1 files changed, 803 insertions, 0 deletions

diff --git a/drivers/net/ppp_synctty.c b/drivers/net/ppp_synctty.c
new file mode 100644
index 000000000000..7d0150b4c629
--- /dev/null
+++ b/drivers/net/ppp_synctty.c
@@ -0,0 +1,803 @@
+/*
+ * PPP synchronous tty channel driver for Linux.
+ *
+ * This is a ppp channel driver that can be used with tty device drivers
+ * that are frame oriented, such as synchronous HDLC devices.
+ *
+ * Complete PPP frames without encoding/decoding are exchanged between
+ * the channel driver and the device driver.
+ * 
+ * The async map IOCTL codes are implemented to keep the user mode
+ * applications happy if they call them. Synchronous PPP does not use
+ * the async maps.
+ *
+ * Copyright 1999 Paul Mackerras.
+ *
+ * Also touched by the grubby hands of Paul Fulghum paulkf@microgate.com
+ *
+ *  This program is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU General Public License
+ *  as published by the Free Software Foundation; either version
+ *  2 of the License, or (at your option) any later version.
+ *
+ * This driver provides the encapsulation and framing for sending
+ * and receiving PPP frames over sync serial lines.  It relies on
+ * the generic PPP layer to give it frames to send and to process
+ * received frames.  It implements the PPP line discipline.
+ *
+ * Part of the code in this driver was inspired by the old async-only
+ * PPP driver, written by Michael Callahan and Al Longyear, and
+ * subsequently hacked by Paul Mackerras.
+ *
+ * ==FILEVERSION 20040616==
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/skbuff.h>
+#include <linux/tty.h>
+#include <linux/netdevice.h>
+#include <linux/poll.h>
+#include <linux/ppp_defs.h>
+#include <linux/if_ppp.h>
+#include <linux/ppp_channel.h>
+#include <linux/spinlock.h>
+#include <linux/init.h>
+#include <asm/uaccess.h>
+#include <asm/semaphore.h>
+
+#define PPP_VERSION     "2.4.2"
+
+/* Structure for storing local state. */
+struct syncppp {
+        struct tty_struct *tty;
+        unsigned int    flags;
+        unsigned int    rbits;
+        int             mru;
+        spinlock_t      xmit_lock;
+        spinlock_t      recv_lock;
+        unsigned long   xmit_flags;
+        u32             xaccm[8];
+        u32             raccm;
+        unsigned int    bytes_sent;
+        unsigned int    bytes_rcvd;
+
+        struct sk_buff  *tpkt;
+        unsigned long   last_xmit;
+
+        struct sk_buff_head rqueue;
+
+        struct tasklet_struct tsk;
+
+        atomic_t        refcnt;
+        struct semaphore dead_sem;
+        struct ppp_channel chan;        /* interface to generic ppp layer */
+};
+
+/* Bit numbers in xmit_flags */
+#define XMIT_WAKEUP     0
+#define XMIT_FULL       1
+
+/* Bits in rbits */
+#define SC_RCV_BITS     (SC_RCV_B7_1|SC_RCV_B7_0|SC_RCV_ODDP|SC_RCV_EVNP)
+
+#define PPPSYNC_MAX_RQLEN       32      /* arbitrary */
+
+/*
+ * Prototypes.
+ */
+static struct sk_buff* ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *);
+static int ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb);
+static int ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd,
+                          unsigned long arg);
+static void ppp_sync_process(unsigned long arg);
+static int ppp_sync_push(struct syncppp *ap);
+static void ppp_sync_flush_output(struct syncppp *ap);
+static void ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
+                           char *flags, int count);
+
+static struct ppp_channel_ops sync_ops = {
+        ppp_sync_send,
+        ppp_sync_ioctl
+};
+
+/*
+ * Utility procedures to print a buffer in hex/ascii
+ */
+static void
+ppp_print_hex (register __u8 * out, const __u8 * in, int count)
+{
+        register __u8 next_ch;
+        static char hex[] = "0123456789ABCDEF";
+
+        while (count-- > 0) {
+                next_ch = *in++;
+                *out++ = hex[(next_ch >> 4) & 0x0F];
+                *out++ = hex[next_ch & 0x0F];
+                ++out;
+        }
+}
+
+static void
+ppp_print_char (register __u8 * out, const __u8 * in, int count)
+{
+        register __u8 next_ch;
+
+        while (count-- > 0) {
+                next_ch = *in++;
+
+                if (next_ch < 0x20 || next_ch > 0x7e)
+                        *out++ = '.';
+                else {
+                        *out++ = next_ch;
+                        if (next_ch == '%')   /* printk/syslogd has a bug !! */
+                                *out++ = '%';
+                }
+        }
+        *out = '\0';
+}
+
+static void
+ppp_print_buffer (const char *name, const __u8 *buf, int count)
+{
+        __u8 line[44];
+
+        if (name != NULL)
+                printk(KERN_DEBUG "ppp_synctty: %s, count = %d\n", name, count);
+
+        while (count > 8) {
+                memset (line, 32, 44);
+                ppp_print_hex (line, buf, 8);
+                ppp_print_char (&line[8 * 3], buf, 8);
+                printk(KERN_DEBUG "%s\n", line);
+                count -= 8;
+                buf += 8;
+        }
+
+        if (count > 0) {
+                memset (line, 32, 44);
+                ppp_print_hex (line, buf, count);
+                ppp_print_char (&line[8 * 3], buf, count);
+                printk(KERN_DEBUG "%s\n", line);
+        }
+}
+
+
+/*
+ * Routines implementing the synchronous PPP line discipline.
+ */
+
+/*
+ * We have a potential race on dereferencing tty->disc_data,
+ * because the tty layer provides no locking at all - thus one
+ * cpu could be running ppp_synctty_receive while another
+ * calls ppp_synctty_close, which zeroes tty->disc_data and
+ * frees the memory that ppp_synctty_receive is using.  The best
+ * way to fix this is to use a rwlock in the tty struct, but for now
+ * we use a single global rwlock for all ttys in ppp line discipline.
+ *
+ * FIXME: Fixed in tty_io nowdays.
+ */
+static DEFINE_RWLOCK(disc_data_lock);
+
+static struct syncppp *sp_get(struct tty_struct *tty)
+{
+        struct syncppp *ap;
+
+        read_lock(&disc_data_lock);
+        ap = tty->disc_data;
+        if (ap != NULL)
+                atomic_inc(&ap->refcnt);
+        read_unlock(&disc_data_lock);
+        return ap;
+}
+
+static void sp_put(struct syncppp *ap)
+{
+        if (atomic_dec_and_test(&ap->refcnt))
+                up(&ap->dead_sem);
+}
+
+/*
+ * Called when a tty is put into sync-PPP line discipline.
+ */
+static int
+ppp_sync_open(struct tty_struct *tty)
+{
+        struct syncppp *ap;
+        int err;
+
+        ap = kmalloc(sizeof(*ap), GFP_KERNEL);
+        err = -ENOMEM;
+        if (ap == 0)
+                goto out;
+
+        /* initialize the syncppp structure */
+        memset(ap, 0, sizeof(*ap));
+        ap->tty = tty;
+        ap->mru = PPP_MRU;
+        spin_lock_init(&ap->xmit_lock);
+        spin_lock_init(&ap->recv_lock);
+        ap->xaccm[0] = ~0U;
+        ap->xaccm[3] = 0x60000000U;
+        ap->raccm = ~0U;
+
+        skb_queue_head_init(&ap->rqueue);
+        tasklet_init(&ap->tsk, ppp_sync_process, (unsigned long) ap);
+
+        atomic_set(&ap->refcnt, 1);
+        init_MUTEX_LOCKED(&ap->dead_sem);
+
+        ap->chan.private = ap;
+        ap->chan.ops = &sync_ops;
+        ap->chan.mtu = PPP_MRU;
+        ap->chan.hdrlen = 2;    /* for A/C bytes */
+        err = ppp_register_channel(&ap->chan);
+        if (err)
+                goto out_free;
+
+        tty->disc_data = ap;
+
+        return 0;
+
+ out_free:
+        kfree(ap);
+ out:
+        return err;
+}
+
+/*
+ * Called when the tty is put into another line discipline
+ * or it hangs up.  We have to wait for any cpu currently
+ * executing in any of the other ppp_synctty_* routines to
+ * finish before we can call ppp_unregister_channel and free
+ * the syncppp struct.  This routine must be called from
+ * process context, not interrupt or softirq context.
+ */
+static void
+ppp_sync_close(struct tty_struct *tty)
+{
+        struct syncppp *ap;
+
+        write_lock_irq(&disc_data_lock);
+        ap = tty->disc_data;
+        tty->disc_data = NULL;
+        write_unlock_irq(&disc_data_lock);
+        if (ap == 0)
+                return;
+
+        /*
+         * We have now ensured that nobody can start using ap from now
+         * on, but we have to wait for all existing users to finish.
+         * Note that ppp_unregister_channel ensures that no calls to
+         * our channel ops (i.e. ppp_sync_send/ioctl) are in progress
+         * by the time it returns.
+         */
+        if (!atomic_dec_and_test(&ap->refcnt))
+                down(&ap->dead_sem);
+        tasklet_kill(&ap->tsk);
+
+        ppp_unregister_channel(&ap->chan);
+        skb_queue_purge(&ap->rqueue);
+        if (ap->tpkt != 0)
+                kfree_skb(ap->tpkt);
+        kfree(ap);
+}
+
+/*
+ * Called on tty hangup in process context.
+ *
+ * Wait for I/O to driver to complete and unregister PPP channel.
+ * This is already done by the close routine, so just call that.
+ */
+static int ppp_sync_hangup(struct tty_struct *tty)
+{
+        ppp_sync_close(tty);
+        return 0;
+}
+
+/*
+ * Read does nothing - no data is ever available this way.
+ * Pppd reads and writes packets via /dev/ppp instead.
+ */
+static ssize_t
+ppp_sync_read(struct tty_struct *tty, struct file *file,
+               unsigned char __user *buf, size_t count)
+{
+        return -EAGAIN;
+}
+
+/*
+ * Write on the tty does nothing, the packets all come in
+ * from the ppp generic stuff.
+ */
+static ssize_t
+ppp_sync_write(struct tty_struct *tty, struct file *file,
+                const unsigned char *buf, size_t count)
+{
+        return -EAGAIN;
+}
+
+static int
+ppp_synctty_ioctl(struct tty_struct *tty, struct file *file,
+                  unsigned int cmd, unsigned long arg)
+{
+        struct syncppp *ap = sp_get(tty);
+        int __user *p = (int __user *)arg;
+        int err, val;
+
+        if (ap == 0)
+                return -ENXIO;
+        err = -EFAULT;
+        switch (cmd) {
+        case PPPIOCGCHAN:
+                err = -ENXIO;
+                if (ap == 0)
+                        break;
+                err = -EFAULT;
+                if (put_user(ppp_channel_index(&ap->chan), p))
+                        break;
+                err = 0;
+                break;
+
+        case PPPIOCGUNIT:
+                err = -ENXIO;
+                if (ap == 0)
+                        break;
+                err = -EFAULT;
+                if (put_user(ppp_unit_number(&ap->chan), p))
+                        break;
+                err = 0;
+                break;
+
+        case TCGETS:
+        case TCGETA:
+                err = n_tty_ioctl(tty, file, cmd, arg);
+                break;
+
+        case TCFLSH:
+                /* flush our buffers and the serial port's buffer */
+                if (arg == TCIOFLUSH || arg == TCOFLUSH)
+                        ppp_sync_flush_output(ap);
+                err = n_tty_ioctl(tty, file, cmd, arg);
+                break;
+
+        case FIONREAD:
+                val = 0;
+                if (put_user(val, p))
+                        break;
+                err = 0;
+                break;
+
+        default:
+                err = -ENOIOCTLCMD;
+        }
+
+        sp_put(ap);
+        return err;
+}
+
+/* No kernel lock - fine */
+static unsigned int
+ppp_sync_poll(struct tty_struct *tty, struct file *file, poll_table *wait)
+{
+        return 0;
+}
+
+static int
+ppp_sync_room(struct tty_struct *tty)
+{
+        return 65535;
+}
+
+/*
+ * This can now be called from hard interrupt level as well
+ * as soft interrupt level or mainline.
+ */
+static void
+ppp_sync_receive(struct tty_struct *tty, const unsigned char *buf,
+                  char *cflags, int count)
+{
+        struct syncppp *ap = sp_get(tty);
+        unsigned long flags;
+
+        if (ap == 0)
+                return;
+        spin_lock_irqsave(&ap->recv_lock, flags);
+        ppp_sync_input(ap, buf, cflags, count);
+        spin_unlock_irqrestore(&ap->recv_lock, flags);
+        if (skb_queue_len(&ap->rqueue))
+                tasklet_schedule(&ap->tsk);
+        sp_put(ap);
+        if (test_and_clear_bit(TTY_THROTTLED, &tty->flags)
+            && tty->driver->unthrottle)
+                tty->driver->unthrottle(tty);
+}
+
+static void
+ppp_sync_wakeup(struct tty_struct *tty)
+{
+        struct syncppp *ap = sp_get(tty);
+
+        clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+        if (ap == 0)
+                return;
+        set_bit(XMIT_WAKEUP, &ap->xmit_flags);
+        tasklet_schedule(&ap->tsk);
+        sp_put(ap);
+}
+
+
+static struct tty_ldisc ppp_sync_ldisc = {
+        .owner  = THIS_MODULE,
+        .magic  = TTY_LDISC_MAGIC,
+        .name   = "pppsync",
+        .open   = ppp_sync_open,
+        .close  = ppp_sync_close,
+        .hangup = ppp_sync_hangup,
+        .read   = ppp_sync_read,
+        .write  = ppp_sync_write,
+        .ioctl  = ppp_synctty_ioctl,
+        .poll   = ppp_sync_poll,
+        .receive_room = ppp_sync_room,
+        .receive_buf = ppp_sync_receive,
+        .write_wakeup = ppp_sync_wakeup,
+};
+
+static int __init
+ppp_sync_init(void)
+{
+        int err;
+
+        err = tty_register_ldisc(N_SYNC_PPP, &ppp_sync_ldisc);
+        if (err != 0)
+                printk(KERN_ERR "PPP_sync: error %d registering line disc.\n",
+                       err);
+        return err;
+}
+
+/*
+ * The following routines provide the PPP channel interface.
+ */
+static int
+ppp_sync_ioctl(struct ppp_channel *chan, unsigned int cmd, unsigned long arg)
+{
+        struct syncppp *ap = chan->private;
+        int err, val;
+        u32 accm[8];
+        void __user *argp = (void __user *)arg;
+        u32 __user *p = argp;
+
+        err = -EFAULT;
+        switch (cmd) {
+        case PPPIOCGFLAGS:
+                val = ap->flags | ap->rbits;
+                if (put_user(val, (int __user *) argp))
+                        break;
+                err = 0;
+                break;
+        case PPPIOCSFLAGS:
+                if (get_user(val, (int __user *) argp))
+                        break;
+                ap->flags = val & ~SC_RCV_BITS;
+                spin_lock_irq(&ap->recv_lock);
+                ap->rbits = val & SC_RCV_BITS;
+                spin_unlock_irq(&ap->recv_lock);
+                err = 0;
+                break;
+
+        case PPPIOCGASYNCMAP:
+                if (put_user(ap->xaccm[0], p))
+                        break;
+                err = 0;
+                break;
+        case PPPIOCSASYNCMAP:
+                if (get_user(ap->xaccm[0], p))
+                        break;
+                err = 0;
+                break;
+
+        case PPPIOCGRASYNCMAP:
+                if (put_user(ap->raccm, p))
+                        break;
+                err = 0;
+                break;
+        case PPPIOCSRASYNCMAP:
+                if (get_user(ap->raccm, p))
+                        break;
+                err = 0;
+                break;
+
+        case PPPIOCGXASYNCMAP:
+                if (copy_to_user(argp, ap->xaccm, sizeof(ap->xaccm)))
+                        break;
+                err = 0;
+                break;
+        case PPPIOCSXASYNCMAP:
+                if (copy_from_user(accm, argp, sizeof(accm)))
+                        break;
+                accm[2] &= ~0x40000000U;        /* can't escape 0x5e */
+                accm[3] |= 0x60000000U;         /* must escape 0x7d, 0x7e */
+                memcpy(ap->xaccm, accm, sizeof(ap->xaccm));
+                err = 0;
+                break;
+
+        case PPPIOCGMRU:
+                if (put_user(ap->mru, (int __user *) argp))
+                        break;
+                err = 0;
+                break;
+        case PPPIOCSMRU:
+                if (get_user(val, (int __user *) argp))
+                        break;
+                if (val < PPP_MRU)
+                        val = PPP_MRU;
+                ap->mru = val;
+                err = 0;
+                break;
+
+        default:
+                err = -ENOTTY;
+        }
+        return err;
+}
+
+/*
+ * This is called at softirq level to deliver received packets
+ * to the ppp_generic code, and to tell the ppp_generic code
+ * if we can accept more output now.
+ */
+static void ppp_sync_process(unsigned long arg)
+{
+        struct syncppp *ap = (struct syncppp *) arg;
+        struct sk_buff *skb;
+
+        /* process received packets */
+        while ((skb = skb_dequeue(&ap->rqueue)) != NULL) {
+                if (skb->len == 0) {
+                        /* zero length buffers indicate error */
+                        ppp_input_error(&ap->chan, 0);
+                        kfree_skb(skb);
+                }
+                else
+                        ppp_input(&ap->chan, skb);
+        }
+
+        /* try to push more stuff out */
+        if (test_bit(XMIT_WAKEUP, &ap->xmit_flags) && ppp_sync_push(ap))
+                ppp_output_wakeup(&ap->chan);
+}
+
+/*
+ * Procedures for encapsulation and framing.
+ */
+
+struct sk_buff*
+ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
+{
+        int proto;
+        unsigned char *data;
+        int islcp;
+
+        data  = skb->data;
+        proto = (data[0] << 8) + data[1];
+
+        /* LCP packets with codes between 1 (configure-request)
+         * and 7 (code-reject) must be sent as though no options
+         * have been negotiated.
+         */
+        islcp = proto == PPP_LCP && 1 <= data[2] && data[2] <= 7;
+
+        /* compress protocol field if option enabled */
+        if (data[0] == 0 && (ap->flags & SC_COMP_PROT) && !islcp)
+                skb_pull(skb,1);
+
+        /* prepend address/control fields if necessary */
+        if ((ap->flags & SC_COMP_AC) == 0 || islcp) {
+                if (skb_headroom(skb) < 2) {
+                        struct sk_buff *npkt = dev_alloc_skb(skb->len + 2);
+                        if (npkt == NULL) {
+                                kfree_skb(skb);
+                                return NULL;
+                        }
+                        skb_reserve(npkt,2);
+                        memcpy(skb_put(npkt,skb->len), skb->data, skb->len);
+                        kfree_skb(skb);
+                        skb = npkt;
+                }
+                skb_push(skb,2);
+                skb->data[0] = PPP_ALLSTATIONS;
+                skb->data[1] = PPP_UI;
+        }
+
+        ap->last_xmit = jiffies;
+
+        if (skb && ap->flags & SC_LOG_OUTPKT)
+                ppp_print_buffer ("send buffer", skb->data, skb->len);
+
+        return skb;
+}
+
+/*
+ * Transmit-side routines.
+ */
+
+/*
+ * Send a packet to the peer over an sync tty line.
+ * Returns 1 iff the packet was accepted.
+ * If the packet was not accepted, we will call ppp_output_wakeup
+ * at some later time.
+ */
+static int
+ppp_sync_send(struct ppp_channel *chan, struct sk_buff *skb)
+{
+        struct syncppp *ap = chan->private;
+
+        ppp_sync_push(ap);
+
+        if (test_and_set_bit(XMIT_FULL, &ap->xmit_flags))
+                return 0;       /* already full */
+        skb = ppp_sync_txmunge(ap, skb);
+        if (skb != NULL)
+                ap->tpkt = skb;
+        else
+                clear_bit(XMIT_FULL, &ap->xmit_flags);
+
+        ppp_sync_push(ap);
+        return 1;
+}
+
+/*
+ * Push as much data as possible out to the tty.
+ */
+static int
+ppp_sync_push(struct syncppp *ap)
+{
+        int sent, done = 0;
+        struct tty_struct *tty = ap->tty;
+        int tty_stuffed = 0;
+
+        if (!spin_trylock_bh(&ap->xmit_lock))
+                return 0;
+        for (;;) {
+                if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
+                        tty_stuffed = 0;
+                if (!tty_stuffed && ap->tpkt != 0) {
+                        set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+                        sent = tty->driver->write(tty, ap->tpkt->data, ap->tpkt->len);
+                        if (sent < 0)
+                                goto flush;     /* error, e.g. loss of CD */
+                        if (sent < ap->tpkt->len) {
+                                tty_stuffed = 1;
+                        } else {
+                                kfree_skb(ap->tpkt);
+                                ap->tpkt = NULL;
+                                clear_bit(XMIT_FULL, &ap->xmit_flags);
+                                done = 1;
+                        }
+                        continue;
+                }
+                /* haven't made any progress */
+                spin_unlock_bh(&ap->xmit_lock);
+                if (!(test_bit(XMIT_WAKEUP, &ap->xmit_flags)
+                      || (!tty_stuffed && ap->tpkt != 0)))
+                        break;
+                if (!spin_trylock_bh(&ap->xmit_lock))
+                        break;
+        }
+        return done;
+
+flush:
+        if (ap->tpkt != 0) {
+                kfree_skb(ap->tpkt);
+                ap->tpkt = NULL;
+                clear_bit(XMIT_FULL, &ap->xmit_flags);
+                done = 1;
+        }
+        spin_unlock_bh(&ap->xmit_lock);
+        return done;
+}
+
+/*
+ * Flush output from our internal buffers.
+ * Called for the TCFLSH ioctl.
+ */
+static void
+ppp_sync_flush_output(struct syncppp *ap)
+{
+        int done = 0;
+
+        spin_lock_bh(&ap->xmit_lock);
+        if (ap->tpkt != NULL) {
+                kfree_skb(ap->tpkt);
+                ap->tpkt = NULL;
+                clear_bit(XMIT_FULL, &ap->xmit_flags);
+                done = 1;
+        }
+        spin_unlock_bh(&ap->xmit_lock);
+        if (done)
+                ppp_output_wakeup(&ap->chan);
+}
+
+/*
+ * Receive-side routines.
+ */
+
+/* called when the tty driver has data for us.
+ *
+ * Data is frame oriented: each call to ppp_sync_input is considered
+ * a whole frame. If the 1st flag byte is non-zero then the whole
+ * frame is considered to be in error and is tossed.
+ */
+static void
+ppp_sync_input(struct syncppp *ap, const unsigned char *buf,
+                char *flags, int count)
+{
+        struct sk_buff *skb;
+        unsigned char *p;
+
+        if (count == 0)
+                return;
+
+        if (ap->flags & SC_LOG_INPKT)
+                ppp_print_buffer ("receive buffer", buf, count);
+
+        /* stuff the chars in the skb */
+        if ((skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2)) == 0) {
+                printk(KERN_ERR "PPPsync: no memory (input pkt)\n");
+                goto err;
+        }
+        /* Try to get the payload 4-byte aligned */
+        if (buf[0] != PPP_ALLSTATIONS)
+                skb_reserve(skb, 2 + (buf[0] & 1));
+
+        if (flags != 0 && *flags) {
+                /* error flag set, ignore frame */
+                goto err;
+        } else if (count > skb_tailroom(skb)) {
+                /* packet overflowed MRU */
+                goto err;
+        }
+
+        p = skb_put(skb, count);
+        memcpy(p, buf, count);
+
+        /* strip address/control field if present */
+        p = skb->data;
+        if (p[0] == PPP_ALLSTATIONS && p[1] == PPP_UI) {
+                /* chop off address/control */
+                if (skb->len < 3)
+                        goto err;
+                p = skb_pull(skb, 2);
+        }
+
+        /* decompress protocol field if compressed */
+        if (p[0] & 1) {
+                /* protocol is compressed */
+                skb_push(skb, 1)[0] = 0;
+        } else if (skb->len < 2)
+                goto err;
+
+        /* queue the frame to be processed */
+        skb_queue_tail(&ap->rqueue, skb);
+        return;
+
+err:
+        /* queue zero length packet as error indication */
+        if (skb || (skb = dev_alloc_skb(0))) {
+                skb_trim(skb, 0);
+                skb_queue_tail(&ap->rqueue, skb);
+        }
+}
+
+static void __exit
+ppp_sync_cleanup(void)
+{
+        if (tty_register_ldisc(N_SYNC_PPP, NULL) != 0)
+                printk(KERN_ERR "failed to unregister Sync PPP line discipline\n");
+}
+
+module_init(ppp_sync_init);
+module_exit(ppp_sync_cleanup);
+MODULE_LICENSE("GPL");
+MODULE_ALIAS_LDISC(N_SYNC_PPP);