drivers/net/ppp/ppp_synctty.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * PPP synchronous tty channel driver for Linux.
   4  *
   5  * This is a ppp channel driver that can be used with tty device drivers
   6  * that are frame oriented, such as synchronous HDLC devices.
   7  *
   8  * Complete PPP frames without encoding/decoding are exchanged between
   9  * the channel driver and the device driver.
  10  *
  11  * The async map IOCTL codes are implemented to keep the user mode
  12  * applications happy if they call them. Synchronous PPP does not use
  13  * the async maps.
  14  *
  15  * Copyright 1999 Paul Mackerras.
  16  *
  17  * Also touched by the grubby hands of Paul Fulghum paulkf@microgate.com
  18  *
  19  * This driver provides the encapsulation and framing for sending
  20  * and receiving PPP frames over sync serial lines.  It relies on
  21  * the generic PPP layer to give it frames to send and to process
  22  * received frames.  It implements the PPP line discipline.
  23  *
  24  * Part of the code in this driver was inspired by the old async-only
  25  * PPP driver, written by Michael Callahan and Al Longyear, and
  26  * subsequently hacked by Paul Mackerras.
  27  *
  28  * ==FILEVERSION 20040616==
  29  */
  30
  31 #include <linux/module.h>
  32 #include <linux/kernel.h>
  33 #include <linux/skbuff.h>
  34 #include <linux/tty.h>
  35 #include <linux/netdevice.h>
  36 #include <linux/poll.h>
  37 #include <linux/ppp_defs.h>
  38 #include <linux/ppp-ioctl.h>
  39 #include <linux/ppp_channel.h>
  40 #include <linux/spinlock.h>
  41 #include <linux/completion.h>
  42 #include <linux/init.h>
  43 #include <linux/interrupt.h>
  44 #include <linux/slab.h>
  45 #include <linux/refcount.h>
  46 #include <linux/unaligned.h>
  47 #include <linux/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         refcount_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(struct tasklet_struct *t);
  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 u8 *buf, const u8 *flags,
  97                            int count);
  98
  99 static const struct ppp_channel_ops sync_ops = {
 100         .start_xmit = ppp_sync_send,
 101         .ioctl      = ppp_sync_ioctl,
 102 };
 103
 104 /*
 105  * Utility procedure to print a buffer in hex/ascii
 106  */
 107 static void
 108 ppp_print_buffer (const char *name, const __u8 *buf, int count)
 109 {
 110         if (name != NULL)
 111                 printk(KERN_DEBUG "ppp_synctty: %s, count = %d\n", name, count);
 112
 113         print_hex_dump_bytes("", DUMP_PREFIX_NONE, buf, count);
 114 }
 115
 116
 117 /*
 118  * Routines implementing the synchronous PPP line discipline.
 119  */
 120
 121 /*
 122  * We have a potential race on dereferencing tty->disc_data,
 123  * because the tty layer provides no locking at all - thus one
 124  * cpu could be running ppp_synctty_receive while another
 125  * calls ppp_synctty_close, which zeroes tty->disc_data and
 126  * frees the memory that ppp_synctty_receive is using.  The best
 127  * way to fix this is to use a rwlock in the tty struct, but for now
 128  * we use a single global rwlock for all ttys in ppp line discipline.
 129  *
 130  * FIXME: Fixed in tty_io nowadays.
 131  */
 132 static DEFINE_RWLOCK(disc_data_lock);
 133
 134 static struct syncppp *sp_get(struct tty_struct *tty)
 135 {
 136         struct syncppp *ap;
 137
 138         read_lock(&disc_data_lock);
 139         ap = tty->disc_data;
 140         if (ap != NULL)
 141                 refcount_inc(&ap->refcnt);
 142         read_unlock(&disc_data_lock);
 143         return ap;
 144 }
 145
 146 static void sp_put(struct syncppp *ap)
 147 {
 148         if (refcount_dec_and_test(&ap->refcnt))
 149                 complete(&ap->dead_cmp);
 150 }
 151
 152 /*
 153  * Called when a tty is put into sync-PPP line discipline.
 154  */
 155 static int
 156 ppp_sync_open(struct tty_struct *tty)
 157 {
 158         struct syncppp *ap;
 159         int err;
 160         int speed;
 161
 162         if (tty->ops->write == NULL)
 163                 return -EOPNOTSUPP;
 164
 165         ap = kzalloc(sizeof(*ap), GFP_KERNEL);
 166         err = -ENOMEM;
 167         if (!ap)
 168                 goto out;
 169
 170         /* initialize the syncppp structure */
 171         ap->tty = tty;
 172         ap->mru = PPP_MRU;
 173         spin_lock_init(&ap->xmit_lock);
 174         spin_lock_init(&ap->recv_lock);
 175         ap->xaccm[0] = ~0U;
 176         ap->xaccm[3] = 0x60000000U;
 177         ap->raccm = ~0U;
 178
 179         skb_queue_head_init(&ap->rqueue);
 180         tasklet_setup(&ap->tsk, ppp_sync_process);
 181
 182         refcount_set(&ap->refcnt, 1);
 183         init_completion(&ap->dead_cmp);
 184
 185         ap->chan.private = ap;
 186         ap->chan.ops = &sync_ops;
 187         ap->chan.mtu = PPP_MRU;
 188         ap->chan.hdrlen = 2;    /* for A/C bytes */
 189         speed = tty_get_baud_rate(tty);
 190         ap->chan.speed = speed;
 191         err = ppp_register_channel(&ap->chan);
 192         if (err)
 193                 goto out_free;
 194
 195         tty->disc_data = ap;
 196         tty->receive_room = 65536;
 197         return 0;
 198
 199  out_free:
 200         kfree(ap);
 201  out:
 202         return err;
 203 }
 204
 205 /*
 206  * Called when the tty is put into another line discipline
 207  * or it hangs up.  We have to wait for any cpu currently
 208  * executing in any of the other ppp_synctty_* routines to
 209  * finish before we can call ppp_unregister_channel and free
 210  * the syncppp struct.  This routine must be called from
 211  * process context, not interrupt or softirq context.
 212  */
 213 static void
 214 ppp_sync_close(struct tty_struct *tty)
 215 {
 216         struct syncppp *ap;
 217
 218         write_lock_irq(&disc_data_lock);
 219         ap = tty->disc_data;
 220         tty->disc_data = NULL;
 221         write_unlock_irq(&disc_data_lock);
 222         if (!ap)
 223                 return;
 224
 225         /*
 226          * We have now ensured that nobody can start using ap from now
 227          * on, but we have to wait for all existing users to finish.
 228          * Note that ppp_unregister_channel ensures that no calls to
 229          * our channel ops (i.e. ppp_sync_send/ioctl) are in progress
 230          * by the time it returns.
 231          */
 232         if (!refcount_dec_and_test(&ap->refcnt))
 233                 wait_for_completion(&ap->dead_cmp);
 234         tasklet_kill(&ap->tsk);
 235
 236         ppp_unregister_channel(&ap->chan);
 237         skb_queue_purge(&ap->rqueue);
 238         kfree_skb(ap->tpkt);
 239         kfree(ap);
 240 }
 241
 242 /*
 243  * Called on tty hangup in process context.
 244  *
 245  * Wait for I/O to driver to complete and unregister PPP channel.
 246  * This is already done by the close routine, so just call that.
 247  */
 248 static void ppp_sync_hangup(struct tty_struct *tty)
 249 {
 250         ppp_sync_close(tty);
 251 }
 252
 253 /*
 254  * Read does nothing - no data is ever available this way.
 255  * Pppd reads and writes packets via /dev/ppp instead.
 256  */
 257 static ssize_t
 258 ppp_sync_read(struct tty_struct *tty, struct file *file, u8 *buf, size_t count,
 259               void **cookie, unsigned long offset)
 260 {
 261         return -EAGAIN;
 262 }
 263
 264 /*
 265  * Write on the tty does nothing, the packets all come in
 266  * from the ppp generic stuff.
 267  */
 268 static ssize_t
 269 ppp_sync_write(struct tty_struct *tty, struct file *file, const u8 *buf,
 270                size_t count)
 271 {
 272         return -EAGAIN;
 273 }
 274
 275 static int
 276 ppp_synctty_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
 277 {
 278         struct syncppp *ap = sp_get(tty);
 279         int __user *p = (int __user *)arg;
 280         int err, val;
 281
 282         if (!ap)
 283                 return -ENXIO;
 284         err = -EFAULT;
 285         switch (cmd) {
 286         case PPPIOCGCHAN:
 287                 err = -EFAULT;
 288                 if (put_user(ppp_channel_index(&ap->chan), p))
 289                         break;
 290                 err = 0;
 291                 break;
 292
 293         case PPPIOCGUNIT:
 294                 err = -EFAULT;
 295                 if (put_user(ppp_unit_number(&ap->chan), p))
 296                         break;
 297                 err = 0;
 298                 break;
 299
 300         case TCFLSH:
 301                 /* flush our buffers and the serial port's buffer */
 302                 if (arg == TCIOFLUSH || arg == TCOFLUSH)
 303                         ppp_sync_flush_output(ap);
 304                 err = n_tty_ioctl_helper(tty, cmd, arg);
 305                 break;
 306
 307         case FIONREAD:
 308                 val = 0;
 309                 if (put_user(val, p))
 310                         break;
 311                 err = 0;
 312                 break;
 313
 314         default:
 315                 err = tty_mode_ioctl(tty, cmd, arg);
 316                 break;
 317         }
 318
 319         sp_put(ap);
 320         return err;
 321 }
 322
 323 /* May sleep, don't call from interrupt level or with interrupts disabled */
 324 static void
 325 ppp_sync_receive(struct tty_struct *tty, const u8 *buf, const u8 *cflags,
 326                  size_t count)
 327 {
 328         struct syncppp *ap = sp_get(tty);
 329         unsigned long flags;
 330
 331         if (!ap)
 332                 return;
 333         spin_lock_irqsave(&ap->recv_lock, flags);
 334         ppp_sync_input(ap, buf, cflags, count);
 335         spin_unlock_irqrestore(&ap->recv_lock, flags);
 336         if (!skb_queue_empty(&ap->rqueue))
 337                 tasklet_schedule(&ap->tsk);
 338         sp_put(ap);
 339         tty_unthrottle(tty);
 340 }
 341
 342 static void
 343 ppp_sync_wakeup(struct tty_struct *tty)
 344 {
 345         struct syncppp *ap = sp_get(tty);
 346
 347         clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 348         if (!ap)
 349                 return;
 350         set_bit(XMIT_WAKEUP, &ap->xmit_flags);
 351         tasklet_schedule(&ap->tsk);
 352         sp_put(ap);
 353 }
 354
 355
 356 static struct tty_ldisc_ops ppp_sync_ldisc = {
 357         .owner  = THIS_MODULE,
 358         .num    = N_SYNC_PPP,
 359         .name   = "pppsync",
 360         .open   = ppp_sync_open,
 361         .close  = ppp_sync_close,
 362         .hangup = ppp_sync_hangup,
 363         .read   = ppp_sync_read,
 364         .write  = ppp_sync_write,
 365         .ioctl  = ppp_synctty_ioctl,
 366         .receive_buf = ppp_sync_receive,
 367         .write_wakeup = ppp_sync_wakeup,
 368 };
 369
 370 static int __init
 371 ppp_sync_init(void)
 372 {
 373         int err;
 374
 375         err = tty_register_ldisc(&ppp_sync_ldisc);
 376         if (err != 0)
 377                 printk(KERN_ERR "PPP_sync: error %d registering line disc.\n",
 378                        err);
 379         return err;
 380 }
 381
 382 /*
 383  * The following routines provide the PPP channel interface.
 384  */
 385 static int
 386 ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd, unsigned long arg)
 387 {
 388         struct syncppp *ap = chan->private;
 389         int err, val;
 390         u32 accm[8];
 391         void __user *argp = (void __user *)arg;
 392         u32 __user *p = argp;
 393
 394         err = -EFAULT;
 395         switch (cmd) {
 396         case PPPIOCGFLAGS:
 397                 val = ap->flags | ap->rbits;
 398                 if (put_user(val, (int __user *) argp))
 399                         break;
 400                 err = 0;
 401                 break;
 402         case PPPIOCSFLAGS:
 403                 if (get_user(val, (int __user *) argp))
 404                         break;
 405                 ap->flags = val & ~SC_RCV_BITS;
 406                 spin_lock_irq(&ap->recv_lock);
 407                 ap->rbits = val & SC_RCV_BITS;
 408                 spin_unlock_irq(&ap->recv_lock);
 409                 err = 0;
 410                 break;
 411
 412         case PPPIOCGASYNCMAP:
 413                 if (put_user(ap->xaccm[0], p))
 414                         break;
 415                 err = 0;
 416                 break;
 417         case PPPIOCSASYNCMAP:
 418                 if (get_user(ap->xaccm[0], p))
 419                         break;
 420                 err = 0;
 421                 break;
 422
 423         case PPPIOCGRASYNCMAP:
 424                 if (put_user(ap->raccm, p))
 425                         break;
 426                 err = 0;
 427                 break;
 428         case PPPIOCSRASYNCMAP:
 429                 if (get_user(ap->raccm, p))
 430                         break;
 431                 err = 0;
 432                 break;
 433
 434         case PPPIOCGXASYNCMAP:
 435                 if (copy_to_user(argp, ap->xaccm, sizeof(ap->xaccm)))
 436                         break;
 437                 err = 0;
 438                 break;
 439         case PPPIOCSXASYNCMAP:
 440                 if (copy_from_user(accm, argp, sizeof(accm)))
 441                         break;
 442                 accm[2] &= ~0x40000000U;        /* can't escape 0x5e */
 443                 accm[3] |= 0x60000000U;         /* must escape 0x7d, 0x7e */
 444                 memcpy(ap->xaccm, accm, sizeof(ap->xaccm));
 445                 err = 0;
 446                 break;
 447
 448         case PPPIOCGMRU:
 449                 if (put_user(ap->mru, (int __user *) argp))
 450                         break;
 451                 err = 0;
 452                 break;
 453         case PPPIOCSMRU:
 454                 if (get_user(val, (int __user *) argp))
 455                         break;
 456                 if (val > U16_MAX) {
 457                         err = -EINVAL;
 458                         break;
 459                 }
 460                 if (val < PPP_MRU)
 461                         val = PPP_MRU;
 462                 ap->mru = val;
 463                 err = 0;
 464                 break;
 465
 466         default:
 467                 err = -ENOTTY;
 468         }
 469         return err;
 470 }
 471
 472 /*
 473  * This is called at softirq level to deliver received packets
 474  * to the ppp_generic code, and to tell the ppp_generic code
 475  * if we can accept more output now.
 476  */
 477 static void ppp_sync_process(struct tasklet_struct *t)
 478 {
 479         struct syncppp *ap = from_tasklet(ap, t, tsk);
 480         struct sk_buff *skb;
 481
 482         /* process received packets */
 483         while ((skb = skb_dequeue(&ap->rqueue)) != NULL) {
 484                 if (skb->len == 0) {
 485                         /* zero length buffers indicate error */
 486                         ppp_input_error(&ap->chan, 0);
 487                         kfree_skb(skb);
 488                 }
 489                 else
 490                         ppp_input(&ap->chan, skb);
 491         }
 492
 493         /* try to push more stuff out */
 494         if (test_bit(XMIT_WAKEUP, &ap->xmit_flags) && ppp_sync_push(ap))
 495                 ppp_output_wakeup(&ap->chan);
 496 }
 497
 498 /*
 499  * Procedures for encapsulation and framing.
 500  */
 501
 502 static struct sk_buff*
 503 ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
 504 {
 505         int proto;
 506         unsigned char *data;
 507         int islcp;
 508
 509         data  = skb->data;
 510         proto = get_unaligned_be16(data);
 511
 512         /* LCP packets with codes between 1 (configure-request)
 513          * and 7 (code-reject) must be sent as though no options
 514          * have been negotiated.
 515          */
 516         islcp = proto == PPP_LCP && 1 <= data[2] && data[2] <= 7;
 517
 518         /* compress protocol field if option enabled */
 519         if (data[0] == 0 && (ap->flags & SC_COMP_PROT) && !islcp)
 520                 skb_pull(skb,1);
 521
 522         /* prepend address/control fields if necessary */
 523         if ((ap->flags & SC_COMP_AC) == 0 || islcp) {
 524                 if (skb_headroom(skb) < 2) {
 525                         struct sk_buff *npkt = dev_alloc_skb(skb->len + 2);
 526                         if (npkt == NULL) {
 527                                 kfree_skb(skb);
 528                                 return NULL;
 529                         }
 530                         skb_reserve(npkt,2);
 531                         skb_copy_from_linear_data(skb,
 532                                       skb_put(npkt, skb->len), skb->len);
 533                         consume_skb(skb);
 534                         skb = npkt;
 535                 }
 536                 skb_push(skb,2);
 537                 skb->data[0] = PPP_ALLSTATIONS;
 538                 skb->data[1] = PPP_UI;
 539         }
 540
 541         ap->last_xmit = jiffies;
 542
 543         if (skb && ap->flags & SC_LOG_OUTPKT)
 544                 ppp_print_buffer ("send buffer", skb->data, skb->len);
 545
 546         return skb;
 547 }
 548
 549 /*
 550  * Transmit-side routines.
 551  */
 552
 553 /*
 554  * Send a packet to the peer over an sync tty line.
 555  * Returns 1 iff the packet was accepted.
 556  * If the packet was not accepted, we will call ppp_output_wakeup
 557  * at some later time.
 558  */
 559 static int
 560 ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb)
 561 {
 562         struct syncppp *ap = chan->private;
 563
 564         ppp_sync_push(ap);
 565
 566         if (test_and_set_bit(XMIT_FULL, &ap->xmit_flags))
 567                 return 0;       /* already full */
 568         skb = ppp_sync_txmunge(ap, skb);
 569         if (skb != NULL)
 570                 ap->tpkt = skb;
 571         else
 572                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 573
 574         ppp_sync_push(ap);
 575         return 1;
 576 }
 577
 578 /*
 579  * Push as much data as possible out to the tty.
 580  */
 581 static int
 582 ppp_sync_push(struct syncppp *ap)
 583 {
 584         int sent, done = 0;
 585         struct tty_struct *tty = ap->tty;
 586         int tty_stuffed = 0;
 587
 588         if (!spin_trylock_bh(&ap->xmit_lock))
 589                 return 0;
 590         for (;;) {
 591                 if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
 592                         tty_stuffed = 0;
 593                 if (!tty_stuffed && ap->tpkt) {
 594                         set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 595                         sent = tty->ops->write(tty, ap->tpkt->data, ap->tpkt->len);
 596                         if (sent < 0)
 597                                 goto flush;     /* error, e.g. loss of CD */
 598                         if (sent < ap->tpkt->len) {
 599                                 tty_stuffed = 1;
 600                         } else {
 601                                 consume_skb(ap->tpkt);
 602                                 ap->tpkt = NULL;
 603                                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 604                                 done = 1;
 605                         }
 606                         continue;
 607                 }
 608                 /* haven't made any progress */
 609                 spin_unlock_bh(&ap->xmit_lock);
 610                 if (!(test_bit(XMIT_WAKEUP, &ap->xmit_flags) ||
 611                       (!tty_stuffed && ap->tpkt)))
 612                         break;
 613                 if (!spin_trylock_bh(&ap->xmit_lock))
 614                         break;
 615         }
 616         return done;
 617
 618 flush:
 619         if (ap->tpkt) {
 620                 kfree_skb(ap->tpkt);
 621                 ap->tpkt = NULL;
 622                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 623                 done = 1;
 624         }
 625         spin_unlock_bh(&ap->xmit_lock);
 626         return done;
 627 }
 628
 629 /*
 630  * Flush output from our internal buffers.
 631  * Called for the TCFLSH ioctl.
 632  */
 633 static void
 634 ppp_sync_flush_output(struct syncppp *ap)
 635 {
 636         int done = 0;
 637
 638         spin_lock_bh(&ap->xmit_lock);
 639         if (ap->tpkt != NULL) {
 640                 kfree_skb(ap->tpkt);
 641                 ap->tpkt = NULL;
 642                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 643                 done = 1;
 644         }
 645         spin_unlock_bh(&ap->xmit_lock);
 646         if (done)
 647                 ppp_output_wakeup(&ap->chan);
 648 }
 649
 650 /*
 651  * Receive-side routines.
 652  */
 653
 654 /* called when the tty driver has data for us.
 655  *
 656  * Data is frame oriented: each call to ppp_sync_input is considered
 657  * a whole frame. If the 1st flag byte is non-zero then the whole
 658  * frame is considered to be in error and is tossed.
 659  */
 660 static void
 661 ppp_sync_input(struct syncppp *ap, const u8 *buf, const u8 *flags, int count)
 662 {
 663         struct sk_buff *skb;
 664         unsigned char *p;
 665
 666         if (count == 0)
 667                 return;
 668
 669         if (ap->flags & SC_LOG_INPKT)
 670                 ppp_print_buffer ("receive buffer", buf, count);
 671
 672         /* stuff the chars in the skb */
 673         skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2);
 674         if (!skb) {
 675                 printk(KERN_ERR "PPPsync: no memory (input pkt)\n");
 676                 goto err;
 677         }
 678         /* Try to get the payload 4-byte aligned */
 679         if (buf[0] != PPP_ALLSTATIONS)
 680                 skb_reserve(skb, 2 + (buf[0] & 1));
 681
 682         if (flags && *flags) {
 683                 /* error flag set, ignore frame */
 684                 goto err;
 685         } else if (count > skb_tailroom(skb)) {
 686                 /* packet overflowed MRU */
 687                 goto err;
 688         }
 689
 690         skb_put_data(skb, buf, count);
 691
 692         /* strip address/control field if present */
 693         p = skb->data;
 694         if (skb->len >= 2 && p[0] == PPP_ALLSTATIONS && p[1] == PPP_UI) {
 695                 /* chop off address/control */
 696                 if (skb->len < 3)
 697                         goto err;
 698                 p = skb_pull(skb, 2);
 699         }
 700
 701         /* PPP packet length should be >= 2 bytes when protocol field is not
 702          * compressed.
 703          */
 704         if (!(p[0] & 0x01) && skb->len < 2)
 705                 goto err;
 706
 707         /* queue the frame to be processed */
 708         skb_queue_tail(&ap->rqueue, skb);
 709         return;
 710
 711 err:
 712         /* queue zero length packet as error indication */
 713         if (skb || (skb = dev_alloc_skb(0))) {
 714                 skb_trim(skb, 0);
 715                 skb_queue_tail(&ap->rqueue, skb);
 716         }
 717 }
 718
 719 static void __exit
 720 ppp_sync_cleanup(void)
 721 {
 722         tty_unregister_ldisc(&ppp_sync_ldisc);
 723 }
 724
 725 module_init(ppp_sync_init);
 726 module_exit(ppp_sync_cleanup);
 727 MODULE_DESCRIPTION("PPP synchronous TTY channel module");
 728 MODULE_LICENSE("GPL");
 729 MODULE_ALIAS_LDISC(N_SYNC_PPP);