1 /* 2 * safe_finger - finger client wrapper that protects against nasty stuff 3 * from finger servers. Use this program for automatic reverse finger 4 * probes, not the raw finger command. 5 * 6 * Build with: cc -o safe_finger safe_finger.c 7 * 8 * The problem: some programs may react to stuff in the first column. Other 9 * programs may get upset by thrash anywhere on a line. File systems may 10 * fill up as the finger server keeps sending data. Text editors may bomb 11 * out on extremely long lines. The finger server may take forever because 12 * it is somehow wedged. The code below takes care of all this badness. 13 * 14 * Author: Wietse Venema, Eindhoven University of Technology, The Netherlands. 15 */ 16 17 #ifndef lint 18 static char sccsid[] = "@(#) safe_finger.c 1.4 94/12/28 17:42:41"; 19 #endif 20 21 /* System libraries */ 22 23 #include <sys/types.h> 24 #include <sys/stat.h> 25 #include <signal.h> 26 #include <stdio.h> 27 #include <ctype.h> 28 #include <pwd.h> 29 30 extern void exit(); 31 32 /* Local stuff */ 33 34 char path[] = "PATH=/bin:/usr/bin:/usr/ucb:/usr/bsd:/etc:/usr/etc:/usr/sbin"; 35 36 #define TIME_LIMIT 60 /* Do not keep listinging forever */ 37 #define INPUT_LENGTH 100000 /* Do not keep listinging forever */ 38 #define LINE_LENGTH 128 /* Editors can choke on long lines */ 39 #define FINGER_PROGRAM "finger" /* Most, if not all, UNIX systems */ 40 #define UNPRIV_NAME "nobody" /* Preferred privilege level */ 41 #define UNPRIV_UGID 32767 /* Default uid and gid */ 42 43 int finger_pid; 44 45 void cleanup(sig) 46 int sig; 47 { 48 kill(finger_pid, SIGKILL); 49 exit(0); 50 } 51 52 main(int argc, char **argv) 53 { 54 int c; 55 int line_length = 0; 56 int finger_status; 57 int wait_pid; 58 int input_count = 0; 59 struct passwd *pwd; 60 61 /* 62 * First of all, let's don't run with superuser privileges. 63 */ 64 if (getuid() == 0 || geteuid() == 0) { 65 if ((pwd = getpwnam(UNPRIV_NAME)) && pwd->pw_uid > 0) { 66 setgid(pwd->pw_gid); 67 setuid(pwd->pw_uid); 68 } else { 69 setgid(UNPRIV_UGID); 70 setuid(UNPRIV_UGID); 71 } 72 } 73 74 /* 75 * Redirect our standard input through the raw finger command. 76 */ 77 if (putenv(path)) { 78 fprintf(stderr, "%s: putenv: out of memory", argv[0]); 79 exit(1); 80 } 81 argv[0] = FINGER_PROGRAM; 82 finger_pid = pipe_stdin(argv); 83 84 /* 85 * Don't wait forever (Peter Wemm <peter@gecko.DIALix.oz.au>). 86 */ 87 signal(SIGALRM, cleanup); 88 (void) alarm(TIME_LIMIT); 89 90 /* 91 * Main filter loop. 92 */ 93 while ((c = getchar()) != EOF) { 94 if (input_count++ >= INPUT_LENGTH) { /* don't listen forever */ 95 fclose(stdin); 96 printf("\n\n Input truncated to %d bytes...\n", input_count - 1); 97 break; 98 } 99 if (c == '\n') { /* good: end of line */ 100 putchar(c); 101 line_length = 0; 102 } else { 103 if (line_length >= LINE_LENGTH) { /* force end of line */ 104 printf("\\\n"); 105 line_length = 0; 106 } 107 if (line_length == 0) { /* protect left margin */ 108 putchar(' '); 109 line_length++; 110 } 111 if (isascii(c) && (isprint(c) || isspace(c))) { /* text */ 112 if (c == '\\') { 113 putchar(c); 114 line_length++; 115 } 116 putchar(c); 117 line_length++; 118 } else { /* quote all other thash */ 119 printf("\\%03o", c & 0377); 120 line_length += 4; 121 } 122 } 123 } 124 125 /* 126 * Wait until the finger child process has terminated and account for its 127 * exit status. Which will always be zero on most systems. 128 */ 129 while ((wait_pid = wait(&finger_status)) != -1 && wait_pid != finger_pid) 130 /* void */ ; 131 return (wait_pid != finger_pid || finger_status != 0); 132 } 133 134 /* perror_exit - report system error text and terminate */ 135 136 void perror_exit(char *text) 137 { 138 perror(text); 139 exit(1); 140 } 141 142 /* pipe_stdin - pipe stdin through program (from my ANSI to OLD C converter) */ 143 144 int pipe_stdin(char **argv) 145 { 146 int pipefds[2]; 147 int pid; 148 int i; 149 struct stat st; 150 151 /* 152 * The code that sets up the pipe requires that file descriptors 0,1,2 153 * are already open. All kinds of mysterious things will happen if that 154 * is not the case. The following loops makes sure that descriptors 0,1,2 155 * are set up properly. 156 */ 157 158 for (i = 0; i < 3; i++) { 159 if (fstat(i, &st) == -1 && open("/dev/null", 2) != i) 160 perror_exit("open /dev/null"); 161 } 162 163 /* 164 * Set up the pipe that interposes the command into our standard input 165 * stream. 166 */ 167 168 if (pipe(pipefds)) 169 perror_exit("pipe"); 170 171 switch (pid = fork()) { 172 case -1: /* error */ 173 perror_exit("fork"); 174 /* NOTREACHED */ 175 case 0: /* child */ 176 (void) close(pipefds[0]); /* close reading end */ 177 (void) close(1); /* connect stdout to pipe */ 178 if (dup(pipefds[1]) != 1) 179 perror_exit("dup"); 180 (void) close(pipefds[1]); /* close redundant fd */ 181 (void) execvp(argv[0], argv); 182 perror_exit(argv[0]); 183 /* NOTREACHED */ 184 default: /* parent */ 185 (void) close(pipefds[1]); /* close writing end */ 186 (void) close(0); /* connect stdin to pipe */ 187 if (dup(pipefds[0]) != 0) 188 perror_exit("dup"); 189 (void) close(pipefds[0]); /* close redundant fd */ 190 return (pid); 191 } 192 } 193