up the pipe that interposes the command into our standard input
     * stream.
     */

    if (pipe(pipefds))
    perror_exit("pipe");

    switch (pid = fork()) {
    case -1:                    /* error */
    perror_exit("fork");
    /* NOTREACHED */
    case 0:                 /* child */
    (void) close(pipefds[0]);       /* close reading end */
    (void) close(1);            /* connect stdout to pipe */
    if (dup(pipefds[1]) != 1)
        perror_exit("dup");
    (void) close(pipefds[1]);       /* close redundant fd */
    (void) execvp(argv[0], argv);
    perror_exit(argv[0]);
    /* NOTREACHED */
    default:                    /* parent */
    (void) close(pipefds[1]);       /* close writing end */
    (void) close(0);            /* connect stdin to pipe */
    if (dup(pipefds[0]) != 0)
        perror_exit("dup");
    (void) close(pipefds[0]);       /* close redundant fd */
    return (pid);
    }
}