drivers/net/ppp_synctty.c

   1 /*
   2  * PPP synchronous tty channel driver for Linux.
   3  *
   4  * This is a ppp channel driver that can be used with tty device drivers
   5  * that are frame oriented, such as synchronous HDLC devices.
   6  *
   7  * Complete PPP frames without encoding/decoding are exchanged between
   8  * the channel driver and the device driver.
   9  *
  10  * The async map IOCTL codes are implemented to keep the user mode
  11  * applications happy if they call them. Synchronous PPP does not use
  12  * the async maps.
  13  *
  14  * Copyright 1999 Paul Mackerras.
  15  *
  16  * Also touched by the grubby hands of Paul Fulghum paulkf@microgate.com
  17  *
  18  *  This program is free software; you can redistribute it and/or
  19  *  modify it under the terms of the GNU General Public License
  20  *  as published by the Free Software Foundation; either version
  21  *  2 of the License, or (at your option) any later version.
  22  *
  23  * This driver provides the encapsulation and framing for sending
  24  * and receiving PPP frames over sync serial lines.  It relies on
  25  * the generic PPP layer to give it frames to send and to process
  26  * received frames.  It implements the PPP line discipline.
  27  *
  28  * Part of the code in this driver was inspired by the old async-only
  29  * PPP driver, written by Michael Callahan and Al Longyear, and
  30  * subsequently hacked by Paul Mackerras.
  31  *
  32  * ==FILEVERSION 20040616==
  33  */
  34
  35 #include <linux/module.h>
  36 #include <linux/kernel.h>
  37 #include <linux/skbuff.h>
  38 #include <linux/tty.h>
  39 #include <linux/netdevice.h>
  40 #include <linux/poll.h>
  41 #include <linux/ppp_defs.h>
  42 #include <linux/if_ppp.h>
  43 #include <linux/ppp_channel.h>
  44 #include <linux/spinlock.h>
  45 #include <linux/completion.h>
  46 #include <linux/init.h>
  47 #include <asm/uaccess.h>
  48
  49 #define PPP_VERSION     "2.4.2"
  50
  51 /* Structure for storing local state. */
  52 struct syncppp {
  53         struct tty_struct *tty;
  54         unsigned int    flags;
  55         unsigned int    rbits;
  56         int             mru;
  57         spinlock_t      xmit_lock;
  58         spinlock_t      recv_lock;
  59         unsigned long   xmit_flags;
  60         u32             xaccm[8];
  61         u32             raccm;
  62         unsigned int    bytes_sent;
  63         unsigned int    bytes_rcvd;
  64
  65         struct sk_buff  *tpkt;
  66         unsigned long   last_xmit;
  67
  68         struct sk_buff_head rqueue;
  69
  70         struct tasklet_struct tsk;
  71
  72         atomic_t        refcnt;
  73         struct completion dead_cmp;
  74         struct ppp_channel chan;        /* interface to generic ppp layer */
  75 };
  76
  77 /* Bit numbers in xmit_flags */
  78 #define XMIT_WAKEUP     0
  79 #define XMIT_FULL       1
  80
  81 /* Bits in rbits */
  82 #define SC_RCV_BITS     (SC_RCV_B7_1|SC_RCV_B7_0|SC_RCV_ODDP|SC_RCV_EVNP)
  83
  84 #define PPPSYNC_MAX_RQLEN       32      /* arbitrary */
  85
  86 /*
  87  * Prototypes.
  88  */
  89 static struct sk_buff* ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *);
  90 static int ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb);
  91 static int ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd,
  92                           unsigned long arg);
  93 static void ppp_sync_process(unsigned long arg);
  94 static int ppp_sync_push(struct syncppp *ap);
  95 static void ppp_sync_flush_output(struct syncppp *ap);
  96 static void ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
  97                            char *flags, int count);
  98
  99 static struct ppp_channel_ops sync_ops = {
 100         ppp_sync_send,
 101         ppp_sync_ioctl
 102 };
 103
 104 /*
 105  * Utility procedures to print a buffer in hex/ascii
 106  */
 107 static void
 108 ppp_print_hex (register __u8 * out, const __u8 * in, int count)
 109 {
 110         register __u8 next_ch;
 111         static const char hex[] = "0123456789ABCDEF";
 112
 113         while (count-- > 0) {
 114                 next_ch = *in++;
 115                 *out++ = hex[(next_ch >> 4) & 0x0F];
 116                 *out++ = hex[next_ch & 0x0F];
 117                 ++out;
 118         }
 119 }
 120
 121 static void
 122 ppp_print_char (register __u8 * out, const __u8 * in, int count)
 123 {
 124         register __u8 next_ch;
 125
 126         while (count-- > 0) {
 127                 next_ch = *in++;
 128
 129                 if (next_ch < 0x20 || next_ch > 0x7e)
 130                         *out++ = '.';
 131                 else {
 132                         *out++ = next_ch;
 133                         if (next_ch == '%')   /* printk/syslogd has a bug !! */
 134                                 *out++ = '%';
 135                 }
 136         }
 137         *out = '\0';
 138 }
 139
 140 static void
 141 ppp_print_buffer (const char *name, const __u8 *buf, int count)
 142 {
 143         __u8 line[44];
 144
 145         if (name != NULL)
 146                 printk(KERN_DEBUG "ppp_synctty: %s, count = %d\n", name, count);
 147
 148         while (count > 8) {
 149                 memset (line, 32, 44);
 150                 ppp_print_hex (line, buf, 8);
 151                 ppp_print_char (&line[8 * 3], buf, 8);
 152                 printk(KERN_DEBUG "%s\n", line);
 153                 count -= 8;
 154                 buf += 8;
 155         }
 156
 157         if (count > 0) {
 158                 memset (line, 32, 44);
 159                 ppp_print_hex (line, buf, count);
 160                 ppp_print_char (&line[8 * 3], buf, count);
 161                 printk(KERN_DEBUG "%s\n", line);
 162         }
 163 }
 164
 165
 166 /*
 167  * Routines implementing the synchronous PPP line discipline.
 168  */
 169
 170 /*
 171  * We have a potential race on dereferencing tty->disc_data,
 172  * because the tty layer provides no locking at all - thus one
 173  * cpu could be running ppp_synctty_receive while another
 174  * calls ppp_synctty_close, which zeroes tty->disc_data and
 175  * frees the memory that ppp_synctty_receive is using.  The best
 176  * way to fix this is to use a rwlock in the tty struct, but for now
 177  * we use a single global rwlock for all ttys in ppp line discipline.
 178  *
 179  * FIXME: Fixed in tty_io nowdays.
 180  */
 181 static DEFINE_RWLOCK(disc_data_lock);
 182
 183 static struct syncppp *sp_get(struct tty_struct *tty)
 184 {
 185         struct syncppp *ap;
 186
 187         read_lock(&disc_data_lock);
 188         ap = tty->disc_data;
 189         if (ap != NULL)
 190                 atomic_inc(&ap->refcnt);
 191         read_unlock(&disc_data_lock);
 192         return ap;
 193 }
 194
 195 static void sp_put(struct syncppp *ap)
 196 {
 197         if (atomic_dec_and_test(&ap->refcnt))
 198                 complete(&ap->dead_cmp);
 199 }
 200
 201 /*
 202  * Called when a tty is put into sync-PPP line discipline.
 203  */
 204 static int
 205 ppp_sync_open(struct tty_struct *tty)
 206 {
 207         struct syncppp *ap;
 208         int err;
 209         int speed;
 210
 211         if (tty->ops->write == NULL)
 212                 return -EOPNOTSUPP;
 213
 214         ap = kzalloc(sizeof(*ap), GFP_KERNEL);
 215         err = -ENOMEM;
 216         if (!ap)
 217                 goto out;
 218
 219         /* initialize the syncppp structure */
 220         ap->tty = tty;
 221         ap->mru = PPP_MRU;
 222         spin_lock_init(&ap->xmit_lock);
 223         spin_lock_init(&ap->recv_lock);
 224         ap->xaccm[0] = ~0U;
 225         ap->xaccm[3] = 0x60000000U;
 226         ap->raccm = ~0U;
 227
 228         skb_queue_head_init(&ap->rqueue);
 229         tasklet_init(&ap->tsk, ppp_sync_process, (unsigned long) ap);
 230
 231         atomic_set(&ap->refcnt, 1);
 232         init_completion(&ap->dead_cmp);
 233
 234         ap->chan.private = ap;
 235         ap->chan.ops = &sync_ops;
 236         ap->chan.mtu = PPP_MRU;
 237         ap->chan.hdrlen = 2;    /* for A/C bytes */
 238         speed = tty_get_baud_rate(tty);
 239         ap->chan.speed = speed;
 240         err = ppp_register_channel(&ap->chan);
 241         if (err)
 242                 goto out_free;
 243
 244         tty->disc_data = ap;
 245         tty->receive_room = 65536;
 246         return 0;
 247
 248  out_free:
 249         kfree(ap);
 250  out:
 251         return err;
 252 }
 253
 254 /*
 255  * Called when the tty is put into another line discipline
 256  * or it hangs up.  We have to wait for any cpu currently
 257  * executing in any of the other ppp_synctty_* routines to
 258  * finish before we can call ppp_unregister_channel and free
 259  * the syncppp struct.  This routine must be called from
 260  * process context, not interrupt or softirq context.
 261  */
 262 static void
 263 ppp_sync_close(struct tty_struct *tty)
 264 {
 265         struct syncppp *ap;
 266
 267         write_lock_irq(&disc_data_lock);
 268         ap = tty->disc_data;
 269         tty->disc_data = NULL;
 270         write_unlock_irq(&disc_data_lock);
 271         if (!ap)
 272                 return;
 273
 274         /*
 275          * We have now ensured that nobody can start using ap from now
 276          * on, but we have to wait for all existing users to finish.
 277          * Note that ppp_unregister_channel ensures that no calls to
 278          * our channel ops (i.e. ppp_sync_send/ioctl) are in progress
 279          * by the time it returns.
 280          */
 281         if (!atomic_dec_and_test(&ap->refcnt))
 282                 wait_for_completion(&ap->dead_cmp);
 283         tasklet_kill(&ap->tsk);
 284
 285         ppp_unregister_channel(&ap->chan);
 286         skb_queue_purge(&ap->rqueue);
 287         kfree_skb(ap->tpkt);
 288         kfree(ap);
 289 }
 290
 291 /*
 292  * Called on tty hangup in process context.
 293  *
 294  * Wait for I/O to driver to complete and unregister PPP channel.
 295  * This is already done by the close routine, so just call that.
 296  */
 297 static int ppp_sync_hangup(struct tty_struct *tty)
 298 {
 299         ppp_sync_close(tty);
 300         return 0;
 301 }
 302
 303 /*
 304  * Read does nothing - no data is ever available this way.
 305  * Pppd reads and writes packets via /dev/ppp instead.
 306  */
 307 static ssize_t
 308 ppp_sync_read(struct tty_struct *tty, struct file *file,
 309                unsigned char __user *buf, size_t count)
 310 {
 311         return -EAGAIN;
 312 }
 313
 314 /*
 315  * Write on the tty does nothing, the packets all come in
 316  * from the ppp generic stuff.
 317  */
 318 static ssize_t
 319 ppp_sync_write(struct tty_struct *tty, struct file *file,
 320                 const unsigned char *buf, size_t count)
 321 {
 322         return -EAGAIN;
 323 }
 324
 325 static int
 326 ppp_synctty_ioctl(struct tty_struct *tty, struct file *file,
 327                   unsigned int cmd, unsigned long arg)
 328 {
 329         struct syncppp *ap = sp_get(tty);
 330         int __user *p = (int __user *)arg;
 331         int err, val;
 332
 333         if (!ap)
 334                 return -ENXIO;
 335         err = -EFAULT;
 336         switch (cmd) {
 337         case PPPIOCGCHAN:
 338                 err = -EFAULT;
 339                 if (put_user(ppp_channel_index(&ap->chan), p))
 340                         break;
 341                 err = 0;
 342                 break;
 343
 344         case PPPIOCGUNIT:
 345                 err = -EFAULT;
 346                 if (put_user(ppp_unit_number(&ap->chan), p))
 347                         break;
 348                 err = 0;
 349                 break;
 350
 351         case TCFLSH:
 352                 /* flush our buffers and the serial port's buffer */
 353                 if (arg == TCIOFLUSH || arg == TCOFLUSH)
 354                         ppp_sync_flush_output(ap);
 355                 err = tty_perform_flush(tty, arg);
 356                 break;
 357
 358         case FIONREAD:
 359                 val = 0;
 360                 if (put_user(val, p))
 361                         break;
 362                 err = 0;
 363                 break;
 364
 365         default:
 366                 err = tty_mode_ioctl(tty, file, cmd, arg);
 367                 break;
 368         }
 369
 370         sp_put(ap);
 371         return err;
 372 }
 373
 374 /* No kernel lock - fine */
 375 static unsigned int
 376 ppp_sync_poll(struct tty_struct *tty, struct file *file, poll_table *wait)
 377 {
 378         return 0;
 379 }
 380
 381 /* May sleep, don't call from interrupt level or with interrupts disabled */
 382 static void
 383 ppp_sync_receive(struct tty_struct *tty, const unsigned char *buf,
 384                   char *cflags, int count)
 385 {
 386         struct syncppp *ap = sp_get(tty);
 387         unsigned long flags;
 388
 389         if (!ap)
 390                 return;
 391         spin_lock_irqsave(&ap->recv_lock, flags);
 392         ppp_sync_input(ap, buf, cflags, count);
 393         spin_unlock_irqrestore(&ap->recv_lock, flags);
 394         if (!skb_queue_empty(&ap->rqueue))
 395                 tasklet_schedule(&ap->tsk);
 396         sp_put(ap);
 397         tty_unthrottle(tty);
 398 }
 399
 400 static void
 401 ppp_sync_wakeup(struct tty_struct *tty)
 402 {
 403         struct syncppp *ap = sp_get(tty);
 404
 405         clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 406         if (!ap)
 407                 return;
 408         set_bit(XMIT_WAKEUP, &ap->xmit_flags);
 409         tasklet_schedule(&ap->tsk);
 410         sp_put(ap);
 411 }
 412
 413
 414 static struct tty_ldisc_ops ppp_sync_ldisc = {
 415         .owner  = THIS_MODULE,
 416         .magic  = TTY_LDISC_MAGIC,
 417         .name   = "pppsync",
 418         .open   = ppp_sync_open,
 419         .close  = ppp_sync_close,
 420         .hangup = ppp_sync_hangup,
 421         .read   = ppp_sync_read,
 422         .write  = ppp_sync_write,
 423         .ioctl  = ppp_synctty_ioctl,
 424         .poll   = ppp_sync_poll,
 425         .receive_buf = ppp_sync_receive,
 426         .write_wakeup = ppp_sync_wakeup,
 427 };
 428
 429 static int __init
 430 ppp_sync_init(void)
 431 {
 432         int err;
 433
 434         err = tty_register_ldisc(N_SYNC_PPP, &ppp_sync_ldisc);
 435         if (err != 0)
 436                 printk(KERN_ERR "PPP_sync: error %d registering line disc.\n",
 437                        err);
 438         return err;
 439 }
 440
 441 /*
 442  * The following routines provide the PPP channel interface.
 443  */
 444 static int
 445 ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd, unsigned long arg)
 446 {
 447         struct syncppp *ap = chan->private;
 448         int err, val;
 449         u32 accm[8];
 450         void __user *argp = (void __user *)arg;
 451         u32 __user *p = argp;
 452
 453         err = -EFAULT;
 454         switch (cmd) {
 455         case PPPIOCGFLAGS:
 456                 val = ap->flags | ap->rbits;
 457                 if (put_user(val, (int __user *) argp))
 458                         break;
 459                 err = 0;
 460                 break;
 461         case PPPIOCSFLAGS:
 462                 if (get_user(val, (int __user *) argp))
 463                         break;
 464                 ap->flags = val & ~SC_RCV_BITS;
 465                 spin_lock_irq(&ap->recv_lock);
 466                 ap->rbits = val & SC_RCV_BITS;
 467                 spin_unlock_irq(&ap->recv_lock);
 468                 err = 0;
 469                 break;
 470
 471         case PPPIOCGASYNCMAP:
 472                 if (put_user(ap->xaccm[0], p))
 473                         break;
 474                 err = 0;
 475                 break;
 476         case PPPIOCSASYNCMAP:
 477                 if (get_user(ap->xaccm[0], p))
 478                         break;
 479                 err = 0;
 480                 break;
 481
 482         case PPPIOCGRASYNCMAP:
 483                 if (put_user(ap->raccm, p))
 484                         break;
 485                 err = 0;
 486                 break;
 487         case PPPIOCSRASYNCMAP:
 488                 if (get_user(ap->raccm, p))
 489                         break;
 490                 err = 0;
 491                 break;
 492
 493         case PPPIOCGXASYNCMAP:
 494                 if (copy_to_user(argp, ap->xaccm, sizeof(ap->xaccm)))
 495                         break;
 496                 err = 0;
 497                 break;
 498         case PPPIOCSXASYNCMAP:
 499                 if (copy_from_user(accm, argp, sizeof(accm)))
 500                         break;
 501                 accm[2] &= ~0x40000000U;        /* can't escape 0x5e */
 502                 accm[3] |= 0x60000000U;         /* must escape 0x7d, 0x7e */
 503                 memcpy(ap->xaccm, accm, sizeof(ap->xaccm));
 504                 err = 0;
 505                 break;
 506
 507         case PPPIOCGMRU:
 508                 if (put_user(ap->mru, (int __user *) argp))
 509                         break;
 510                 err = 0;
 511                 break;
 512         case PPPIOCSMRU:
 513                 if (get_user(val, (int __user *) argp))
 514                         break;
 515                 if (val < PPP_MRU)
 516                         val = PPP_MRU;
 517                 ap->mru = val;
 518                 err = 0;
 519                 break;
 520
 521         default:
 522                 err = -ENOTTY;
 523         }
 524         return err;
 525 }
 526
 527 /*
 528  * This is called at softirq level to deliver received packets
 529  * to the ppp_generic code, and to tell the ppp_generic code
 530  * if we can accept more output now.
 531  */
 532 static void ppp_sync_process(unsigned long arg)
 533 {
 534         struct syncppp *ap = (struct syncppp *) arg;
 535         struct sk_buff *skb;
 536
 537         /* process received packets */
 538         while ((skb = skb_dequeue(&ap->rqueue)) != NULL) {
 539                 if (skb->len == 0) {
 540                         /* zero length buffers indicate error */
 541                         ppp_input_error(&ap->chan, 0);
 542                         kfree_skb(skb);
 543                 }
 544                 else
 545                         ppp_input(&ap->chan, skb);
 546         }
 547
 548         /* try to push more stuff out */
 549         if (test_bit(XMIT_WAKEUP, &ap->xmit_flags) && ppp_sync_push(ap))
 550                 ppp_output_wakeup(&ap->chan);
 551 }
 552
 553 /*
 554  * Procedures for encapsulation and framing.
 555  */
 556
 557 static struct sk_buff*
 558 ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
 559 {
 560         int proto;
 561         unsigned char *data;
 562         int islcp;
 563
 564         data  = skb->data;
 565         proto = (data[0] << 8) + data[1];
 566
 567         /* LCP packets with codes between 1 (configure-request)
 568          * and 7 (code-reject) must be sent as though no options
 569          * have been negotiated.
 570          */
 571         islcp = proto == PPP_LCP && 1 <= data[2] && data[2] <= 7;
 572
 573         /* compress protocol field if option enabled */
 574         if (data[0] == 0 && (ap->flags & SC_COMP_PROT) && !islcp)
 575                 skb_pull(skb,1);
 576
 577         /* prepend address/control fields if necessary */
 578         if ((ap->flags & SC_COMP_AC) == 0 || islcp) {
 579                 if (skb_headroom(skb) < 2) {
 580                         struct sk_buff *npkt = dev_alloc_skb(skb->len + 2);
 581                         if (npkt == NULL) {
 582                                 kfree_skb(skb);
 583                                 return NULL;
 584                         }
 585                         skb_reserve(npkt,2);
 586                         skb_copy_from_linear_data(skb,
 587                                       skb_put(npkt, skb->len), skb->len);
 588                         kfree_skb(skb);
 589                         skb = npkt;
 590                 }
 591                 skb_push(skb,2);
 592                 skb->data[0] = PPP_ALLSTATIONS;
 593                 skb->data[1] = PPP_UI;
 594         }
 595
 596         ap->last_xmit = jiffies;
 597
 598         if (skb && ap->flags & SC_LOG_OUTPKT)
 599                 ppp_print_buffer ("send buffer", skb->data, skb->len);
 600
 601         return skb;
 602 }
 603
 604 /*
 605  * Transmit-side routines.
 606  */
 607
 608 /*
 609  * Send a packet to the peer over an sync tty line.
 610  * Returns 1 iff the packet was accepted.
 611  * If the packet was not accepted, we will call ppp_output_wakeup
 612  * at some later time.
 613  */
 614 static int
 615 ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb)
 616 {
 617         struct syncppp *ap = chan->private;
 618
 619         ppp_sync_push(ap);
 620
 621         if (test_and_set_bit(XMIT_FULL, &ap->xmit_flags))
 622                 return 0;       /* already full */
 623         skb = ppp_sync_txmunge(ap, skb);
 624         if (skb != NULL)
 625                 ap->tpkt = skb;
 626         else
 627                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 628
 629         ppp_sync_push(ap);
 630         return 1;
 631 }
 632
 633 /*
 634  * Push as much data as possible out to the tty.
 635  */
 636 static int
 637 ppp_sync_push(struct syncppp *ap)
 638 {
 639         int sent, done = 0;
 640         struct tty_struct *tty = ap->tty;
 641         int tty_stuffed = 0;
 642
 643         if (!spin_trylock_bh(&ap->xmit_lock))
 644                 return 0;
 645         for (;;) {
 646                 if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
 647                         tty_stuffed = 0;
 648                 if (!tty_stuffed && ap->tpkt) {
 649                         set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 650                         sent = tty->ops->write(tty, ap->tpkt->data, ap->tpkt->len);
 651                         if (sent < 0)
 652                                 goto flush;     /* error, e.g. loss of CD */
 653                         if (sent < ap->tpkt->len) {
 654                                 tty_stuffed = 1;
 655                         } else {
 656                                 kfree_skb(ap->tpkt);
 657                                 ap->tpkt = NULL;
 658                                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 659                                 done = 1;
 660                         }
 661                         continue;
 662                 }
 663                 /* haven't made any progress */
 664                 spin_unlock_bh(&ap->xmit_lock);
 665                 if (!(test_bit(XMIT_WAKEUP, &ap->xmit_flags) ||
 666                       (!tty_stuffed && ap->tpkt)))
 667                         break;
 668                 if (!spin_trylock_bh(&ap->xmit_lock))
 669                         break;
 670         }
 671         return done;
 672
 673 flush:
 674         if (ap->tpkt) {
 675                 kfree_skb(ap->tpkt);
 676                 ap->tpkt = NULL;
 677                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 678                 done = 1;
 679         }
 680         spin_unlock_bh(&ap->xmit_lock);
 681         return done;
 682 }
 683
 684 /*
 685  * Flush output from our internal buffers.
 686  * Called for the TCFLSH ioctl.
 687  */
 688 static void
 689 ppp_sync_flush_output(struct syncppp *ap)
 690 {
 691         int done = 0;
 692
 693         spin_lock_bh(&ap->xmit_lock);
 694         if (ap->tpkt != NULL) {
 695                 kfree_skb(ap->tpkt);
 696                 ap->tpkt = NULL;
 697                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 698                 done = 1;
 699         }
 700         spin_unlock_bh(&ap->xmit_lock);
 701         if (done)
 702                 ppp_output_wakeup(&ap->chan);
 703 }
 704
 705 /*
 706  * Receive-side routines.
 707  */
 708
 709 /* called when the tty driver has data for us.
 710  *
 711  * Data is frame oriented: each call to ppp_sync_input is considered
 712  * a whole frame. If the 1st flag byte is non-zero then the whole
 713  * frame is considered to be in error and is tossed.
 714  */
 715 static void
 716 ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
 717                 char *flags, int count)
 718 {
 719         struct sk_buff *skb;
 720         unsigned char *p;
 721
 722         if (count == 0)
 723                 return;
 724
 725         if (ap->flags & SC_LOG_INPKT)
 726                 ppp_print_buffer ("receive buffer", buf, count);
 727
 728         /* stuff the chars in the skb */
 729         skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2);
 730         if (!skb) {
 731                 printk(KERN_ERR "PPPsync: no memory (input pkt)\n");
 732                 goto err;
 733         }
 734         /* Try to get the payload 4-byte aligned */
 735         if (buf[0] != PPP_ALLSTATIONS)
 736                 skb_reserve(skb, 2 + (buf[0] & 1));
 737
 738         if (flags && *flags) {
 739                 /* error flag set, ignore frame */
 740                 goto err;
 741         } else if (count > skb_tailroom(skb)) {
 742                 /* packet overflowed MRU */
 743                 goto err;
 744         }
 745
 746         p = skb_put(skb, count);
 747         memcpy(p, buf, count);
 748
 749         /* strip address/control field if present */
 750         p = skb->data;
 751         if (p[0] == PPP_ALLSTATIONS && p[1] == PPP_UI) {
 752                 /* chop off address/control */
 753                 if (skb->len < 3)
 754                         goto err;
 755                 p = skb_pull(skb, 2);
 756         }
 757
 758         /* decompress protocol field if compressed */
 759         if (p[0] & 1) {
 760                 /* protocol is compressed */
 761                 skb_push(skb, 1)[0] = 0;
 762         } else if (skb->len < 2)
 763                 goto err;
 764
 765         /* queue the frame to be processed */
 766         skb_queue_tail(&ap->rqueue, skb);
 767         return;
 768
 769 err:
 770         /* queue zero length packet as error indication */
 771         if (skb || (skb = dev_alloc_skb(0))) {
 772                 skb_trim(skb, 0);
 773                 skb_queue_tail(&ap->rqueue, skb);
 774         }
 775 }
 776
 777 static void __exit
 778 ppp_sync_cleanup(void)
 779 {
 780         if (tty_unregister_ldisc(N_SYNC_PPP) != 0)
 781                 printk(KERN_ERR "failed to unregister Sync PPP line discipline\n");
 782 }
 783
 784 module_init(ppp_sync_init);
 785 module_exit(ppp_sync_cleanup);
 786 MODULE_LICENSE("GPL");
 787 MODULE_ALIAS_LDISC(N_SYNC_PPP);