Linux 6.14-rc1

[linux.git] / drivers / net / ppp / ppp_async.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * PPP async serial channel driver for Linux.
 *
 * Copyright 1999 Paul Mackerras.
 *
 * 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 <linux/unaligned.h>
#include <linux/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;

        refcount_t      refcnt;
        struct completion dead;
        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_DESCRIPTION("PPP async serial channel module");
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,
                            const u8 *flags, int count);
static int ppp_async_ioctl(struct ppp_channel *chan, unsigned int cmd,
                           unsigned long arg);
static void ppp_async_process(struct tasklet_struct *t);

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)
                refcount_inc(&ap->refcnt);
        read_unlock(&disc_data_lock);
        return ap;
}

static void ap_put(struct asyncppp *ap)
{
        if (refcount_dec_and_test(&ap->refcnt))
                complete(&ap->dead);
}

/*
 * 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_setup(&ap->tsk, ppp_async_process);

        refcount_set(&ap->refcnt, 1);
        init_completion(&ap->dead);

        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 (!refcount_dec_and_test(&ap->refcnt))
                wait_for_completion(&ap->dead);
        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 void ppp_asynctty_hangup(struct tty_struct *tty)
{
        ppp_asynctty_close(tty);
}

/*
 * 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, u8 *buf,
                  size_t count, void **cookie, unsigned long offset)
{
        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 u8 *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, 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 = n_tty_ioctl_helper(tty, cmd, 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, cmd, arg);
        }

        ap_put(ap);
        return err;
}

/* May sleep, don't call from interrupt level or with interrupts disabled */
static void
ppp_asynctty_receive(struct tty_struct *tty, const u8 *buf, const u8 *cflags,
                     size_t 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,
        .num    = N_PPP,
        .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,
        .receive_buf = ppp_asynctty_receive,
        .write_wakeup = ppp_asynctty_wakeup,
};

static int __init
ppp_async_init(void)
{
        int err;

        err = tty_register_ldisc(&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 > U16_MAX) {
                        err = -EINVAL;
                        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(struct tasklet_struct *t)
{
        struct asyncppp *ap = from_tasklet(ap, t, tsk);
        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-request)
         * and 7 (code-reject) must be sent as though no options
         * had been negotiated.
         */
        islcp = proto == PPP_LCP && count >= 3 && 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;

        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);
        }

        /* If protocol field is not compressed, it can be LCP packet */
        if (!(p[0] & 0x01)) {
                unsigned int proto;

                if (skb->len < 2)
                        goto err;
                proto = (p[0] << 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 u8 *buf, const u8 *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_data(skb, 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)
{
        tty_unregister_ldisc(&ppp_ldisc);
}

module_init(ppp_async_init);
module_exit(ppp_async_cleanup);