release/src/router/busybox/loginutils/getty.c

   1 /* vi: set sw=4 ts=4: */
   2 /* agetty.c - another getty program for Linux. By W. Z. Venema 1989
   3  * Ported to Linux by Peter Orbaek <poe@daimi.aau.dk>
   4  * This program is freely distributable. The entire man-page used to
   5  * be here. Now read the real man-page agetty.8 instead.
   6  *
   7  * option added by Eric Rasmussen <ear@usfirst.org> - 12/28/95
   8  *
   9  * 1999-02-22 Arkadiusz Mickiewicz <misiek@misiek.eu.org>
  10  * - added Native Language Support
  11  *
  12  * 1999-05-05 Thorsten Kranzkowski <dl8bcu@gmx.net>
  13  * - enable hardware flow control before displaying /etc/issue
  14  *
  15  * Licensed under GPLv2 or later, see file LICENSE in this source tree.
  16  */
  17
  18 #include "libbb.h"
  19 #include <syslog.h>
  20
  21 #if ENABLE_FEATURE_UTMP
  22 # include <utmp.h> /* LOGIN_PROCESS */
  23 #endif
  24
  25 #ifndef IUCLC
  26 # define IUCLC 0
  27 #endif
  28
  29 /*
  30  * Some heuristics to find out what environment we are in: if it is not
  31  * System V, assume it is SunOS 4.
  32  */
  33 #ifdef LOGIN_PROCESS                    /* defined in System V utmp.h */
  34 #include <sys/utsname.h>
  35 #else /* if !sysV style, wtmp/utmp code is off */
  36 #undef ENABLE_FEATURE_UTMP
  37 #undef ENABLE_FEATURE_WTMP
  38 #define ENABLE_FEATURE_UTMP 0
  39 #define ENABLE_FEATURE_WTMP 0
  40 #endif  /* LOGIN_PROCESS */
  41
  42 /*
  43  * Things you may want to modify.
  44  *
  45  * You may disagree with the default line-editing etc. characters defined
  46  * below. Note, however, that DEL cannot be used for interrupt generation
  47  * and for line editing at the same time.
  48  */
  49
  50 /* I doubt there are systems which still need this */
  51 #undef HANDLE_ALLCAPS
  52 #undef ANCIENT_BS_KILL_CHARS
  53
  54 #define _PATH_LOGIN "/bin/login"
  55
  56 /* If ISSUE is not defined, getty will never display the contents of the
  57  * /etc/issue file. You will not want to spit out large "issue" files at the
  58  * wrong baud rate.
  59  */
  60 #define ISSUE "/etc/issue"              /* displayed before the login prompt */
  61
  62 /* Some shorthands for control characters. */
  63 #define CTL(x)          ((x) ^ 0100)    /* Assumes ASCII dialect */
  64 #define CR              CTL('M')        /* carriage return */
  65 #define NL              CTL('J')        /* line feed */
  66 #define BS              CTL('H')        /* back space */
  67 #define DEL             CTL('?')        /* delete */
  68
  69 /* Defaults for line-editing etc. characters; you may want to change this. */
  70 #define DEF_ERASE       DEL             /* default erase character */
  71 #define DEF_INTR        CTL('C')        /* default interrupt character */
  72 #define DEF_QUIT        CTL('\\')       /* default quit char */
  73 #define DEF_KILL        CTL('U')        /* default kill char */
  74 #define DEF_EOF         CTL('D')        /* default EOF char */
  75 #define DEF_EOL         '\n'
  76 #define DEF_SWITCH      0               /* default switch char */
  77
  78 /*
  79  * When multiple baud rates are specified on the command line, the first one
  80  * we will try is the first one specified.
  81  */
  82 #define MAX_SPEED       10              /* max. nr. of baud rates */
  83
  84 /* Storage for command-line options. */
  85 struct options {
  86         int flags;                      /* toggle switches, see below */
  87         unsigned timeout;               /* time-out period */
  88         const char *login;              /* login program */
  89         const char *tty;                /* name of tty */
  90         const char *initstring;         /* modem init string */
  91         const char *issue;              /* alternative issue file */
  92         int numspeed;                   /* number of baud rates to try */
  93         int speeds[MAX_SPEED];          /* baud rates to be tried */
  94 };
  95
  96 /* Storage for things detected while the login name was read. */
  97 struct chardata {
  98         unsigned char erase;    /* erase character */
  99         unsigned char kill;     /* kill character */
 100         unsigned char eol;      /* end-of-line character */
 101         unsigned char parity;   /* what parity did we see */
 102         /* (parity & 1): saw odd parity char with 7th bit set */
 103         /* (parity & 2): saw even parity char with 7th bit set */
 104         /* parity == 0: probably 7-bit, space parity? */
 105         /* parity == 1: probably 7-bit, odd parity? */
 106         /* parity == 2: probably 7-bit, even parity? */
 107         /* parity == 3: definitely 8 bit, no parity! */
 108         /* Hmm... with any value of "parity" 8 bit, no parity is possible */
 109 #ifdef HANDLE_ALLCAPS
 110         unsigned char capslock; /* upper case without lower case */
 111 #endif
 112 };
 113
 114
 115 /* Initial values for the above. */
 116 static const struct chardata init_chardata = {
 117         DEF_ERASE,                              /* default erase character */
 118         DEF_KILL,                               /* default kill character */
 119         13,                                     /* default eol char */
 120         0,                                      /* space parity */
 121 #ifdef HANDLE_ALLCAPS
 122         0,                                      /* no capslock */
 123 #endif
 124 };
 125
 126 static const char opt_string[] ALIGN1 = "I:LH:f:hil:mt:wn";
 127 #define F_INITSTRING    (1 << 0)        /* -I initstring is set */
 128 #define F_LOCAL         (1 << 1)        /* -L force local */
 129 #define F_FAKEHOST      (1 << 2)        /* -H fake hostname */
 130 #define F_CUSTISSUE     (1 << 3)        /* -f give alternative issue file */
 131 #define F_RTSCTS        (1 << 4)        /* -h enable RTS/CTS flow control */
 132 #define F_ISSUE         (1 << 5)        /* -i display /etc/issue */
 133 #define F_LOGIN         (1 << 6)        /* -l non-default login program */
 134 #define F_PARSE         (1 << 7)        /* -m process modem status messages */
 135 #define F_TIMEOUT       (1 << 8)        /* -t time out */
 136 #define F_WAITCRLF      (1 << 9)        /* -w wait for CR or LF */
 137 #define F_NOPROMPT      (1 << 10)       /* -n don't ask for login name */
 138
 139
 140 #define line_buf bb_common_bufsiz1
 141
 142 /* The following is used for understandable diagnostics. */
 143 #ifdef DEBUGGING
 144 static FILE *dbf;
 145 #define DEBUGTERM "/dev/ttyp0"
 146 #define debug(...) do { fprintf(dbf, __VA_ARGS__); fflush(dbf); } while (0)
 147 #else
 148 #define debug(...) ((void)0)
 149 #endif
 150
 151
 152 /* bcode - convert speed string to speed code; return <= 0 on failure */
 153 static int bcode(const char *s)
 154 {
 155         int value = bb_strtou(s, NULL, 10); /* yes, int is intended! */
 156         if (value < 0) /* bad terminating char, overflow, etc */
 157                 return value;
 158         return tty_value_to_baud(value);
 159 }
 160
 161 /* parse_speeds - parse alternate baud rates */
 162 static void parse_speeds(struct options *op, char *arg)
 163 {
 164         char *cp;
 165
 166         /* NB: at least one iteration is always done */
 167         debug("entered parse_speeds\n");
 168         while ((cp = strsep(&arg, ",")) != NULL) {
 169                 op->speeds[op->numspeed] = bcode(cp);
 170                 if (op->speeds[op->numspeed] < 0)
 171                         bb_error_msg_and_die("bad speed: %s", cp);
 172                 /* note: arg "0" turns into speed B0 */
 173                 op->numspeed++;
 174                 if (op->numspeed > MAX_SPEED)
 175                         bb_error_msg_and_die("too many alternate speeds");
 176         }
 177         debug("exiting parse_speeds\n");
 178 }
 179
 180 /* parse_args - parse command-line arguments */
 181 static void parse_args(char **argv, struct options *op, char **fakehost_p)
 182 {
 183         char *ts;
 184
 185         opt_complementary = "-2:t+"; /* at least 2 args; -t N */
 186         op->flags = getopt32(argv, opt_string,
 187                 &(op->initstring), fakehost_p, &(op->issue),
 188                 &(op->login), &op->timeout);
 189         argv += optind;
 190         if (op->flags & F_INITSTRING) {
 191                 op->initstring = xstrdup(op->initstring);
 192                 /* decode \ddd octal codes into chars */
 193                 strcpy_and_process_escape_sequences((char*)op->initstring, op->initstring);
 194         }
 195         op->flags ^= F_ISSUE;           /* invert flag "show /etc/issue" */
 196         debug("after getopt\n");
 197
 198         /* we loosen up a bit and accept both "baudrate tty" and "tty baudrate" */
 199         op->tty = argv[0];      /* tty name */
 200         ts = argv[1];           /* baud rate(s) */
 201         if (isdigit(argv[0][0])) {
 202                 /* a number first, assume it's a speed (BSD style) */
 203                 op->tty = ts;   /* tty name is in argv[1] */
 204                 ts = argv[0];   /* baud rate(s) */
 205         }
 206         parse_speeds(op, ts);
 207         applet_name = xasprintf("getty: %s", op->tty);
 208
 209         if (argv[2])
 210                 xsetenv("TERM", argv[2]);
 211
 212         debug("exiting parse_args\n");
 213 }
 214
 215 /* open_tty - set up tty as standard { input, output, error } */
 216 static void open_tty(const char *tty)
 217 {
 218         /* Set up new standard input, unless we are given an already opened port. */
 219         if (NOT_LONE_DASH(tty)) {
 220 //              struct stat st;
 221 //              int cur_dir_fd;
 222 //              int fd;
 223
 224                 /* Sanity checks... */
 225 //              cur_dir_fd = xopen(".", O_DIRECTORY | O_NONBLOCK);
 226 //              xchdir("/dev");
 227 //              xstat(tty, &st);
 228 //              if (!S_ISCHR(st.st_mode))
 229 //                      bb_error_msg_and_die("not a character device");
 230
 231                 if (tty[0] != '/')
 232                         tty = xasprintf("/dev/%s", tty); /* will leak it */
 233
 234                 /* Open the tty as standard input. */
 235                 debug("open(2)\n");
 236                 close(0);
 237                 /*fd =*/ xopen(tty, O_RDWR | O_NONBLOCK); /* uses fd 0 */
 238
 239 //              /* Restore current directory */
 240 //              fchdir(cur_dir_fd);
 241
 242                 /* Open the tty as standard input, continued */
 243 //              xmove_fd(fd, 0);
 244 //              /* fd is >= cur_dir_fd, and cur_dir_fd gets closed too here: */
 245 //              while (fd > 2)
 246 //                      close(fd--);
 247
 248                 /* Set proper protections and ownership. */
 249                 fchown(0, 0, 0);        /* 0:0 */
 250                 fchmod(0, 0620);        /* crw--w---- */
 251         } else {
 252                 /*
 253                  * Standard input should already be connected to an open port. Make
 254                  * sure it is open for read/write.
 255                  */
 256                 if ((fcntl(0, F_GETFL) & O_RDWR) != O_RDWR)
 257                         bb_error_msg_and_die("stdin is not open for read/write");
 258         }
 259 }
 260
 261 /* termios_init - initialize termios settings */
 262 static void termios_init(struct termios *tp, int speed, struct options *op)
 263 {
 264         speed_t ispeed, ospeed;
 265         /*
 266          * Initial termios settings: 8-bit characters, raw-mode, blocking i/o.
 267          * Special characters are set after we have read the login name; all
 268          * reads will be done in raw mode anyway. Errors will be dealt with
 269          * later on.
 270          */
 271         /* flush input and output queues, important for modems! */
 272         tcflush(0, TCIOFLUSH);
 273         ispeed = ospeed = speed;
 274         if (speed == B0) {
 275                 /* Speed was specified as "0" on command line.
 276                  * Just leave it unchanged */
 277                 ispeed = cfgetispeed(tp);
 278                 ospeed = cfgetospeed(tp);
 279         }
 280         tp->c_cflag = CS8 | HUPCL | CREAD;
 281         if (op->flags & F_LOCAL)
 282                 tp->c_cflag |= CLOCAL;
 283         cfsetispeed(tp, ispeed);
 284         cfsetospeed(tp, ospeed);
 285
 286         tp->c_iflag = tp->c_lflag = 0;
 287         tp->c_oflag = OPOST | ONLCR;
 288         tp->c_cc[VMIN] = 1;
 289         tp->c_cc[VTIME] = 0;
 290 #ifdef __linux__
 291         tp->c_line = 0;
 292 #endif
 293
 294         /* Optionally enable hardware flow control */
 295 #ifdef CRTSCTS
 296         if (op->flags & F_RTSCTS)
 297                 tp->c_cflag |= CRTSCTS;
 298 #endif
 299
 300         tcsetattr_stdin_TCSANOW(tp);
 301
 302         debug("term_io 2\n");
 303 }
 304
 305 /* auto_baud - extract baud rate from modem status message */
 306 static void auto_baud(char *buf, unsigned size_buf, struct termios *tp)
 307 {
 308         int speed;
 309         int vmin;
 310         unsigned iflag;
 311         char *bp;
 312         int nread;
 313
 314         /*
 315          * This works only if the modem produces its status code AFTER raising
 316          * the DCD line, and if the computer is fast enough to set the proper
 317          * baud rate before the message has gone by. We expect a message of the
 318          * following format:
 319          *
 320          * <junk><number><junk>
 321          *
 322          * The number is interpreted as the baud rate of the incoming call. If the
 323          * modem does not tell us the baud rate within one second, we will keep
 324          * using the current baud rate. It is advisable to enable BREAK
 325          * processing (comma-separated list of baud rates) if the processing of
 326          * modem status messages is enabled.
 327          */
 328
 329         /*
 330          * Use 7-bit characters, don't block if input queue is empty. Errors will
 331          * be dealt with later on.
 332          */
 333         iflag = tp->c_iflag;
 334         tp->c_iflag |= ISTRIP;          /* enable 8th-bit stripping */
 335         vmin = tp->c_cc[VMIN];
 336         tp->c_cc[VMIN] = 0;             /* don't block if queue empty */
 337         tcsetattr_stdin_TCSANOW(tp);
 338
 339         /*
 340          * Wait for a while, then read everything the modem has said so far and
 341          * try to extract the speed of the dial-in call.
 342          */
 343         sleep(1);
 344         nread = safe_read(STDIN_FILENO, buf, size_buf - 1);
 345         if (nread > 0) {
 346                 buf[nread] = '\0';
 347                 for (bp = buf; bp < buf + nread; bp++) {
 348                         if (isdigit(*bp)) {
 349                                 speed = bcode(bp);
 350                                 if (speed > 0)
 351                                         cfsetspeed(tp, speed);
 352                                 break;
 353                         }
 354                 }
 355         }
 356
 357         /* Restore terminal settings. Errors will be dealt with later on. */
 358         tp->c_iflag = iflag;
 359         tp->c_cc[VMIN] = vmin;
 360         tcsetattr_stdin_TCSANOW(tp);
 361 }
 362
 363 /* do_prompt - show login prompt, optionally preceded by /etc/issue contents */
 364 static void do_prompt(struct options *op)
 365 {
 366 #ifdef ISSUE
 367         print_login_issue(op->issue, op->tty);
 368 #endif
 369         print_login_prompt();
 370 }
 371
 372 #ifdef HANDLE_ALLCAPS
 373 /* all_is_upcase - string contains upper case without lower case */
 374 /* returns 1 if true, 0 if false */
 375 static int all_is_upcase(const char *s)
 376 {
 377         while (*s)
 378                 if (islower(*s++))
 379                         return 0;
 380         return 1;
 381 }
 382 #endif
 383
 384 /* get_logname - get user name, establish parity, speed, erase, kill, eol;
 385  * return NULL on BREAK, logname on success */
 386 static char *get_logname(char *logname, unsigned size_logname,
 387                 struct options *op, struct chardata *cp)
 388 {
 389         char *bp;
 390         char c;                         /* input character, full eight bits */
 391         char ascval;                    /* low 7 bits of input character */
 392         int bits;                       /* # of "1" bits per character */
 393         int mask;                       /* mask with 1 bit up */
 394         static const char erase[][3] = {/* backspace-space-backspace */
 395                 "\010\040\010",                 /* space parity */
 396                 "\010\040\010",                 /* odd parity */
 397                 "\210\240\210",                 /* even parity */
 398                 "\010\040\010",                 /* 8 bit no parity */
 399         };
 400
 401         /* NB: *cp is pre-initialized with init_chardata */
 402
 403         /* Flush pending input (esp. after parsing or switching the baud rate). */
 404         sleep(1);
 405         tcflush(0, TCIOFLUSH);
 406
 407         /* Prompt for and read a login name. */
 408         logname[0] = '\0';
 409         while (!logname[0]) {
 410                 /* Write issue file and prompt, with "parity" bit == 0. */
 411                 do_prompt(op);
 412
 413                 /* Read name, watch for break, parity, erase, kill, end-of-line. */
 414                 bp = logname;
 415                 cp->eol = '\0';
 416                 while (cp->eol == '\0') {
 417
 418                         /* Do not report trivial EINTR/EIO errors. */
 419                         errno = EINTR; /* make read of 0 bytes be silent too */
 420                         if (read(STDIN_FILENO, &c, 1) < 1) {
 421                                 if (errno == EINTR || errno == EIO)
 422                                         exit(EXIT_SUCCESS);
 423                                 bb_perror_msg_and_die(bb_msg_read_error);
 424                         }
 425
 426                         /* BREAK. If we have speeds to try,
 427                          * return NULL (will switch speeds and return here) */
 428                         if (c == '\0' && op->numspeed > 1)
 429                                 return NULL;
 430
 431                         /* Do parity bit handling. */
 432                         if (!(op->flags & F_LOCAL) && (c & 0x80)) {       /* "parity" bit on? */
 433                                 bits = 1;
 434                                 mask = 1;
 435                                 while (mask & 0x7f) {
 436                                         if (mask & c)
 437                                                 bits++; /* count "1" bits */
 438                                         mask <<= 1;
 439                                 }
 440                                 /* ... |= 2 - even, 1 - odd */
 441                                 cp->parity |= 2 - (bits & 1);
 442                         }
 443
 444                         /* Do erase, kill and end-of-line processing. */
 445                         ascval = c & 0x7f;
 446                         switch (ascval) {
 447                         case CR:
 448                         case NL:
 449                                 *bp = '\0';             /* terminate logname */
 450                                 cp->eol = ascval;       /* set end-of-line char */
 451                                 break;
 452                         case BS:
 453                         case DEL:
 454 #ifdef ANCIENT_BS_KILL_CHARS
 455                         case '#':
 456 #endif
 457                                 cp->erase = ascval;     /* set erase character */
 458                                 if (bp > logname) {
 459                                         full_write(STDOUT_FILENO, erase[cp->parity], 3);
 460                                         bp--;
 461                                 }
 462                                 break;
 463                         case CTL('U'):
 464 #ifdef ANCIENT_BS_KILL_CHARS
 465                         case '@':
 466 #endif
 467                                 cp->kill = ascval;      /* set kill character */
 468                                 while (bp > logname) {
 469                                         full_write(STDOUT_FILENO, erase[cp->parity], 3);
 470                                         bp--;
 471                                 }
 472                                 break;
 473                         case CTL('D'):
 474                                 exit(EXIT_SUCCESS);
 475                         default:
 476                                 if (ascval < ' ') {
 477                                         /* ignore garbage characters */
 478                                 } else if ((int)(bp - logname) >= size_logname - 1) {
 479                                         bb_error_msg_and_die("input overrun");
 480                                 } else {
 481                                         full_write(STDOUT_FILENO, &c, 1); /* echo the character */
 482                                         *bp++ = ascval; /* and store it */
 483                                 }
 484                                 break;
 485                         }
 486                 }
 487         }
 488         /* Handle names with upper case and no lower case. */
 489
 490 #ifdef HANDLE_ALLCAPS
 491         cp->capslock = all_is_upcase(logname);
 492         if (cp->capslock) {
 493                 for (bp = logname; *bp; bp++)
 494                         if (isupper(*bp))
 495                                 *bp = tolower(*bp);     /* map name to lower case */
 496         }
 497 #endif
 498         return logname;
 499 }
 500
 501 /* termios_final - set the final tty mode bits */
 502 static void termios_final(struct options *op, struct termios *tp, struct chardata *cp)
 503 {
 504         /* General terminal-independent stuff. */
 505         tp->c_iflag |= IXON | IXOFF;    /* 2-way flow control */
 506         tp->c_lflag |= ICANON | ISIG | ECHO | ECHOE | ECHOK | ECHOKE;
 507         /* no longer| ECHOCTL | ECHOPRT */
 508         tp->c_oflag |= OPOST;
 509         /* tp->c_cflag = 0; */
 510         tp->c_cc[VINTR] = DEF_INTR;     /* default interrupt */
 511         tp->c_cc[VQUIT] = DEF_QUIT;     /* default quit */
 512         tp->c_cc[VEOF] = DEF_EOF;       /* default EOF character */
 513         tp->c_cc[VEOL] = DEF_EOL;
 514 #ifdef VSWTC
 515         tp->c_cc[VSWTC] = DEF_SWITCH;   /* default switch character */
 516 #endif
 517
 518         /* Account for special characters seen in input. */
 519         if (cp->eol == CR) {
 520                 tp->c_iflag |= ICRNL;   /* map CR in input to NL */
 521                 tp->c_oflag |= ONLCR;   /* map NL in output to CR-NL */
 522         }
 523         tp->c_cc[VERASE] = cp->erase;   /* set erase character */
 524         tp->c_cc[VKILL] = cp->kill;     /* set kill character */
 525
 526         /* Account for the presence or absence of parity bits in input. */
 527         switch (cp->parity) {
 528         case 0:                                 /* space (always 0) parity */
 529 // I bet most people go here - they use only 7-bit chars in usernames....
 530                 break;
 531         case 1:                                 /* odd parity */
 532                 tp->c_cflag |= PARODD;
 533                 /* FALLTHROUGH */
 534         case 2:                                 /* even parity */
 535                 tp->c_cflag |= PARENB;
 536                 tp->c_iflag |= INPCK | ISTRIP;
 537                 /* FALLTHROUGH */
 538         case (1 | 2):                           /* no parity bit */
 539                 tp->c_cflag &= ~CSIZE;
 540                 tp->c_cflag |= CS7;
 541 // FIXME: wtf? case 3: we saw both even and odd 8-bit bytes -
 542 // it's probably some umlauts etc, but definitely NOT 7-bit!!!
 543 // Entire parity detection madness here just begs for deletion...
 544                 break;
 545         }
 546
 547         /* Account for upper case without lower case. */
 548 #ifdef HANDLE_ALLCAPS
 549         if (cp->capslock) {
 550                 tp->c_iflag |= IUCLC;
 551                 tp->c_lflag |= XCASE;
 552                 tp->c_oflag |= OLCUC;
 553         }
 554 #endif
 555         /* Optionally enable hardware flow control */
 556 #ifdef CRTSCTS
 557         if (op->flags & F_RTSCTS)
 558                 tp->c_cflag |= CRTSCTS;
 559 #endif
 560
 561         /* Finally, make the new settings effective */
 562         if (tcsetattr_stdin_TCSANOW(tp) < 0)
 563                 bb_perror_msg_and_die("tcsetattr");
 564 }
 565
 566 int getty_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 567 int getty_main(int argc UNUSED_PARAM, char **argv)
 568 {
 569         int n;
 570         pid_t pid;
 571         char *fakehost = NULL;          /* Fake hostname for ut_host */
 572         char *logname;                  /* login name, given to /bin/login */
 573         /* Merging these into "struct local" may _seem_ to reduce
 574          * parameter passing, but today's gcc will inline
 575          * statics which are called once anyway, so don't do that */
 576         struct chardata chardata;       /* set by get_logname() */
 577         struct termios termios;         /* terminal mode bits */
 578         struct options options;
 579
 580         chardata = init_chardata;
 581
 582         memset(&options, 0, sizeof(options));
 583         options.login = _PATH_LOGIN;    /* default login program */
 584         options.tty = "tty1";           /* default tty line */
 585         options.initstring = "";        /* modem init string */
 586 #ifdef ISSUE
 587         options.issue = ISSUE;          /* default issue file */
 588 #endif
 589
 590         /* Parse command-line arguments. */
 591         parse_args(argv, &options, &fakehost);
 592
 593         logmode = LOGMODE_NONE;
 594
 595         /* Create new session, lose controlling tty, if any */
 596         /* docs/ctty.htm says:
 597          * "This is allowed only when the current process
 598          *  is not a process group leader" - is this a problem? */
 599         setsid();
 600         /* close stdio, and stray descriptors, just in case */
 601         n = xopen(bb_dev_null, O_RDWR);
 602         /* dup2(n, 0); - no, we need to handle "getty - 9600" too */
 603         xdup2(n, 1);
 604         xdup2(n, 2);
 605         while (n > 2)
 606                 close(n--);
 607
 608         /* Logging. We want special flavor of error_msg_and_die */
 609         die_sleep = 10;
 610         msg_eol = "\r\n";
 611         /* most likely will internally use fd #3 in CLOEXEC mode: */
 612         openlog(applet_name, LOG_PID, LOG_AUTH);
 613         logmode = LOGMODE_BOTH;
 614
 615 #ifdef DEBUGGING
 616         dbf = xfopen_for_write(DEBUGTERM);
 617         for (n = 1; argv[n]; n++) {
 618                 debug(argv[n]);
 619                 debug("\n");
 620         }
 621 #endif
 622
 623         /* Open the tty as standard input, if it is not "-" */
 624         /* If it's not "-" and not taken yet, it will become our ctty */
 625         debug("calling open_tty\n");
 626         open_tty(options.tty);
 627         ndelay_off(0);
 628         debug("duping\n");
 629         xdup2(0, 1);
 630         xdup2(0, 2);
 631
 632         /*
 633          * The following ioctl will fail if stdin is not a tty, but also when
 634          * there is noise on the modem control lines. In the latter case, the
 635          * common course of action is (1) fix your cables (2) give the modem more
 636          * time to properly reset after hanging up. SunOS users can achieve (2)
 637          * by patching the SunOS kernel variable "zsadtrlow" to a larger value;
 638          * 5 seconds seems to be a good value.
 639          */
 640         if (tcgetattr(0, &termios) < 0)
 641                 bb_perror_msg_and_die("tcgetattr");
 642
 643         pid = getpid();
 644 #ifdef __linux__
 645 // FIXME: do we need this? Otherwise "-" case seems to be broken...
 646         // /* Forcibly make fd 0 our controlling tty, even if another session
 647         //  * has it as a ctty. (Another session loses ctty). */
 648         // ioctl(0, TIOCSCTTY, (void*)1);
 649         /* Make ourself a foreground process group within our session */
 650         tcsetpgrp(0, pid);
 651 #endif
 652
 653         /* Update the utmp file. This tty is ours now! */
 654         update_utmp(pid, LOGIN_PROCESS, options.tty, "LOGIN", fakehost);
 655
 656         /* Initialize the termios settings (raw mode, eight-bit, blocking i/o). */
 657         debug("calling termios_init\n");
 658         termios_init(&termios, options.speeds[0], &options);
 659
 660         /* Write the modem init string and DON'T flush the buffers */
 661         if (options.flags & F_INITSTRING) {
 662                 debug("writing init string\n");
 663                 full_write1_str(options.initstring);
 664         }
 665
 666         /* Optionally detect the baud rate from the modem status message */
 667         debug("before autobaud\n");
 668         if (options.flags & F_PARSE)
 669                 auto_baud(line_buf, sizeof(line_buf), &termios);
 670
 671         /* Set the optional timer */
 672         alarm(options.timeout); /* if 0, alarm is not set */
 673
 674         /* Optionally wait for CR or LF before writing /etc/issue */
 675         if (options.flags & F_WAITCRLF) {
 676                 char ch;
 677
 678                 debug("waiting for cr-lf\n");
 679                 while (safe_read(STDIN_FILENO, &ch, 1) == 1) {
 680                         debug("read %x\n", (unsigned char)ch);
 681                         ch &= 0x7f;                     /* strip "parity bit" */
 682                         if (ch == '\n' || ch == '\r')
 683                                 break;
 684                 }
 685         }
 686
 687         logname = NULL;
 688         if (!(options.flags & F_NOPROMPT)) {
 689                 /* NB:termios_init already set line speed
 690                  * to options.speeds[0] */
 691                 int baud_index = 0;
 692
 693                 while (1) {
 694                         /* Read the login name. */
 695                         debug("reading login name\n");
 696                         logname = get_logname(line_buf, sizeof(line_buf),
 697                                         &options, &chardata);
 698                         if (logname)
 699                                 break;
 700                         /* we are here only if options.numspeed > 1 */
 701                         baud_index = (baud_index + 1) % options.numspeed;
 702                         cfsetispeed(&termios, options.speeds[baud_index]);
 703                         cfsetospeed(&termios, options.speeds[baud_index]);
 704                         tcsetattr_stdin_TCSANOW(&termios);
 705                 }
 706         }
 707
 708         /* Disable timer. */
 709         alarm(0);
 710
 711         /* Finalize the termios settings. */
 712         termios_final(&options, &termios, &chardata);
 713
 714         /* Now the newline character should be properly written. */
 715         full_write(STDOUT_FILENO, "\n", 1);
 716
 717         /* Let the login program take care of password validation. */
 718         /* We use PATH because we trust that root doesn't set "bad" PATH,
 719          * and getty is not suid-root applet. */
 720         /* With -n, logname == NULL, and login will ask for username instead */
 721         BB_EXECLP(options.login, options.login, "--", logname, NULL);
 722         bb_error_msg_and_die("can't execute '%s'", options.login);
 723 }