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
 210         if (tty->ops->write == NULL)
 211                 return -EOPNOTSUPP;
 212
 213         ap = kzalloc(sizeof(*ap), GFP_KERNEL);
 214         err = -ENOMEM;
 215         if (!ap)
 216                 goto out;
 217
 218         /* initialize the syncppp structure */
 219         ap->tty = tty;
 220         ap->mru = PPP_MRU;
 221         spin_lock_init(&ap->xmit_lock);
 222         spin_lock_init(&ap->recv_lock);
 223         ap->xaccm[0] = ~0U;
 224         ap->xaccm[3] = 0x60000000U;
 225         ap->raccm = ~0U;
 226
 227         skb_queue_head_init(&ap->rqueue);
 228         tasklet_init(&ap->tsk, ppp_sync_process, (unsigned long) ap);
 229
 230         atomic_set(&ap->refcnt, 1);
 231         init_completion(&ap->dead_cmp);
 232
 233         ap->chan.private = ap;
 234         ap->chan.ops = &sync_ops;
 235         ap->chan.mtu = PPP_MRU;
 236         ap->chan.hdrlen = 2;    /* for A/C bytes */
 237         err = ppp_register_channel(&ap->chan);
 238         if (err)
 239                 goto out_free;
 240
 241         tty->disc_data = ap;
 242         tty->receive_room = 65536;
 243         return 0;
 244
 245  out_free:
 246         kfree(ap);
 247  out:
 248         return err;
 249 }
 250
 251 /*
 252  * Called when the tty is put into another line discipline
 253  * or it hangs up.  We have to wait for any cpu currently
 254  * executing in any of the other ppp_synctty_* routines to
 255  * finish before we can call ppp_unregister_channel and free
 256  * the syncppp struct.  This routine must be called from
 257  * process context, not interrupt or softirq context.
 258  */
 259 static void
 260 ppp_sync_close(struct tty_struct *tty)
 261 {
 262         struct syncppp *ap;
 263
 264         write_lock_irq(&disc_data_lock);
 265         ap = tty->disc_data;
 266         tty->disc_data = NULL;
 267         write_unlock_irq(&disc_data_lock);
 268         if (!ap)
 269                 return;
 270
 271         /*
 272          * We have now ensured that nobody can start using ap from now
 273          * on, but we have to wait for all existing users to finish.
 274          * Note that ppp_unregister_channel ensures that no calls to
 275          * our channel ops (i.e. ppp_sync_send/ioctl) are in progress
 276          * by the time it returns.
 277          */
 278         if (!atomic_dec_and_test(&ap->refcnt))
 279                 wait_for_completion(&ap->dead_cmp);
 280         tasklet_kill(&ap->tsk);
 281
 282         ppp_unregister_channel(&ap->chan);
 283         skb_queue_purge(&ap->rqueue);
 284         if (ap->tpkt)
 285                 kfree_skb(ap->tpkt);
 286         kfree(ap);
 287 }
 288
 289 /*
 290  * Called on tty hangup in process context.
 291  *
 292  * Wait for I/O to driver to complete and unregister PPP channel.
 293  * This is already done by the close routine, so just call that.
 294  */
 295 static int ppp_sync_hangup(struct tty_struct *tty)
 296 {
 297         ppp_sync_close(tty);
 298         return 0;
 299 }
 300
 301 /*
 302  * Read does nothing - no data is ever available this way.
 303  * Pppd reads and writes packets via /dev/ppp instead.
 304  */
 305 static ssize_t
 306 ppp_sync_read(struct tty_struct *tty, struct file *file,
 307                unsigned char __user *buf, size_t count)
 308 {
 309         return -EAGAIN;
 310 }
 311
 312 /*
 313  * Write on the tty does nothing, the packets all come in
 314  * from the ppp generic stuff.
 315  */
 316 static ssize_t
 317 ppp_sync_write(struct tty_struct *tty, struct file *file,
 318                 const unsigned char *buf, size_t count)
 319 {
 320         return -EAGAIN;
 321 }
 322
 323 static int
 324 ppp_synctty_ioctl(struct tty_struct *tty, struct file *file,
 325                   unsigned int cmd, unsigned long arg)
 326 {
 327         struct syncppp *ap = sp_get(tty);
 328         int __user *p = (int __user *)arg;
 329         int err, val;
 330
 331         if (!ap)
 332                 return -ENXIO;
 333         err = -EFAULT;
 334         switch (cmd) {
 335         case PPPIOCGCHAN:
 336                 err = -EFAULT;
 337                 if (put_user(ppp_channel_index(&ap->chan), p))
 338                         break;
 339                 err = 0;
 340                 break;
 341
 342         case PPPIOCGUNIT:
 343                 err = -EFAULT;
 344                 if (put_user(ppp_unit_number(&ap->chan), p))
 345                         break;
 346                 err = 0;
 347                 break;
 348
 349         case TCFLSH:
 350                 /* flush our buffers and the serial port's buffer */
 351                 if (arg == TCIOFLUSH || arg == TCOFLUSH)
 352                         ppp_sync_flush_output(ap);
 353                 err = tty_perform_flush(tty, arg);
 354                 break;
 355
 356         case FIONREAD:
 357                 val = 0;
 358                 if (put_user(val, p))
 359                         break;
 360                 err = 0;
 361                 break;
 362
 363         default:
 364                 err = tty_mode_ioctl(tty, file, cmd, arg);
 365                 break;
 366         }
 367
 368         sp_put(ap);
 369         return err;
 370 }
 371
 372 /* No kernel lock - fine */
 373 static unsigned int
 374 ppp_sync_poll(struct tty_struct *tty, struct file *file, poll_table *wait)
 375 {
 376         return 0;
 377 }
 378
 379 /*
 380  * This can now be called from hard interrupt level as well
 381  * as soft interrupt level or mainline.
 382  */
 383 static void
 384 ppp_sync_receive(struct tty_struct *tty, const unsigned char *buf,
 385                   char *cflags, int count)
 386 {
 387         struct syncppp *ap = sp_get(tty);
 388         unsigned long flags;
 389
 390         if (!ap)
 391                 return;
 392         spin_lock_irqsave(&ap->recv_lock, flags);
 393         ppp_sync_input(ap, buf, cflags, count);
 394         spin_unlock_irqrestore(&ap->recv_lock, flags);
 395         if (!skb_queue_empty(&ap->rqueue))
 396                 tasklet_schedule(&ap->tsk);
 397         sp_put(ap);
 398         tty_unthrottle(tty);
 399 }
 400
 401 static void
 402 ppp_sync_wakeup(struct tty_struct *tty)
 403 {
 404         struct syncppp *ap = sp_get(tty);
 405
 406         clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 407         if (!ap)
 408                 return;
 409         set_bit(XMIT_WAKEUP, &ap->xmit_flags);
 410         tasklet_schedule(&ap->tsk);
 411         sp_put(ap);
 412 }
 413
 414
 415 static struct tty_ldisc_ops ppp_sync_ldisc = {
 416         .owner  = THIS_MODULE,
 417         .magic  = TTY_LDISC_MAGIC,
 418         .name   = "pppsync",
 419         .open   = ppp_sync_open,
 420         .close  = ppp_sync_close,
 421         .hangup = ppp_sync_hangup,
 422         .read   = ppp_sync_read,
 423         .write  = ppp_sync_write,
 424         .ioctl  = ppp_synctty_ioctl,
 425         .poll   = ppp_sync_poll,
 426         .receive_buf = ppp_sync_receive,
 427         .write_wakeup = ppp_sync_wakeup,
 428 };
 429
 430 static int __init
 431 ppp_sync_init(void)
 432 {
 433         int err;
 434
 435         err = tty_register_ldisc(N_SYNC_PPP, &ppp_sync_ldisc);
 436         if (err != 0)
 437                 printk(KERN_ERR "PPP_sync: error %d registering line disc.\n",
 438                        err);
 439         return err;
 440 }
 441
 442 /*
 443  * The following routines provide the PPP channel interface.
 444  */
 445 static int
 446 ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd, unsigned long arg)
 447 {
 448         struct syncppp *ap = chan->private;
 449         int err, val;
 450         u32 accm[8];
 451         void __user *argp = (void __user *)arg;
 452         u32 __user *p = argp;
 453
 454         err = -EFAULT;
 455         switch (cmd) {
 456         case PPPIOCGFLAGS:
 457                 val = ap->flags | ap->rbits;
 458                 if (put_user(val, (int __user *) argp))
 459                         break;
 460                 err = 0;
 461                 break;
 462         case PPPIOCSFLAGS:
 463                 if (get_user(val, (int __user *) argp))
 464                         break;
 465                 ap->flags = val & ~SC_RCV_BITS;
 466                 spin_lock_irq(&ap->recv_lock);
 467                 ap->rbits = val & SC_RCV_BITS;
 468                 spin_unlock_irq(&ap->recv_lock);
 469                 err = 0;
 470                 break;
 471
 472         case PPPIOCGASYNCMAP:
 473                 if (put_user(ap->xaccm[0], p))
 474                         break;
 475                 err = 0;
 476                 break;
 477         case PPPIOCSASYNCMAP:
 478                 if (get_user(ap->xaccm[0], p))
 479                         break;
 480                 err = 0;
 481                 break;
 482
 483         case PPPIOCGRASYNCMAP:
 484                 if (put_user(ap->raccm, p))
 485                         break;
 486                 err = 0;
 487                 break;
 488         case PPPIOCSRASYNCMAP:
 489                 if (get_user(ap->raccm, p))
 490                         break;
 491                 err = 0;
 492                 break;
 493
 494         case PPPIOCGXASYNCMAP:
 495                 if (copy_to_user(argp, ap->xaccm, sizeof(ap->xaccm)))
 496                         break;
 497                 err = 0;
 498                 break;
 499         case PPPIOCSXASYNCMAP:
 500                 if (copy_from_user(accm, argp, sizeof(accm)))
 501                         break;
 502                 accm[2] &= ~0x40000000U;        /* can't escape 0x5e */
 503                 accm[3] |= 0x60000000U;         /* must escape 0x7d, 0x7e */
 504                 memcpy(ap->xaccm, accm, sizeof(ap->xaccm));
 505                 err = 0;
 506                 break;
 507
 508         case PPPIOCGMRU:
 509                 if (put_user(ap->mru, (int __user *) argp))
 510                         break;
 511                 err = 0;
 512                 break;
 513         case PPPIOCSMRU:
 514                 if (get_user(val, (int __user *) argp))
 515                         break;
 516                 if (val < PPP_MRU)
 517                         val = PPP_MRU;
 518                 ap->mru = val;
 519                 err = 0;
 520                 break;
 521
 522         default:
 523                 err = -ENOTTY;
 524         }
 525         return err;
 526 }
 527
 528 /*
 529  * This is called at softirq level to deliver received packets
 530  * to the ppp_generic code, and to tell the ppp_generic code
 531  * if we can accept more output now.
 532  */
 533 static void ppp_sync_process(unsigned long arg)
 534 {
 535         struct syncppp *ap = (struct syncppp *) arg;
 536         struct sk_buff *skb;
 537
 538         /* process received packets */
 539         while ((skb = skb_dequeue(&ap->rqueue)) != NULL) {
 540                 if (skb->len == 0) {
 541                         /* zero length buffers indicate error */
 542                         ppp_input_error(&ap->chan, 0);
 543                         kfree_skb(skb);
 544                 }
 545                 else
 546                         ppp_input(&ap->chan, skb);
 547         }
 548
 549         /* try to push more stuff out */
 550         if (test_bit(XMIT_WAKEUP, &ap->xmit_flags) && ppp_sync_push(ap))
 551                 ppp_output_wakeup(&ap->chan);
 552 }
 553
 554 /*
 555  * Procedures for encapsulation and framing.
 556  */
 557
 558 static struct sk_buff*
 559 ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
 560 {
 561         int proto;
 562         unsigned char *data;
 563         int islcp;
 564
 565         data  = skb->data;
 566         proto = (data[0] << 8) + data[1];
 567
 568         /* LCP packets with codes between 1 (configure-request)
 569          * and 7 (code-reject) must be sent as though no options
 570          * have been negotiated.
 571          */
 572         islcp = proto == PPP_LCP && 1 <= data[2] && data[2] <= 7;
 573
 574         /* compress protocol field if option enabled */
 575         if (data[0] == 0 && (ap->flags & SC_COMP_PROT) && !islcp)
 576                 skb_pull(skb,1);
 577
 578         /* prepend address/control fields if necessary */
 579         if ((ap->flags & SC_COMP_AC) == 0 || islcp) {
 580                 if (skb_headroom(skb) < 2) {
 581                         struct sk_buff *npkt = dev_alloc_skb(skb->len + 2);
 582                         if (npkt == NULL) {
 583                                 kfree_skb(skb);
 584                                 return NULL;
 585                         }
 586                         skb_reserve(npkt,2);
 587                         skb_copy_from_linear_data(skb,
 588                                       skb_put(npkt, skb->len), skb->len);
 589                         kfree_skb(skb);
 590                         skb = npkt;
 591                 }
 592                 skb_push(skb,2);
 593                 skb->data[0] = PPP_ALLSTATIONS;
 594                 skb->data[1] = PPP_UI;
 595         }
 596
 597         ap->last_xmit = jiffies;
 598
 599         if (skb && ap->flags & SC_LOG_OUTPKT)
 600                 ppp_print_buffer ("send buffer", skb->data, skb->len);
 601
 602         return skb;
 603 }
 604
 605 /*
 606  * Transmit-side routines.
 607  */
 608
 609 /*
 610  * Send a packet to the peer over an sync tty line.
 611  * Returns 1 iff the packet was accepted.
 612  * If the packet was not accepted, we will call ppp_output_wakeup
 613  * at some later time.
 614  */
 615 static int
 616 ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb)
 617 {
 618         struct syncppp *ap = chan->private;
 619
 620         ppp_sync_push(ap);
 621
 622         if (test_and_set_bit(XMIT_FULL, &ap->xmit_flags))
 623                 return 0;       /* already full */
 624         skb = ppp_sync_txmunge(ap, skb);
 625         if (skb != NULL)
 626                 ap->tpkt = skb;
 627         else
 628                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 629
 630         ppp_sync_push(ap);
 631         return 1;
 632 }
 633
 634 /*
 635  * Push as much data as possible out to the tty.
 636  */
 637 static int
 638 ppp_sync_push(struct syncppp *ap)
 639 {
 640         int sent, done = 0;
 641         struct tty_struct *tty = ap->tty;
 642         int tty_stuffed = 0;
 643
 644         if (!spin_trylock_bh(&ap->xmit_lock))
 645                 return 0;
 646         for (;;) {
 647                 if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
 648                         tty_stuffed = 0;
 649                 if (!tty_stuffed && ap->tpkt) {
 650                         set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 651                         sent = tty->ops->write(tty, ap->tpkt->data, ap->tpkt->len);
 652                         if (sent < 0)
 653                                 goto flush;     /* error, e.g. loss of CD */
 654                         if (sent < ap->tpkt->len) {
 655                                 tty_stuffed = 1;
 656                         } else {
 657                                 kfree_skb(ap->tpkt);
 658                                 ap->tpkt = NULL;
 659                                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 660                                 done = 1;
 661                         }
 662                         continue;
 663                 }
 664                 /* haven't made any progress */
 665                 spin_unlock_bh(&ap->xmit_lock);
 666                 if (!(test_bit(XMIT_WAKEUP, &ap->xmit_flags)
 667                       || (!tty_stuffed && ap->tpkt)))
 668                         break;
 669                 if (!spin_trylock_bh(&ap->xmit_lock))
 670                         break;
 671         }
 672         return done;
 673
 674 flush:
 675         if (ap->tpkt) {
 676                 kfree_skb(ap->tpkt);
 677                 ap->tpkt = NULL;
 678                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 679                 done = 1;
 680         }
 681         spin_unlock_bh(&ap->xmit_lock);
 682         return done;
 683 }
 684
 685 /*
 686  * Flush output from our internal buffers.
 687  * Called for the TCFLSH ioctl.
 688  */
 689 static void
 690 ppp_sync_flush_output(struct syncppp *ap)
 691 {
 692         int done = 0;
 693
 694         spin_lock_bh(&ap->xmit_lock);
 695         if (ap->tpkt != NULL) {
 696                 kfree_skb(ap->tpkt);
 697                 ap->tpkt = NULL;
 698                 clear_bit(XMIT_FULL, &ap->xmit_flags);
 699                 done = 1;
 700         }
 701         spin_unlock_bh(&ap->xmit_lock);
 702         if (done)
 703                 ppp_output_wakeup(&ap->chan);
 704 }
 705
 706 /*
 707  * Receive-side routines.
 708  */
 709
 710 /* called when the tty driver has data for us.
 711  *
 712  * Data is frame oriented: each call to ppp_sync_input is considered
 713  * a whole frame. If the 1st flag byte is non-zero then the whole
 714  * frame is considered to be in error and is tossed.
 715  */
 716 static void
 717 ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
 718                 char *flags, int count)
 719 {
 720         struct sk_buff *skb;
 721         unsigned char *p;
 722
 723         if (count == 0)
 724                 return;
 725
 726         if (ap->flags & SC_LOG_INPKT)
 727                 ppp_print_buffer ("receive buffer", buf, count);
 728
 729         /* stuff the chars in the skb */
 730         skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2);
 731         if (!skb) {
 732                 printk(KERN_ERR "PPPsync: no memory (input pkt)\n");
 733                 goto err;
 734         }
 735         /* Try to get the payload 4-byte aligned */
 736         if (buf[0] != PPP_ALLSTATIONS)
 737                 skb_reserve(skb, 2 + (buf[0] & 1));
 738
 739         if (flags && *flags) {
 740                 /* error flag set, ignore frame */
 741                 goto err;
 742         } else if (count > skb_tailroom(skb)) {
 743                 /* packet overflowed MRU */
 744                 goto err;
 745         }
 746
 747         p = skb_put(skb, count);
 748         memcpy(p, buf, count);
 749
 750         /* strip address/control field if present */
 751         p = skb->data;
 752         if (p[0] == PPP_ALLSTATIONS && p[1] == PPP_UI) {
 753                 /* chop off address/control */
 754                 if (skb->len < 3)
 755                         goto err;
 756                 p = skb_pull(skb, 2);
 757         }
 758
 759         /* decompress protocol field if compressed */
 760         if (p[0] & 1) {
 761                 /* protocol is compressed */
 762                 skb_push(skb, 1)[0] = 0;
 763         } else if (skb->len < 2)
 764                 goto err;
 765
 766         /* queue the frame to be processed */
 767         skb_queue_tail(&ap->rqueue, skb);
 768         return;
 769
 770 err:
 771         /* queue zero length packet as error indication */
 772         if (skb || (skb = dev_alloc_skb(0))) {
 773                 skb_trim(skb, 0);
 774                 skb_queue_tail(&ap->rqueue, skb);
 775         }
 776 }
 777
 778 static void __exit
 779 ppp_sync_cleanup(void)
 780 {
 781         if (tty_unregister_ldisc(N_SYNC_PPP) != 0)
 782                 printk(KERN_ERR "failed to unregister Sync PPP line discipline\n");
 783 }
 784
 785 module_init(ppp_sync_init);
 786 module_exit(ppp_sync_cleanup);
 787 MODULE_LICENSE("GPL");
 788 MODULE_ALIAS_LDISC(N_SYNC_PPP);