drivers/net/ppp/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/ppp-ioctl.h>
  43 #include <linux/ppp_channel.h>
  44 #include <linux/spinlock.h>
  45 #include <linux/completion.h>
  46 #include <linux/init.h>
  47 #include <linux/interrupt.h>
  48 #include <linux/slab.h>
  49 #include <linux/refcount.h>
  50 #include <asm/unaligned.h>
  51 #include <linux/uaccess.h>
  52
  53 #define PPP_VERSION     "2.4.2"
  54
  55 /* Structure for storing local state. */
  56 struct syncppp {
  57         struct tty_struct *tty;
  58         unsigned int    flags;
  59         unsigned int    rbits;
  60         int             mru;
  61         spinlock_t      xmit_lock;
  62         spinlock_t      recv_lock;
  63         unsigned long   xmit_flags;
  64         u32             xaccm[8];
  65         u32             raccm;
  66         unsigned int    bytes_sent;
  67         unsigned int    bytes_rcvd;
  68
  69         struct sk_buff  *tpkt;
  70         unsigned long   last_xmit;
  71
  72         struct sk_buff_head rqueue;
  73
  74         struct tasklet_struct tsk;
  75
  76         refcount_t      refcnt;
  77         struct completion dead_cmp;
  78         struct ppp_channel chan;        /* interface to generic ppp layer */
  79 };
  80
  81 /* Bit numbers in xmit_flags */
  82 #define XMIT_WAKEUP     0
  83 #define XMIT_FULL       1
  84
  85 /* Bits in rbits */
  86 #define SC_RCV_BITS     (SC_RCV_B7_1|SC_RCV_B7_0|SC_RCV_ODDP|SC_RCV_EVNP)
  87
  88 #define PPPSYNC_MAX_RQLEN       32      /* arbitrary */
  89
  90 /*
  91  * Prototypes.
  92  */
  93 static struct sk_buff* ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *);
  94 static int ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb);
  95 static int ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd,
  96                           unsigned long arg);
  97 static void ppp_sync_process(unsigned long arg);
  98 static int ppp_sync_push(struct syncppp *ap);
  99 static void ppp_sync_flush_output(struct syncppp *ap);
 100 static void ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
 101                            char *flags, int count);
 102
 103 static const struct ppp_channel_ops sync_ops = {
 104         .start_xmit = ppp_sync_send,
 105         .ioctl      = ppp_sync_ioctl,
 106 };
 107
 108 /*
 109  * Utility procedure to print a buffer in hex/ascii
 110  */
 111 static void
 112 ppp_print_buffer (const char *name, const __u8 *buf, int count)
 113 {
 114         if (name != NULL)
 115                 printk(KERN_DEBUG "ppp_synctty: %s, count = %d\n", name, count);
 116
 117         print_hex_dump_bytes("", DUMP_PREFIX_NONE, buf, count);
 118 }
 119
 120
 121 /*
 122  * Routines implementing the synchronous PPP line discipline.
 123  */
 124
 125 /*
 126  * We have a potential race on dereferencing tty->disc_data,
 127  * because the tty layer provides no locking at all - thus one
 128  * cpu could be running ppp_synctty_receive while another
 129  * calls ppp_synctty_close, which zeroes tty->disc_data and
 130  * frees the memory that ppp_synctty_receive is using.  The best
 131  * way to fix this is to use a rwlock in the tty struct, but for now
 132  * we use a single global rwlock for all ttys in ppp line discipline.
 133  *
 134  * FIXME: Fixed in tty_io nowadays.
 135  */
 136 static DEFINE_RWLOCK(disc_data_lock);
 137
 138 static struct syncppp *sp_get(struct tty_struct *tty)
 139 {
 140         struct syncppp *ap;
 141
 142         read_lock(&disc_data_lock);
 143         ap = tty->disc_data;
 144         if (ap != NULL)
 145                 refcount_inc(&ap->refcnt);
 146         read_unlock(&disc_data_lock);
 147         return ap;
 148 }
 149
 150 static void sp_put(struct syncppp *ap)
 151 {
 152         if (refcount_dec_and_test(&ap->refcnt))
 153                 complete(&ap->dead_cmp);
 154 }
 155
 156 /*
 157  * Called when a tty is put into sync-PPP line discipline.
 158  */
 159 static int
 160 ppp_sync_open(struct tty_struct *tty)
 161 {
 162         struct syncppp *ap;
 163         int err;
 164         int speed;
 165
 166         if (tty->ops->write == NULL)
 167                 return -EOPNOTSUPP;
 168
 169         ap = kzalloc(sizeof(*ap), GFP_KERNEL);
 170         err = -ENOMEM;
 171         if (!ap)
 172                 goto out;
 173
 174         /* initialize the syncppp structure */
 175         ap->tty = tty;
 176         ap->mru = PPP_MRU;
 177         spin_lock_init(&ap->xmit_lock);
 178         spin_lock_init(&ap->recv_lock);
 179         ap->xaccm[0] = ~0U;
 180         ap->xaccm[3] = 0x60000000U;
 181         ap->raccm = ~0U;
 182
 183         skb_queue_head_init(&ap->rqueue);
 184         tasklet_init(&ap->tsk, ppp_sync_process, (unsigned long) ap);
 185
 186         refcount_set(&ap->refcnt, 1);
 187         init_completion(&ap->dead_cmp);
 188
 189         ap->chan.private = ap;
 190         ap->chan.ops = &sync_ops;
 191         ap->chan.mtu = PPP_MRU;
 192         ap->chan.hdrlen = 2;    /* for A/C bytes */
 193         speed = tty_get_baud_rate(tty);
 194         ap->chan.speed = speed;
 195         err = ppp_register_channel(&ap->chan);
 196         if (err)
 197                 goto out_free;
 198
 199         tty->disc_data = ap;
 200         tty->receive_room = 65536;
 201         return 0;
 202
 203  out_free:
 204         kfree(ap);
 205  out:
 206         return err;
 207 }
 208
 209 /*
 210  * Called when the tty is put into another line discipline
 211  * or it hangs up.  We have to wait for any cpu currently
 212  * executing in any of the other ppp_synctty_* routines to
 213  * finish before we can call ppp_unregister_channel and free
 214  * the syncppp struct.  This routine must be called from
 215  * process context, not interrupt or softirq context.
 216  */
 217 static void
 218 ppp_sync_close(struct tty_struct *tty)
 219 {
 220         struct syncppp *ap;
 221
 222         write_lock_irq(&disc_data_lock);
 223         ap = tty->disc_data;
 224         tty->disc_data = NULL;
 225         write_unlock_irq(&disc_data_lock);
 226         if (!ap)
 227                 return;
 228
 229         /*
 230          * We have now ensured that nobody can start using ap from now
 231          * on, but we have to wait for all existing users to finish.
 232          * Note that ppp_unregister_channel ensures that no calls to
 233          * our channel ops (i.e. ppp_sync_send/ioctl) are in progress
 234          * by the time it returns.
 235          */
 236         if (!refcount_dec_and_test(&ap->refcnt))
 237                 wait_for_completion(&ap->dead_cmp);
 238         tasklet_kill(&ap->tsk);
 239
 240         ppp_unregister_channel(&ap->chan);
 241         skb_queue_purge(&ap->rqueue);
 242         kfree_skb(ap->tpkt);
 243         kfree(ap);
 244 }
 245
 246 /*
 247  * Called on tty hangup in process context.
 248  *
 249  * Wait for I/O to driver to complete and unregister PPP channel.
 250  * This is already done by the close routine, so just call that.
 251  */
 252 static int ppp_sync_hangup(struct tty_struct *tty)
 253 {
 254         ppp_sync_close(tty);
 255         return 0;
 256 }
 257
 258 /*
 259  * Read does nothing - no data is ever available this way.
 260  * Pppd reads and writes packets via /dev/ppp instead.
 261  */
 262 static ssize_t
 263 ppp_sync_read(struct tty_struct *tty, struct file *file,
 264                unsigned char __user *buf, size_t count)
 265 {
 266         return -EAGAIN;
 267 }
 268
 269 /*
 270  * Write on the tty does nothing, the packets all come in
 271  * from the ppp generic stuff.
 272  */
 273 static ssize_t
 274 ppp_sync_write(struct tty_struct *tty, struct file *file,
 275                 const unsigned char *buf, size_t count)
 276 {
 277         return -EAGAIN;
 278 }
 279
 280 static int
 281 ppp_synctty_ioctl(struct tty_struct *tty, struct file *file,
 282                   unsigned int cmd, unsigned long arg)
 283 {
 284         struct syncppp *ap = sp_get(tty);
 285         int __user *p = (int __user *)arg;
 286         int err, val;
 287
 288         if (!ap)
 289                 return -ENXIO;
 290         err = -EFAULT;
 291         switch (cmd) {
 292         case PPPIOCGCHAN:
 293                 err = -EFAULT;
 294                 if (put_user(ppp_channel_index(&ap->chan), p))
 295                         break;
 296                 err = 0;
 297                 break;
 298
 299         case PPPIOCGUNIT:
 300                 err = -EFAULT;
 301                 if (put_user(ppp_unit_number(&ap->chan), p))
 302                         break;
 303                 err = 0;
 304                 break;
 305
 306         case TCFLSH:
 307                 /* flush our buffers and the serial port's buffer */
 308                 if (arg == TCIOFLUSH || arg == TCOFLUSH)
 309                         ppp_sync_flush_output(ap);
 310                 err = n_tty_ioctl_helper(tty, file, cmd, arg);
 311                 break;
 312
 313         case FIONREAD:
 314                 val = 0;
 315                 if (put_user(val, p))
 316                         break;
 317                 err = 0;
 318                 break;
 319
 320         default:
 321                 err = tty_mode_ioctl(tty, file, cmd, arg);
 322                 break;
 323         }
 324
 325         sp_put(ap);
 326         return err;
 327 }
 328
 329 /* No kernel lock - fine */
 330 static unsigned int
 331 ppp_sync_poll(struct tty_struct *tty, struct file *file, poll_table *wait)
 332 {
 333         return 0;
 334 }
 335
 336 /* May sleep, don't call from interrupt level or with interrupts disabled */
 337 static void
 338 ppp_sync_receive(struct tty_struct *tty, const unsigned char *buf,
 339                   char *cflags, int count)
 340 {
 341         struct syncppp *ap = sp_get(tty);
 342         unsigned long flags;
 343
 344         if (!ap)
 345                 return;
 346         spin_lock_irqsave(&ap->recv_lock, flags);
 347         ppp_sync_input(ap, buf, cflags, count);
 348         spin_unlock_irqrestore(&ap->recv_lock, flags);
 349         if (!skb_queue_empty(&ap->rqueue))
 350                 tasklet_schedule(&ap->tsk);
 351         sp_put(ap);
 352         tty_unthrottle(tty);
 353 }
 354
 355 static void
 356 ppp_sync_wakeup(struct tty_struct *tty)
 357 {
 358         struct syncppp *ap = sp_get(tty);
 359
 360         clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 361         if (!ap)
 362                 return;
 363         set_bit(XMIT_WAKEUP, &ap->xmit_flags);
 364         tasklet_schedule(&ap->tsk);
 365         sp_put(ap);
 366 }
 367
 368
 369 static struct tty_ldisc_ops ppp_sync_ldisc = {
 370         .owner  = THIS_MODULE,
 371         .magic  = TTY_LDISC_MAGIC,
 372         .name   = "pppsync",
 373         .open   = ppp_sync_open,
 374         .close  = ppp_sync_close,
 375         .hangup = ppp_sync_hangup,
 376         .read   = ppp_sync_read,
 377         .write  = ppp_sync_write,
 378         .ioctl  = ppp_synctty_ioctl,
 379         .poll   = ppp_sync_poll,
 380         .receive_buf = ppp_sync_receive,
 381         .write_wakeup = ppp_sync_wakeup,
 382 };
 383
 384 static int __init
 385 ppp_sync_init(void)
 386 {
 387         int err;
 388
 389         err = tty_register_ldisc(N_SYNC_PPP, &ppp_sync_ldisc);
 390         if (err != 0)
 391                 printk(KERN_ERR "PPP_sync: error %d registering line disc.\n",
 392                        err);
 393         return err;
 394 }
 395
 396 /*
 397  * The following routines provide the PPP channel interface.
 398  */
 399 static int
 400 ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd, unsigned long arg)
 401 {
 402         struct syncppp *ap = chan->private;
 403         int err, val;
 404         u32 accm[8];
 405         void __user *argp = (void __user *)arg;
 406         u32 __user *p = argp;
 407
 408         err = -EFAULT;
 409         switch (cmd) {
 410         case PPPIOCGFLAGS:
 411                 val = ap->flags | ap->rbits;
 412                 if (put_user(val, (int __user *) argp))
 413                         break;
 414                 err = 0;
 415                 break;
 416         case PPPIOCSFLAGS:
 417                 if (get_user(val, (int __user *) argp))
 418                         break;
 419                 ap->flags = val & ~SC_RCV_BITS;
 420                 spin_lock_irq(&ap->recv_lock);
 421                 ap->rbits = val & SC_RCV_BITS;
 422                 spin_unlock_irq(&ap->recv_lock);
 423                 err = 0;
 424                 break;
 425
 426         case PPPIOCGASYNCMAP:
 427                 if (put_user(ap->xaccm[0], p))
 428                         break;
 429                 err = 0;
 430                 break;
 431         case PPPIOCSASYNCMAP:
 432                 if (get_user(ap->xaccm[0], p))
 433                         break;
 434                 err = 0;
 435                 break;
 436
 437         case PPPIOCGRASYNCMAP:
 438                 if (put_user(ap->raccm, p))
 439                         break;
 440                 err = 0;
 441                 break;
 442         case PPPIOCSRASYNCMAP:
 443                 if (get_user(ap->raccm, p))
 444                         break;
 445                 err = 0;
 446                 break;
 447
 448         case PPPIOCGXASYNCMAP:
 449                 if (copy_to_user(argp, ap->xaccm, sizeof(ap->xaccm)))
 450                         break;
 451                 err = 0;
 452                 break;
 453         case PPPIOCSXASYNCMAP:
 454                 if (copy_from_user(accm, argp, sizeof(accm)))
 455                         break;
 456                 accm[2] &= ~0x40000000U;        /* can't escape 0x5e */
 457                 accm[3] |= 0x60000000U;         /* must escape 0x7d, 0x7e */
 458                 memcpy(ap->xaccm, accm, sizeof(ap->xaccm));
 459                 err = 0;
 460                 break;
 461
 462         case PPPIOCGMRU:
 463                 if (put_user(ap->mru, (int __user *) argp))
 464                         break;
 465                 err = 0;
 466                 break;
 467         case PPPIOCSMRU:
 468                 if (get_user(val, (int __user *) argp))
 469                         break;
 470                 if (val < PPP_MRU)
 471                         val = PPP_MRU;
 472                 ap->mru = val;
 473                 err = 0;
 474                 break;
 475
 476         default:
 477                 err = -ENOTTY;
 478         }
 479         return err;
 480 }
 481
 482 /*
 483  * This is called at softirq level to deliver received packets
 484  * to the ppp_generic code, and to tell the ppp_generic code
 485  * if we can accept more output now.
 486  */
 487 static void ppp_sync_process(unsigned long arg)
 488 {
 489         struct syncppp *ap = (struct syncppp *) arg;
 490         struct sk_buff *skb;
 491
 492         /* process received packets */
 493         while ((skb = skb_dequeue(&ap->rqueue)) != NULL) {
 494                 if (skb->len == 0) {
 495                         /* zero length buffers indicate error */
 496                         ppp_input_error(&ap->chan, 0);
 497                         kfree_skb(skb);
 498                 }
 499                 else
 500                         ppp_input(&ap->chan, skb);
 501         }
 502
 503         /* try to push more stuff out */
 504         if (test_bit(XMIT_WAKEUP, &ap->xmit_flags) && ppp_sync_push(ap))
 505                 ppp_output_wakeup(&ap->chan);
 506 }
 507
 508 /*
 509  * Procedures for encapsulation and framing.
 510  */
 511
 512 static struct sk_buff*
 513 ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
 514 {
 515         int proto;
 516         unsigned char *data;
 517         int islcp;
 518
 519         data  = skb->data;
 520         proto = get_unaligned_be16(data);
 521
 522         /* LCP packets with codes between 1 (configure-request)
 523          * and 7 (code-reject) must be sent as though no options
 524          * have been negotiated.
 525          */
 526         islcp = proto == PPP_LCP && 1 <= data[2] && data[2] <= 7;
 527
 528         /* compress protocol field if option enabled */
 529         if (data[0] == 0 && (ap->flags & SC_COMP_PROT) && !islcp)
 530                 skb_pull(skb,1);
 531
 532         /* prepend address/control fields if necessary */
 533         if ((ap->flags & SC_COMP_AC) == 0 || islcp) {
 534                 if (skb_headroom(skb) < 2) {
 535                         struct sk_buff *npkt = dev_alloc_skb(skb->len + 2);
 536                         if (npkt == NULL) {
 537                                 kfree_skb(skb);
 538                                 return NULL;
 539                         }
 540                         skb_reserve(npkt,2);
 541                         skb_copy_from_linear_data(skb,
 542                                       skb_put(npkt, skb->len), skb->len);
 543                         consume_skb(skb);
 544                         skb = npkt;
 545                 }
 546                 skb_push(skb,2);
 547                 skb->data[0] = PPP_ALLSTATIONS;
 548                 skb->data[1] = PPP_UI;
 549         }
 550
 551         ap->last_xmit = jiffies;
 552
 553         if (skb && ap->flags & SC_LOG_OUTPKT)
 554                 ppp_print_buffer ("send buffer", skb->data, skb->len);
 555
 556         return skb;
 557 }
 558
 559 /*
 560  * Transmit-side routines.
 561  */
 562
 563 /*
 564  * Send a packet to the peer over an sync tty line.
 565  * Returns 1 iff the packet was accepted.
 566  * If the packet was not accepted, we will call ppp_output_wakeup
 567  * at some later time.
 568  */
 569 static int
 570 ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb)
 571 {
 572         struct syncppp *ap = chan->private;
 573
 574         ppp_sync_push(ap);
 575
 576         if (test_and_set_bit(XMIT_FULL, &ap->xmit_flags))
 577                 return 0;       /* already full */
 578         skb = ppp_sync_txmunge(ap, skb);
 579         if (skb != NULL)
 580                 ap->tpkt = skb;
 581         else
 582                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 583
 584         ppp_sync_push(ap);
 585         return 1;
 586 }
 587
 588 /*
 589  * Push as much data as possible out to the tty.
 590  */
 591 static int
 592 ppp_sync_push(struct syncppp *ap)
 593 {
 594         int sent, done = 0;
 595         struct tty_struct *tty = ap->tty;
 596         int tty_stuffed = 0;
 597
 598         if (!spin_trylock_bh(&ap->xmit_lock))
 599                 return 0;
 600         for (;;) {
 601                 if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
 602                         tty_stuffed = 0;
 603                 if (!tty_stuffed && ap->tpkt) {
 604                         set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 605                         sent = tty->ops->write(tty, ap->tpkt->data, ap->tpkt->len);
 606                         if (sent < 0)
 607                                 goto flush;     /* error, e.g. loss of CD */
 608                         if (sent < ap->tpkt->len) {
 609                                 tty_stuffed = 1;
 610                         } else {
 611                                 consume_skb(ap->tpkt);
 612                                 ap->tpkt = NULL;
 613                                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 614                                 done = 1;
 615                         }
 616                         continue;
 617                 }
 618                 /* haven't made any progress */
 619                 spin_unlock_bh(&ap->xmit_lock);
 620                 if (!(test_bit(XMIT_WAKEUP, &ap->xmit_flags) ||
 621                       (!tty_stuffed && ap->tpkt)))
 622                         break;
 623                 if (!spin_trylock_bh(&ap->xmit_lock))
 624                         break;
 625         }
 626         return done;
 627
 628 flush:
 629         if (ap->tpkt) {
 630                 kfree_skb(ap->tpkt);
 631                 ap->tpkt = NULL;
 632                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 633                 done = 1;
 634         }
 635         spin_unlock_bh(&ap->xmit_lock);
 636         return done;
 637 }
 638
 639 /*
 640  * Flush output from our internal buffers.
 641  * Called for the TCFLSH ioctl.
 642  */
 643 static void
 644 ppp_sync_flush_output(struct syncppp *ap)
 645 {
 646         int done = 0;
 647
 648         spin_lock_bh(&ap->xmit_lock);
 649         if (ap->tpkt != NULL) {
 650                 kfree_skb(ap->tpkt);
 651                 ap->tpkt = NULL;
 652                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 653                 done = 1;
 654         }
 655         spin_unlock_bh(&ap->xmit_lock);
 656         if (done)
 657                 ppp_output_wakeup(&ap->chan);
 658 }
 659
 660 /*
 661  * Receive-side routines.
 662  */
 663
 664 /* called when the tty driver has data for us.
 665  *
 666  * Data is frame oriented: each call to ppp_sync_input is considered
 667  * a whole frame. If the 1st flag byte is non-zero then the whole
 668  * frame is considered to be in error and is tossed.
 669  */
 670 static void
 671 ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
 672                 char *flags, int count)
 673 {
 674         struct sk_buff *skb;
 675         unsigned char *p;
 676
 677         if (count == 0)
 678                 return;
 679
 680         if (ap->flags & SC_LOG_INPKT)
 681                 ppp_print_buffer ("receive buffer", buf, count);
 682
 683         /* stuff the chars in the skb */
 684         skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2);
 685         if (!skb) {
 686                 printk(KERN_ERR "PPPsync: no memory (input pkt)\n");
 687                 goto err;
 688         }
 689         /* Try to get the payload 4-byte aligned */
 690         if (buf[0] != PPP_ALLSTATIONS)
 691                 skb_reserve(skb, 2 + (buf[0] & 1));
 692
 693         if (flags && *flags) {
 694                 /* error flag set, ignore frame */
 695                 goto err;
 696         } else if (count > skb_tailroom(skb)) {
 697                 /* packet overflowed MRU */
 698                 goto err;
 699         }
 700
 701         skb_put_data(skb, buf, count);
 702
 703         /* strip address/control field if present */
 704         p = skb->data;
 705         if (p[0] == PPP_ALLSTATIONS && p[1] == PPP_UI) {
 706                 /* chop off address/control */
 707                 if (skb->len < 3)
 708                         goto err;
 709                 p = skb_pull(skb, 2);
 710         }
 711
 712         /* decompress protocol field if compressed */
 713         if (p[0] & 1) {
 714                 /* protocol is compressed */
 715                 *(u8 *)skb_push(skb, 1) = 0;
 716         } else if (skb->len < 2)
 717                 goto err;
 718
 719         /* queue the frame to be processed */
 720         skb_queue_tail(&ap->rqueue, skb);
 721         return;
 722
 723 err:
 724         /* queue zero length packet as error indication */
 725         if (skb || (skb = dev_alloc_skb(0))) {
 726                 skb_trim(skb, 0);
 727                 skb_queue_tail(&ap->rqueue, skb);
 728         }
 729 }
 730
 731 static void __exit
 732 ppp_sync_cleanup(void)
 733 {
 734         if (tty_unregister_ldisc(N_SYNC_PPP) != 0)
 735                 printk(KERN_ERR "failed to unregister Sync PPP line discipline\n");
 736 }
 737
 738 module_init(ppp_sync_init);
 739 module_exit(ppp_sync_cleanup);
 740 MODULE_LICENSE("GPL");
 741 MODULE_ALIAS_LDISC(N_SYNC_PPP);