ppp_async.c

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/*
 * PPP async serial channel driver for Linux.
 *
 * Copyright 1999 Paul Mackerras.
 *
 *  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 async 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.
 */

#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/crc-ccitt.h>
#include <linux/ppp_defs.h>
#include <linux/ppp-ioctl.h>
#include <linux/ppp_channel.h>
#include <linux/spinlock.h>
#include <linux/init.h>
#include <linux/interrupt.h>
#include <linux/jiffies.h>
#include <linux/slab.h>
#include <asm/unaligned.h>
#include <asm/uaccess.h>
#include <asm/string.h>

#define PPP_VERSION "2.4.2"

#define OBUFSIZE    4096

/* Structure for storing local state. */
struct asyncppp {
    struct tty_struct *tty;
    unsigned int    flags;
    unsigned int    state;
    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;
    int     tpkt_pos;
    u16     tfcs;
    unsigned char   *optr;
    unsigned char   *olim;
    unsigned long   last_xmit;

    struct sk_buff  *rpkt;
    int     lcp_fcs;
    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 */
    unsigned char   obuf[OBUFSIZE];
};

/* Bit numbers in xmit_flags */
#define XMIT_WAKEUP 0
#define XMIT_FULL   1
#define XMIT_BUSY   2

/* State bits */
#define SC_TOSS     1
#define SC_ESCAPE   2
#define SC_PREV_ERROR   4

/* Bits in rbits */
#define SC_RCV_BITS (SC_RCV_B7_1|SC_RCV_B7_0|SC_RCV_ODDP|SC_RCV_EVNP)

static int flag_time = HZ;
module_param(flag_time, int, 0);
MODULE_PARM_DESC(flag_time, "ppp_async: interval between flagged packets (in clock ticks)");
MODULE_LICENSE("GPL");
MODULE_ALIAS_LDISC(N_PPP);

/*
 * Prototypes.
 */
static int ppp_async_encode(struct asyncppp *ap);
static int ppp_async_send(struct ppp_channel *chan, struct sk_buff *skb);
static int ppp_async_push(struct asyncppp *ap);
static void ppp_async_flush_output(struct asyncppp *ap);
static void ppp_async_input(struct asyncppp *ap, const unsigned char *buf,
                char *flags, int count);
static int ppp_async_ioctl(struct ppp_channel *chan, unsigned int cmd,
               unsigned long arg);
static void ppp_async_process(unsigned long arg);

static void async_lcp_peek(struct asyncppp *ap, unsigned char *data,
               int len, int inbound);

static const struct ppp_channel_ops async_ops = {
    .start_xmit = ppp_async_send,
    .ioctl      = ppp_async_ioctl,
};

/*
 * Routines implementing the 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_asynctty_receive while another
 * calls ppp_asynctty_close, which zeroes tty->disc_data and
 * frees the memory that ppp_asynctty_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: this is no longer true. The _close path for the ldisc is
 * now guaranteed to be sane.
 */
static DEFINE_RWLOCK(disc_data_lock);

static struct asyncppp *ap_get(struct tty_struct *tty)
{
    struct asyncppp *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 ap_put(struct asyncppp *ap)
{
    if (atomic_dec_and_test(&ap->refcnt))
        up(&ap->dead_sem);
}

/*
 * Called when a tty is put into PPP line discipline. Called in process
 * context.
 */
static int
ppp_asynctty_open(struct tty_struct *tty)
{
    struct asyncppp *ap;
    int err;
    int speed;

    if (tty->ops->write == NULL)
        return -EOPNOTSUPP;

    err = -ENOMEM;
    ap = kzalloc(sizeof(*ap), GFP_KERNEL);
    if (!ap)
        goto out;

    /* initialize the asyncppp structure */
    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;
    ap->optr = ap->obuf;
    ap->olim = ap->obuf;
    ap->lcp_fcs = -1;

    skb_queue_head_init(&ap->rqueue);
    tasklet_init(&ap->tsk, ppp_async_process, (unsigned long) ap);

    atomic_set(&ap->refcnt, 1);
    sema_init(&ap->dead_sem, 0);

    ap->chan.private = ap;
    ap->chan.ops = &async_ops;
    ap->chan.mtu = PPP_MRU;
    speed = tty_get_baud_rate(tty);
    ap->chan.speed = speed;
    err = ppp_register_channel(&ap->chan);
    if (err)
        goto out_free;

    tty->disc_data = ap;
    tty->receive_room = 65536;
    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_asynctty_* routines to
 * finish before we can call ppp_unregister_channel and free
 * the asyncppp struct.  This routine must be called from
 * process context, not interrupt or softirq context.
 */
static void
ppp_asynctty_close(struct tty_struct *tty)
{
    struct asyncppp *ap;

    write_lock_irq(&disc_data_lock);
    ap = tty->disc_data;
    tty->disc_data = NULL;
    write_unlock_irq(&disc_data_lock);
    if (!ap)
        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_async_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);
    kfree_skb(ap->rpkt);
    skb_queue_purge(&ap->rqueue);
    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_asynctty_hangup(struct tty_struct *tty)
{
    ppp_asynctty_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_asynctty_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_asynctty_write(struct tty_struct *tty, struct file *file,
           const unsigned char *buf, size_t count)
{
    return -EAGAIN;
}

/*
 * Called in process context only. May be re-entered by multiple
 * ioctl calling threads.
 */

static int
ppp_asynctty_ioctl(struct tty_struct *tty, struct file *file,
           unsigned int cmd, unsigned long arg)
{
    struct asyncppp *ap = ap_get(tty);
    int err, val;
    int __user *p = (int __user *)arg;

    if (!ap)
        return -ENXIO;
    err = -EFAULT;
    switch (cmd) {
    case PPPIOCGCHAN:
        err = -EFAULT;
        if (put_user(ppp_channel_index(&ap->chan), p))
            break;
        err = 0;
        break;

    case PPPIOCGUNIT:
        err = -EFAULT;
        if (put_user(ppp_unit_number(&ap->chan), p))
            break;
        err = 0;
        break;

    case TCFLSH:
        /* flush our buffers and the serial port's buffer */
        if (arg == TCIOFLUSH || arg == TCOFLUSH)
            ppp_async_flush_output(ap);
        err = tty_perform_flush(tty, arg);
        break;

    case FIONREAD:
        val = 0;
        if (put_user(val, p))
            break;
        err = 0;
        break;

    default:
        /* Try the various mode ioctls */
        err = tty_mode_ioctl(tty, file, cmd, arg);
    }

    ap_put(ap);
    return err;
}

/* No kernel lock - fine */
static unsigned int
ppp_asynctty_poll(struct tty_struct *tty, struct file *file, poll_table *wait)
{
    return 0;
}

/* May sleep, don't call from interrupt level or with interrupts disabled */
static void
ppp_asynctty_receive(struct tty_struct *tty, const unsigned char *buf,
          char *cflags, int count)
{
    struct asyncppp *ap = ap_get(tty);
    unsigned long flags;

    if (!ap)
        return;
    spin_lock_irqsave(&ap->recv_lock, flags);
    ppp_async_input(ap, buf, cflags, count);
    spin_unlock_irqrestore(&ap->recv_lock, flags);
    if (!skb_queue_empty(&ap->rqueue))
        tasklet_schedule(&ap->tsk);
    ap_put(ap);
    tty_unthrottle(tty);
}

static void
ppp_asynctty_wakeup(struct tty_struct *tty)
{
    struct asyncppp *ap = ap_get(tty);

    clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
    if (!ap)
        return;
    set_bit(XMIT_WAKEUP, &ap->xmit_flags);
    tasklet_schedule(&ap->tsk);
    ap_put(ap);
}


static struct tty_ldisc_ops ppp_ldisc = {
    .owner  = THIS_MODULE,
    .magic  = TTY_LDISC_MAGIC,
    .name   = "ppp",
    .open   = ppp_asynctty_open,
    .close  = ppp_asynctty_close,
    .hangup = ppp_asynctty_hangup,
    .read   = ppp_asynctty_read,
    .write  = ppp_asynctty_write,
    .ioctl  = ppp_asynctty_ioctl,
    .poll   = ppp_asynctty_poll,
    .receive_buf = ppp_asynctty_receive,
    .write_wakeup = ppp_asynctty_wakeup,
};

static int __init
ppp_async_init(void)
{
    int err;

    err = tty_register_ldisc(N_PPP, &ppp_ldisc);
    if (err != 0)
        printk(KERN_ERR "PPP_async: error %d registering line disc.\n",
               err);
    return err;
}

/*
 * The following routines provide the PPP channel interface.
 */
static int
ppp_async_ioctl(struct ppp_channel *chan, unsigned int cmd, unsigned long arg)
{
    struct asyncppp *ap = chan->private;
    void __user *argp = (void __user *)arg;
    int __user *p = argp;
    int err, val;
    u32 accm[8];

    err = -EFAULT;
    switch (cmd) {
    case PPPIOCGFLAGS:
        val = ap->flags | ap->rbits;
        if (put_user(val, p))
            break;
        err = 0;
        break;
    case PPPIOCSFLAGS:
        if (get_user(val, p))
            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], (u32 __user *)argp))
            break;
        err = 0;
        break;
    case PPPIOCSASYNCMAP:
        if (get_user(ap->xaccm[0], (u32 __user *)argp))
            break;
        err = 0;
        break;

    case PPPIOCGRASYNCMAP:
        if (put_user(ap->raccm, (u32 __user *)argp))
            break;
        err = 0;
        break;
    case PPPIOCSRASYNCMAP:
        if (get_user(ap->raccm, (u32 __user *)argp))
            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, p))
            break;
        err = 0;
        break;
    case PPPIOCSMRU:
        if (get_user(val, p))
            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_async_process(unsigned long arg)
{
    struct asyncppp *ap = (struct asyncppp *) arg;
    struct sk_buff *skb;

    /* process received packets */
    while ((skb = skb_dequeue(&ap->rqueue)) != NULL) {
        if (skb->cb[0])
            ppp_input_error(&ap->chan, 0);
        ppp_input(&ap->chan, skb);
    }

    /* try to push more stuff out */
    if (test_bit(XMIT_WAKEUP, &ap->xmit_flags) && ppp_async_push(ap))
        ppp_output_wakeup(&ap->chan);
}

/*
 * Procedures for encapsulation and framing.
 */

/*
 * Procedure to encode the data for async serial transmission.
 * Does octet stuffing (escaping), puts the address/control bytes
 * on if A/C compression is disabled, and does protocol compression.
 * Assumes ap->tpkt != 0 on entry.
 * Returns 1 if we finished the current frame, 0 otherwise.
 */

#define PUT_BYTE(ap, buf, c, islcp) do {        \
    if ((islcp && c < 0x20) || (ap->xaccm[c >> 5] & (1 << (c & 0x1f)))) {\
        *buf++ = PPP_ESCAPE;            \
        *buf++ = c ^ PPP_TRANS;         \
    } else                      \
        *buf++ = c;             \
} while (0)

static int
ppp_async_encode(struct asyncppp *ap)
{
    int fcs, i, count, c, proto;
    unsigned char *buf, *buflim;
    unsigned char *data;
    int islcp;

    buf = ap->obuf;
    ap->olim = buf;
    ap->optr = buf;
    i = ap->tpkt_pos;
    data = ap->tpkt->data;
    count = ap->tpkt->len;
    fcs = ap->tfcs;
    proto = get_unaligned_be16(data);

    /*
     * LCP packets with code values between 1 (configure-reqest)
     * and 7 (code-reject) must be sent as though no options
     * had been negotiated.
     */
    islcp = proto == PPP_LCP && 1 <= data[2] && data[2] <= 7;

    if (i == 0) {
        if (islcp)
            async_lcp_peek(ap, data, count, 0);

        /*
         * Start of a new packet - insert the leading FLAG
         * character if necessary.
         */
        if (islcp || flag_time == 0 ||
            time_after_eq(jiffies, ap->last_xmit + flag_time))
            *buf++ = PPP_FLAG;
        ap->last_xmit = jiffies;
        fcs = PPP_INITFCS;

        /*
         * Put in the address/control bytes if necessary
         */
        if ((ap->flags & SC_COMP_AC) == 0 || islcp) {
            PUT_BYTE(ap, buf, 0xff, islcp);
            fcs = PPP_FCS(fcs, 0xff);
            PUT_BYTE(ap, buf, 0x03, islcp);
            fcs = PPP_FCS(fcs, 0x03);
        }
    }

    /*
     * Once we put in the last byte, we need to put in the FCS
     * and closing flag, so make sure there is at least 7 bytes
     * of free space in the output buffer.
     */
    buflim = ap->obuf + OBUFSIZE - 6;
    while (i < count && buf < buflim) {
        c = data[i++];
        if (i == 1 && c == 0 && (ap->flags & SC_COMP_PROT))
            continue;   /* compress protocol field */
        fcs = PPP_FCS(fcs, c);
        PUT_BYTE(ap, buf, c, islcp);
    }

    if (i < count) {
        /*
         * Remember where we are up to in this packet.
         */
        ap->olim = buf;
        ap->tpkt_pos = i;
        ap->tfcs = fcs;
        return 0;
    }

    /*
     * We have finished the packet.  Add the FCS and flag.
     */
    fcs = ~fcs;
    c = fcs & 0xff;
    PUT_BYTE(ap, buf, c, islcp);
    c = (fcs >> 8) & 0xff;
    PUT_BYTE(ap, buf, c, islcp);
    *buf++ = PPP_FLAG;
    ap->olim = buf;

    consume_skb(ap->tpkt);
    ap->tpkt = NULL;
    return 1;
}

/*
 * Transmit-side routines.
 */

/*
 * Send a packet to the peer over an async 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_async_send(struct ppp_channel *chan, struct sk_buff *skb)
{
    struct asyncppp *ap = chan->private;

    ppp_async_push(ap);

    if (test_and_set_bit(XMIT_FULL, &ap->xmit_flags))
        return 0;   /* already full */
    ap->tpkt = skb;
    ap->tpkt_pos = 0;

    ppp_async_push(ap);
    return 1;
}

/*
 * Push as much data as possible out to the tty.
 */
static int
ppp_async_push(struct asyncppp *ap)
{
    int avail, sent, done = 0;
    struct tty_struct *tty = ap->tty;
    int tty_stuffed = 0;

    /*
     * We can get called recursively here if the tty write
     * function calls our wakeup function.  This can happen
     * for example on a pty with both the master and slave
     * set to PPP line discipline.
     * We use the XMIT_BUSY bit to detect this and get out,
     * leaving the XMIT_WAKEUP bit set to tell the other
     * instance that it may now be able to write more now.
     */
    if (test_and_set_bit(XMIT_BUSY, &ap->xmit_flags))
        return 0;
    spin_lock_bh(&ap->xmit_lock);
    for (;;) {
        if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
            tty_stuffed = 0;
        if (!tty_stuffed && ap->optr < ap->olim) {
            avail = ap->olim - ap->optr;
            set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
            sent = tty->ops->write(tty, ap->optr, avail);
            if (sent < 0)
                goto flush; /* error, e.g. loss of CD */
            ap->optr += sent;
            if (sent < avail)
                tty_stuffed = 1;
            continue;
        }
        if (ap->optr >= ap->olim && ap->tpkt) {
            if (ppp_async_encode(ap)) {
                /* finished processing ap->tpkt */
                clear_bit(XMIT_FULL, &ap->xmit_flags);
                done = 1;
            }
            continue;
        }
        /*
         * We haven't made any progress this time around.
         * Clear XMIT_BUSY to let other callers in, but
         * after doing so we have to check if anyone set
         * XMIT_WAKEUP since we last checked it.  If they
         * did, we should try again to set XMIT_BUSY and go
         * around again in case XMIT_BUSY was still set when
         * the other caller tried.
         */
        clear_bit(XMIT_BUSY, &ap->xmit_flags);
        /* any more work to do? if not, exit the loop */
        if (!(test_bit(XMIT_WAKEUP, &ap->xmit_flags) ||
              (!tty_stuffed && ap->tpkt)))
            break;
        /* more work to do, see if we can do it now */
        if (test_and_set_bit(XMIT_BUSY, &ap->xmit_flags))
            break;
    }
    spin_unlock_bh(&ap->xmit_lock);
    return done;

flush:
    clear_bit(XMIT_BUSY, &ap->xmit_flags);
    if (ap->tpkt) {
        kfree_skb(ap->tpkt);
        ap->tpkt = NULL;
        clear_bit(XMIT_FULL, &ap->xmit_flags);
        done = 1;
    }
    ap->optr = ap->olim;
    spin_unlock_bh(&ap->xmit_lock);
    return done;
}

/*
 * Flush output from our internal buffers.
 * Called for the TCFLSH ioctl. Can be entered in parallel
 * but this is covered by the xmit_lock.
 */
static void
ppp_async_flush_output(struct asyncppp *ap)
{
    int done = 0;

    spin_lock_bh(&ap->xmit_lock);
    ap->optr = ap->olim;
    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.
 */

/* see how many ordinary chars there are at the start of buf */
static inline int
scan_ordinary(struct asyncppp *ap, const unsigned char *buf, int count)
{
    int i, c;

    for (i = 0; i < count; ++i) {
        c = buf[i];
        if (c == PPP_ESCAPE || c == PPP_FLAG ||
            (c < 0x20 && (ap->raccm & (1 << c)) != 0))
            break;
    }
    return i;
}

/* called when a flag is seen - do end-of-packet processing */
static void
process_input_packet(struct asyncppp *ap)
{
    struct sk_buff *skb;
    unsigned char *p;
    unsigned int len, fcs, proto;

    skb = ap->rpkt;
    if (ap->state & (SC_TOSS | SC_ESCAPE))
        goto err;

    if (skb == NULL)
        return;     /* 0-length packet */

    /* check the FCS */
    p = skb->data;
    len = skb->len;
    if (len < 3)
        goto err;   /* too short */
    fcs = PPP_INITFCS;
    for (; len > 0; --len)
        fcs = PPP_FCS(fcs, *p++);
    if (fcs != PPP_GOODFCS)
        goto err;   /* bad FCS */
    skb_trim(skb, skb->len - 2);

    /* check for address/control and protocol compression */
    p = skb->data;
    if (p[0] == PPP_ALLSTATIONS) {
        /* chop off address/control */
        if (p[1] != PPP_UI || skb->len < 3)
            goto err;
        p = skb_pull(skb, 2);
    }
    proto = p[0];
    if (proto & 1) {
        /* protocol is compressed */
        skb_push(skb, 1)[0] = 0;
    } else {
        if (skb->len < 2)
            goto err;
        proto = (proto << 8) + p[1];
        if (proto == PPP_LCP)
            async_lcp_peek(ap, p, skb->len, 1);
    }

    /* queue the frame to be processed */
    skb->cb[0] = ap->state;
    skb_queue_tail(&ap->rqueue, skb);
    ap->rpkt = NULL;
    ap->state = 0;
    return;

 err:
    /* frame had an error, remember that, reset SC_TOSS & SC_ESCAPE */
    ap->state = SC_PREV_ERROR;
    if (skb) {
        /* make skb appear as freshly allocated */
        skb_trim(skb, 0);
        skb_reserve(skb, - skb_headroom(skb));
    }
}

/* Called when the tty driver has data for us. Runs parallel with the
   other ldisc functions but will not be re-entered */

static void
ppp_async_input(struct asyncppp *ap, const unsigned char *buf,
        char *flags, int count)
{
    struct sk_buff *skb;
    int c, i, j, n, s, f;
    unsigned char *sp;

    /* update bits used for 8-bit cleanness detection */
    if (~ap->rbits & SC_RCV_BITS) {
        s = 0;
        for (i = 0; i < count; ++i) {
            c = buf[i];
            if (flags && flags[i] != 0)
                continue;
            s |= (c & 0x80)? SC_RCV_B7_1: SC_RCV_B7_0;
            c = ((c >> 4) ^ c) & 0xf;
            s |= (0x6996 & (1 << c))? SC_RCV_ODDP: SC_RCV_EVNP;
        }
        ap->rbits |= s;
    }

    while (count > 0) {
        /* scan through and see how many chars we can do in bulk */
        if ((ap->state & SC_ESCAPE) && buf[0] == PPP_ESCAPE)
            n = 1;
        else
            n = scan_ordinary(ap, buf, count);

        f = 0;
        if (flags && (ap->state & SC_TOSS) == 0) {
            /* check the flags to see if any char had an error */
            for (j = 0; j < n; ++j)
                if ((f = flags[j]) != 0)
                    break;
        }
        if (f != 0) {
            /* start tossing */
            ap->state |= SC_TOSS;

        } else if (n > 0 && (ap->state & SC_TOSS) == 0) {
            /* stuff the chars in the skb */
            skb = ap->rpkt;
            if (!skb) {
                skb = dev_alloc_skb(ap->mru + PPP_HDRLEN + 2);
                if (!skb)
                    goto nomem;
                ap->rpkt = skb;
            }
            if (skb->len == 0) {
                /* Try to get the payload 4-byte aligned.
                 * This should match the
                 * PPP_ALLSTATIONS/PPP_UI/compressed tests in
                 * process_input_packet, but we do not have
                 * enough chars here to test buf[1] and buf[2].
                 */
                if (buf[0] != PPP_ALLSTATIONS)
                    skb_reserve(skb, 2 + (buf[0] & 1));
            }
            if (n > skb_tailroom(skb)) {
                /* packet overflowed MRU */
                ap->state |= SC_TOSS;
            } else {
                sp = skb_put(skb, n);
                memcpy(sp, buf, n);
                if (ap->state & SC_ESCAPE) {
                    sp[0] ^= PPP_TRANS;
                    ap->state &= ~SC_ESCAPE;
                }
            }
        }

        if (n >= count)
            break;

        c = buf[n];
        if (flags != NULL && flags[n] != 0) {
            ap->state |= SC_TOSS;
        } else if (c == PPP_FLAG) {
            process_input_packet(ap);
        } else if (c == PPP_ESCAPE) {
            ap->state |= SC_ESCAPE;
        } else if (I_IXON(ap->tty)) {
            if (c == START_CHAR(ap->tty))
                start_tty(ap->tty);
            else if (c == STOP_CHAR(ap->tty))
                stop_tty(ap->tty);
        }
        /* otherwise it's a char in the recv ACCM */
        ++n;

        buf += n;
        if (flags)
            flags += n;
        count -= n;
    }
    return;

 nomem:
    printk(KERN_ERR "PPPasync: no memory (input pkt)\n");
    ap->state |= SC_TOSS;
}

/*
 * We look at LCP frames going past so that we can notice
 * and react to the LCP configure-ack from the peer.
 * In the situation where the peer has been sent a configure-ack
 * already, LCP is up once it has sent its configure-ack
 * so the immediately following packet can be sent with the
 * configured LCP options.  This allows us to process the following
 * packet correctly without pppd needing to respond quickly.
 *
 * We only respond to the received configure-ack if we have just
 * sent a configure-request, and the configure-ack contains the
 * same data (this is checked using a 16-bit crc of the data).
 */
#define CONFREQ     1   /* LCP code field values */
#define CONFACK     2
#define LCP_MRU     1   /* LCP option numbers */
#define LCP_ASYNCMAP    2

static void async_lcp_peek(struct asyncppp *ap, unsigned char *data,
               int len, int inbound)
{
    int dlen, fcs, i, code;
    u32 val;

    data += 2;      /* skip protocol bytes */
    len -= 2;
    if (len < 4)        /* 4 = code, ID, length */
        return;
    code = data[0];
    if (code != CONFACK && code != CONFREQ)
        return;
    dlen = get_unaligned_be16(data + 2);
    if (len < dlen)
        return;     /* packet got truncated or length is bogus */

    if (code == (inbound? CONFACK: CONFREQ)) {
        /*
         * sent confreq or received confack:
         * calculate the crc of the data from the ID field on.
         */
        fcs = PPP_INITFCS;
        for (i = 1; i < dlen; ++i)
            fcs = PPP_FCS(fcs, data[i]);

        if (!inbound) {
            /* outbound confreq - remember the crc for later */
            ap->lcp_fcs = fcs;
            return;
        }

        /* received confack, check the crc */
        fcs ^= ap->lcp_fcs;
        ap->lcp_fcs = -1;
        if (fcs != 0)
            return;
    } else if (inbound)
        return; /* not interested in received confreq */

    /* process the options in the confack */
    data += 4;
    dlen -= 4;
    /* data[0] is code, data[1] is length */
    while (dlen >= 2 && dlen >= data[1] && data[1] >= 2) {
        switch (data[0]) {
        case LCP_MRU:
            val = get_unaligned_be16(data + 2);
            if (inbound)
                ap->mru = val;
            else
                ap->chan.mtu = val;
            break;
        case LCP_ASYNCMAP:
            val = get_unaligned_be32(data + 2);
            if (inbound)
                ap->raccm = val;
            else
                ap->xaccm[0] = val;
            break;
        }
        dlen -= data[1];
        data += data[1];
    }
}

static void __exit ppp_async_cleanup(void)
{
    if (tty_unregister_ldisc(N_PPP) != 0)
        printk(KERN_ERR "failed to unregister PPP line discipline\n");
}

module_init(ppp_async_init);
module_exit(ppp_async_cleanup);