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