xref: /linux/drivers/net/ppp/ppp_synctty.c (revision ab59a8605604f71bbbc16077270dc3f39648b7fc)
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
ppp_print_buffer(const char * name,const __u8 * buf,int count)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 
sp_get(struct tty_struct * tty)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 
sp_put(struct syncppp * ap)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
ppp_sync_open(struct tty_struct * tty)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
ppp_sync_close(struct tty_struct * tty)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  */
ppp_sync_hangup(struct tty_struct * tty)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
ppp_sync_read(struct tty_struct * tty,struct file * file,u8 * buf,size_t count,void ** cookie,unsigned long offset)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
ppp_sync_write(struct tty_struct * tty,struct file * file,const u8 * buf,size_t count)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
ppp_synctty_ioctl(struct tty_struct * tty,unsigned int cmd,unsigned long arg)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
ppp_sync_receive(struct tty_struct * tty,const u8 * buf,const u8 * cflags,size_t count)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
ppp_sync_wakeup(struct tty_struct * tty)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
ppp_sync_init(void)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
ppp_sync_ioctl(struct ppp_channel * chan,unsigned int cmd,unsigned long arg)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  */
ppp_sync_process(struct tasklet_struct * t)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*
ppp_sync_txmunge(struct syncppp * ap,struct sk_buff * skb)503 ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
504 {
505 	int proto;
506 	unsigned char *data;
507 	int islcp;
508 
509 	/* Ensure we can safely access protocol field and LCP code */
510 	if (!pskb_may_pull(skb, 3)) {
511 		kfree_skb(skb);
512 		return NULL;
513 	}
514 	data  = skb->data;
515 	proto = get_unaligned_be16(data);
516 
517 	/* LCP packets with codes between 1 (configure-request)
518 	 * and 7 (code-reject) must be sent as though no options
519 	 * have been negotiated.
520 	 */
521 	islcp = proto == PPP_LCP && 1 <= data[2] && data[2] <= 7;
522 
523 	/* compress protocol field if option enabled */
524 	if (data[0] == 0 && (ap->flags & SC_COMP_PROT) && !islcp)
525 		skb_pull(skb,1);
526 
527 	/* prepend address/control fields if necessary */
528 	if ((ap->flags & SC_COMP_AC) == 0 || islcp) {
529 		if (skb_headroom(skb) < 2) {
530 			struct sk_buff *npkt = dev_alloc_skb(skb->len + 2);
531 			if (npkt == NULL) {
532 				kfree_skb(skb);
533 				return NULL;
534 			}
535 			skb_reserve(npkt,2);
536 			skb_copy_from_linear_data(skb,
537 				      skb_put(npkt, skb->len), skb->len);
538 			consume_skb(skb);
539 			skb = npkt;
540 		}
541 		skb_push(skb,2);
542 		skb->data[0] = PPP_ALLSTATIONS;
543 		skb->data[1] = PPP_UI;
544 	}
545 
546 	ap->last_xmit = jiffies;
547 
548 	if (skb && ap->flags & SC_LOG_OUTPKT)
549 		ppp_print_buffer ("send buffer", skb->data, skb->len);
550 
551 	return skb;
552 }
553 
554 /*
555  * Transmit-side routines.
556  */
557 
558 /*
559  * Send a packet to the peer over an sync tty line.
560  * Returns 1 iff the packet was accepted.
561  * If the packet was not accepted, we will call ppp_output_wakeup
562  * at some later time.
563  */
564 static int
ppp_sync_send(struct ppp_channel * chan,struct sk_buff * skb)565 ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb)
566 {
567 	struct syncppp *ap = chan->private;
568 
569 	ppp_sync_push(ap);
570 
571 	if (test_and_set_bit(XMIT_FULL, &ap->xmit_flags))
572 		return 0;	/* already full */
573 	skb = ppp_sync_txmunge(ap, skb);
574 	if (skb != NULL)
575 		ap->tpkt = skb;
576 	else
577 		clear_bit(XMIT_FULL, &ap->xmit_flags);
578 
579 	ppp_sync_push(ap);
580 	return 1;
581 }
582 
583 /*
584  * Push as much data as possible out to the tty.
585  */
586 static int
ppp_sync_push(struct syncppp * ap)587 ppp_sync_push(struct syncppp *ap)
588 {
589 	int sent, done = 0;
590 	struct tty_struct *tty = ap->tty;
591 	int tty_stuffed = 0;
592 
593 	if (!spin_trylock_bh(&ap->xmit_lock))
594 		return 0;
595 	for (;;) {
596 		if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
597 			tty_stuffed = 0;
598 		if (!tty_stuffed && ap->tpkt) {
599 			set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
600 			sent = tty->ops->write(tty, ap->tpkt->data, ap->tpkt->len);
601 			if (sent < 0)
602 				goto flush;	/* error, e.g. loss of CD */
603 			if (sent < ap->tpkt->len) {
604 				tty_stuffed = 1;
605 			} else {
606 				consume_skb(ap->tpkt);
607 				ap->tpkt = NULL;
608 				clear_bit(XMIT_FULL, &ap->xmit_flags);
609 				done = 1;
610 			}
611 			continue;
612 		}
613 		/* haven't made any progress */
614 		spin_unlock_bh(&ap->xmit_lock);
615 		if (!(test_bit(XMIT_WAKEUP, &ap->xmit_flags) ||
616 		      (!tty_stuffed && ap->tpkt)))
617 			break;
618 		if (!spin_trylock_bh(&ap->xmit_lock))
619 			break;
620 	}
621 	return done;
622 
623 flush:
624 	if (ap->tpkt) {
625 		kfree_skb(ap->tpkt);
626 		ap->tpkt = NULL;
627 		clear_bit(XMIT_FULL, &ap->xmit_flags);
628 		done = 1;
629 	}
630 	spin_unlock_bh(&ap->xmit_lock);
631 	return done;
632 }
633 
634 /*
635  * Flush output from our internal buffers.
636  * Called for the TCFLSH ioctl.
637  */
638 static void
ppp_sync_flush_output(struct syncppp * ap)639 ppp_sync_flush_output(struct syncppp *ap)
640 {
641 	int done = 0;
642 
643 	spin_lock_bh(&ap->xmit_lock);
644 	if (ap->tpkt != NULL) {
645 		kfree_skb(ap->tpkt);
646 		ap->tpkt = NULL;
647 		clear_bit(XMIT_FULL, &ap->xmit_flags);
648 		done = 1;
649 	}
650 	spin_unlock_bh(&ap->xmit_lock);
651 	if (done)
652 		ppp_output_wakeup(&ap->chan);
653 }
654 
655 /*
656  * Receive-side routines.
657  */
658 
659 /* called when the tty driver has data for us.
660  *
661  * Data is frame oriented: each call to ppp_sync_input is considered
662  * a whole frame. If the 1st flag byte is non-zero then the whole
663  * frame is considered to be in error and is tossed.
664  */
665 static void
ppp_sync_input(struct syncppp * ap,const u8 * buf,const u8 * flags,int count)666 ppp_sync_input(struct syncppp *ap, const u8 *buf, const u8 *flags, int count)
667 {
668 	struct sk_buff *skb;
669 	unsigned char *p;
670 
671 	if (count == 0)
672 		return;
673 
674 	if (ap->flags & SC_LOG_INPKT)
675 		ppp_print_buffer ("receive buffer", buf, count);
676 
677 	/* stuff the chars in the skb */
678 	skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2);
679 	if (!skb) {
680 		printk(KERN_ERR "PPPsync: no memory (input pkt)\n");
681 		goto err;
682 	}
683 	/* Try to get the payload 4-byte aligned */
684 	if (buf[0] != PPP_ALLSTATIONS)
685 		skb_reserve(skb, 2 + (buf[0] & 1));
686 
687 	if (flags && *flags) {
688 		/* error flag set, ignore frame */
689 		goto err;
690 	} else if (count > skb_tailroom(skb)) {
691 		/* packet overflowed MRU */
692 		goto err;
693 	}
694 
695 	skb_put_data(skb, buf, count);
696 
697 	/* strip address/control field if present */
698 	p = skb->data;
699 	if (skb->len >= 2 && p[0] == PPP_ALLSTATIONS && p[1] == PPP_UI) {
700 		/* chop off address/control */
701 		if (skb->len < 3)
702 			goto err;
703 		p = skb_pull(skb, 2);
704 	}
705 
706 	/* PPP packet length should be >= 2 bytes when protocol field is not
707 	 * compressed.
708 	 */
709 	if (!(p[0] & 0x01) && skb->len < 2)
710 		goto err;
711 
712 	/* queue the frame to be processed */
713 	skb_queue_tail(&ap->rqueue, skb);
714 	return;
715 
716 err:
717 	/* queue zero length packet as error indication */
718 	if (skb || (skb = dev_alloc_skb(0))) {
719 		skb_trim(skb, 0);
720 		skb_queue_tail(&ap->rqueue, skb);
721 	}
722 }
723 
724 static void __exit
ppp_sync_cleanup(void)725 ppp_sync_cleanup(void)
726 {
727 	tty_unregister_ldisc(&ppp_sync_ldisc);
728 }
729 
730 module_init(ppp_sync_init);
731 module_exit(ppp_sync_cleanup);
732 MODULE_DESCRIPTION("PPP synchronous TTY channel module");
733 MODULE_LICENSE("GPL");
734 MODULE_ALIAS_LDISC(N_SYNC_PPP);
735