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n_tty.c

/*
 * n_tty.c --- implements the N_TTY line discipline.
 * 
 * This code used to be in tty_io.c, but things are getting hairy
 * enough that it made sense to split things off.  (The N_TTY
 * processing has changed so much that it's hardly recognizable,
 * anyway...)
 *
 * Note that the open routine for N_TTY is guaranteed never to return
 * an error.  This is because Linux will fall back to setting a line
 * to N_TTY if it can not switch to any other line discipline.  
 *
 * Written by Theodore Ts'o, Copyright 1994.
 * 
 * This file also contains code originally written by Linus Torvalds,
 * Copyright 1991, 1992, 1993, and by Julian Cowley, Copyright 1994.
 * 
 * This file may be redistributed under the terms of the GNU General Public
 * License.
 *
 * Reduced memory usage for older ARM systems  - Russell King.
 *
 * 2000/01/20   Fixed SMP locking on put_tty_queue using bits of 
 *          the patch by Andrew J. Kroll <ag784@freenet.buffalo.edu>
 *          who actually finally proved there really was a race.
 *
 * 2002/03/18   Implemented n_tty_wakeup to send SIGIO POLL_OUTs to
 *          waiting writing processes-Sapan Bhatia <sapan@corewars.org>.
 *          Also fixed a bug in BLOCKING mode where write_chan returns
 *          EAGAIN
 */

#include <linux/types.h>
#include <linux/major.h>
#include <linux/errno.h>
#include <linux/signal.h>
#include <linux/fcntl.h>
#include <linux/sched.h>
#include <linux/interrupt.h>
#include <linux/tty.h>
#include <linux/timer.h>
#include <linux/ctype.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/bitops.h>
#include <linux/audit.h>
#include <linux/file.h>

#include <asm/uaccess.h>
#include <asm/system.h>

/* number of characters left in xmit buffer before select has we have room */
#define WAKEUP_CHARS 256

/*
 * This defines the low- and high-watermarks for throttling and
 * unthrottling the TTY driver.  These watermarks are used for
 * controlling the space in the read buffer.
 */
#define TTY_THRESHOLD_THROTTLE            128 /* now based on remaining room */
#define TTY_THRESHOLD_UNTHROTTLE    128

static inline unsigned char *alloc_buf(void)
{
      gfp_t prio = in_interrupt() ? GFP_ATOMIC : GFP_KERNEL;

      if (PAGE_SIZE != N_TTY_BUF_SIZE)
            return kmalloc(N_TTY_BUF_SIZE, prio);
      else
            return (unsigned char *)__get_free_page(prio);
}

static inline void free_buf(unsigned char *buf)
{
      if (PAGE_SIZE != N_TTY_BUF_SIZE)
            kfree(buf);
      else
            free_page((unsigned long) buf);
}

static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
                         unsigned char __user *ptr)
{
      tty_audit_add_data(tty, &x, 1);
      return put_user(x, ptr);
}

/**
 *    n_tty_set__room   -     receive space
 *    @tty: terminal
 *
 *    Called by the driver to find out how much data it is
 *    permitted to feed to the line discipline without any being lost
 *    and thus to manage flow control. Not serialized. Answers for the
 *    "instant".
 */

static void n_tty_set_room(struct tty_struct *tty)
{
      int   left = N_TTY_BUF_SIZE - tty->read_cnt - 1;

      /*
       * If we are doing input canonicalization, and there are no
       * pending newlines, let characters through without limit, so
       * that erase characters will be handled.  Other excess
       * characters will be beeped.
       */
      if (left <= 0)
            left = tty->icanon && !tty->canon_data;
      tty->receive_room = left;
}

static void put_tty_queue_nolock(unsigned char c, struct tty_struct *tty)
{
      if (tty->read_cnt < N_TTY_BUF_SIZE) {
            tty->read_buf[tty->read_head] = c;
            tty->read_head = (tty->read_head + 1) & (N_TTY_BUF_SIZE-1);
            tty->read_cnt++;
      }
}

static void put_tty_queue(unsigned char c, struct tty_struct *tty)
{
      unsigned long flags;
      /*
       *    The problem of stomping on the buffers ends here.
       *    Why didn't anyone see this one coming? --AJK
      */
      spin_lock_irqsave(&tty->read_lock, flags);
      put_tty_queue_nolock(c, tty);
      spin_unlock_irqrestore(&tty->read_lock, flags);
}

/**
 *    check_unthrottle  -     allow new receive data
 *    @tty; tty device
 *
 *    Check whether to call the driver.unthrottle function.
 *    We test the TTY_THROTTLED bit first so that it always
 *    indicates the current state. The decision about whether
 *    it is worth allowing more input has been taken by the caller.
 *    Can sleep, may be called under the atomic_read_lock mutex but
 *    this is not guaranteed.
 */
 
static void check_unthrottle(struct tty_struct * tty)
{
      if (tty->count &&
          test_and_clear_bit(TTY_THROTTLED, &tty->flags) && 
          tty->driver->unthrottle)
            tty->driver->unthrottle(tty);
}

/**
 *    reset_buffer_flags      -     reset buffer state
 *    @tty: terminal to reset
 *
 *    Reset the read buffer counters, clear the flags, 
 *    and make sure the driver is unthrottled. Called
 *    from n_tty_open() and n_tty_flush_buffer().
 */
static void reset_buffer_flags(struct tty_struct *tty)
{
      unsigned long flags;

      spin_lock_irqsave(&tty->read_lock, flags);
      tty->read_head = tty->read_tail = tty->read_cnt = 0;
      spin_unlock_irqrestore(&tty->read_lock, flags);
      tty->canon_head = tty->canon_data = tty->erasing = 0;
      memset(&tty->read_flags, 0, sizeof tty->read_flags);
      n_tty_set_room(tty);
      check_unthrottle(tty);
}

/**
 *    n_tty_flush_buffer      -     clean input queue
 *    @tty: terminal device
 *
 *    Flush the input buffer. Called when the line discipline is
 *    being closed, when the tty layer wants the buffer flushed (eg
 *    at hangup) or when the N_TTY line discipline internally has to
 *    clean the pending queue (for example some signals).
 *
 *    FIXME: tty->ctrl_status is not spinlocked and relies on
 *    lock_kernel() still.
 */
 
static void n_tty_flush_buffer(struct tty_struct * tty)
{
      /* clear everything and unthrottle the driver */
      reset_buffer_flags(tty);
      
      if (!tty->link)
            return;

      if (tty->link->packet) {
            tty->ctrl_status |= TIOCPKT_FLUSHREAD;
            wake_up_interruptible(&tty->link->read_wait);
      }
}

/**
 *    n_tty_chars_in_buffer   -     report available bytes
 *    @tty: tty device
 *
 *    Report the number of characters buffered to be delivered to user
 *    at this instant in time. 
 */
 
static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
{
      unsigned long flags;
      ssize_t n = 0;

      spin_lock_irqsave(&tty->read_lock, flags);
      if (!tty->icanon) {
            n = tty->read_cnt;
      } else if (tty->canon_data) {
            n = (tty->canon_head > tty->read_tail) ?
                  tty->canon_head - tty->read_tail :
                  tty->canon_head + (N_TTY_BUF_SIZE - tty->read_tail);
      }
      spin_unlock_irqrestore(&tty->read_lock, flags);
      return n;
}

/**
 *    is_utf8_continuation    -     utf8 multibyte check
 *    @c: byte to check
 *
 *    Returns true if the utf8 character 'c' is a multibyte continuation
 *    character. We use this to correctly compute the on screen size
 *    of the character when printing
 */
 
static inline int is_utf8_continuation(unsigned char c)
{
      return (c & 0xc0) == 0x80;
}

/**
 *    is_continuation         -     multibyte check
 *    @c: byte to check
 *
 *    Returns true if the utf8 character 'c' is a multibyte continuation
 *    character and the terminal is in unicode mode.
 */
 
static inline int is_continuation(unsigned char c, struct tty_struct *tty)
{
      return I_IUTF8(tty) && is_utf8_continuation(c);
}

/**
 *    opost             -     output post processor
 *    @c: character (or partial unicode symbol)
 *    @tty: terminal device
 *
 *    Perform OPOST processing.  Returns -1 when the output device is
 *    full and the character must be retried. Note that Linux currently
 *    ignores TABDLY, CRDLY, VTDLY, FFDLY and NLDLY. They simply aren't
 *    relevant in the world today. If you ever need them, add them here.
 *
 *    Called from both the receive and transmit sides and can be called
 *    re-entrantly. Relies on lock_kernel() still.
 */
 
static int opost(unsigned char c, struct tty_struct *tty)
{
      int   space, spaces;

      space = tty->driver->write_room(tty);
      if (!space)
            return -1;

      if (O_OPOST(tty)) {
            switch (c) {
            case '\n':
                  if (O_ONLRET(tty))
                        tty->column = 0;
                  if (O_ONLCR(tty)) {
                        if (space < 2)
                              return -1;
                        tty->driver->put_char(tty, '\r');
                        tty->column = 0;
                  }
                  tty->canon_column = tty->column;
                  break;
            case '\r':
                  if (O_ONOCR(tty) && tty->column == 0)
                        return 0;
                  if (O_OCRNL(tty)) {
                        c = '\n';
                        if (O_ONLRET(tty))
                              tty->canon_column = tty->column = 0;
                        break;
                  }
                  tty->canon_column = tty->column = 0;
                  break;
            case '\t':
                  spaces = 8 - (tty->column & 7);
                  if (O_TABDLY(tty) == XTABS) {
                        if (space < spaces)
                              return -1;
                        tty->column += spaces;
                        tty->driver->write(tty, "        ", spaces);
                        return 0;
                  }
                  tty->column += spaces;
                  break;
            case '\b':
                  if (tty->column > 0)
                        tty->column--;
                  break;
            default:
                  if (O_OLCUC(tty))
                        c = toupper(c);
                  if (!iscntrl(c) && !is_continuation(c, tty))
                        tty->column++;
                  break;
            }
      }
      tty->driver->put_char(tty, c);
      return 0;
}

/**
 *    opost_block       -     block postprocess
 *    @tty: terminal device
 *    @inbuf: user buffer
 *    @nr: number of bytes
 *
 *    This path is used to speed up block console writes, among other
 *    things when processing blocks of output data. It handles only
 *    the simple cases normally found and helps to generate blocks of
 *    symbols for the console driver and thus improve performance.
 *
 *    Called from write_chan under the tty layer write lock.
 */
 
static ssize_t opost_block(struct tty_struct * tty,
                   const unsigned char * buf, unsigned int nr)
{
      int   space;
      int   i;
      const unsigned char *cp;

      space = tty->driver->write_room(tty);
      if (!space)
            return 0;
      if (nr > space)
            nr = space;

      for (i = 0, cp = buf; i < nr; i++, cp++) {
            switch (*cp) {
            case '\n':
                  if (O_ONLRET(tty))
                        tty->column = 0;
                  if (O_ONLCR(tty))
                        goto break_out;
                  tty->canon_column = tty->column;
                  break;
            case '\r':
                  if (O_ONOCR(tty) && tty->column == 0)
                        goto break_out;
                  if (O_OCRNL(tty))
                        goto break_out;
                  tty->canon_column = tty->column = 0;
                  break;
            case '\t':
                  goto break_out;
            case '\b':
                  if (tty->column > 0)
                        tty->column--;
                  break;
            default:
                  if (O_OLCUC(tty))
                        goto break_out;
                  if (!iscntrl(*cp))
                        tty->column++;
                  break;
            }
      }
break_out:
      if (tty->driver->flush_chars)
            tty->driver->flush_chars(tty);
      i = tty->driver->write(tty, buf, i);      
      return i;
}


/**
 *    put_char    -     write character to driver
 *    @c: character (or part of unicode symbol)
 *    @tty: terminal device
 *
 *    Queue a byte to the driver layer for output
 */
 
static inline void put_char(unsigned char c, struct tty_struct *tty)
{
      tty->driver->put_char(tty, c);
}

/**
 *    echo_char   -     echo characters
 *    @c: unicode byte to echo
 *    @tty: terminal device
 *
 *    Echo user input back onto the screen. This must be called only when 
 *    L_ECHO(tty) is true. Called from the driver receive_buf path.
 */

static void echo_char(unsigned char c, struct tty_struct *tty)
{
      if (L_ECHOCTL(tty) && iscntrl(c) && c != '\t') {
            put_char('^', tty);
            put_char(c ^ 0100, tty);
            tty->column += 2;
      } else
            opost(c, tty);
}

static inline void finish_erasing(struct tty_struct *tty)
{
      if (tty->erasing) {
            put_char('/', tty);
            tty->column++;
            tty->erasing = 0;
      }
}

/**
 *    eraser            -     handle erase function
 *    @c: character input
 *    @tty: terminal device
 *
 *    Perform erase and necessary output when an erase character is
 *    present in the stream from the driver layer. Handles the complexities
 *    of UTF-8 multibyte symbols.
 */
 
static void eraser(unsigned char c, struct tty_struct *tty)
{
      enum { ERASE, WERASE, KILL } kill_type;
      int head, seen_alnums, cnt;
      unsigned long flags;

      if (tty->read_head == tty->canon_head) {
            /* opost('\a', tty); */       /* what do you think? */
            return;
      }
      if (c == ERASE_CHAR(tty))
            kill_type = ERASE;
      else if (c == WERASE_CHAR(tty))
            kill_type = WERASE;
      else {
            if (!L_ECHO(tty)) {
                  spin_lock_irqsave(&tty->read_lock, flags);
                  tty->read_cnt -= ((tty->read_head - tty->canon_head) &
                                (N_TTY_BUF_SIZE - 1));
                  tty->read_head = tty->canon_head;
                  spin_unlock_irqrestore(&tty->read_lock, flags);
                  return;
            }
            if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {
                  spin_lock_irqsave(&tty->read_lock, flags);
                  tty->read_cnt -= ((tty->read_head - tty->canon_head) &
                                (N_TTY_BUF_SIZE - 1));
                  tty->read_head = tty->canon_head;
                  spin_unlock_irqrestore(&tty->read_lock, flags);
                  finish_erasing(tty);
                  echo_char(KILL_CHAR(tty), tty);
                  /* Add a newline if ECHOK is on and ECHOKE is off. */
                  if (L_ECHOK(tty))
                        opost('\n', tty);
                  return;
            }
            kill_type = KILL;
      }

      seen_alnums = 0;
      while (tty->read_head != tty->canon_head) {
            head = tty->read_head;

            /* erase a single possibly multibyte character */
            do {
                  head = (head - 1) & (N_TTY_BUF_SIZE-1);
                  c = tty->read_buf[head];
            } while (is_continuation(c, tty) && head != tty->canon_head);

            /* do not partially erase */
            if (is_continuation(c, tty))
                  break;

            if (kill_type == WERASE) {
                  /* Equivalent to BSD's ALTWERASE. */
                  if (isalnum(c) || c == '_')
                        seen_alnums++;
                  else if (seen_alnums)
                        break;
            }
            cnt = (tty->read_head - head) & (N_TTY_BUF_SIZE-1);
            spin_lock_irqsave(&tty->read_lock, flags);
            tty->read_head = head;
            tty->read_cnt -= cnt;
            spin_unlock_irqrestore(&tty->read_lock, flags);
            if (L_ECHO(tty)) {
                  if (L_ECHOPRT(tty)) {
                        if (!tty->erasing) {
                              put_char('\\', tty);
                              tty->column++;
                              tty->erasing = 1;
                        }
                        /* if cnt > 1, output a multi-byte character */
                        echo_char(c, tty);
                        while (--cnt > 0) {
                              head = (head+1) & (N_TTY_BUF_SIZE-1);
                              put_char(tty->read_buf[head], tty);
                        }
                  } else if (kill_type == ERASE && !L_ECHOE(tty)) {
                        echo_char(ERASE_CHAR(tty), tty);
                  } else if (c == '\t') {
                        unsigned int col = tty->canon_column;
                        unsigned long tail = tty->canon_head;

                        /* Find the column of the last char. */
                        while (tail != tty->read_head) {
                              c = tty->read_buf[tail];
                              if (c == '\t')
                                    col = (col | 7) + 1;
                              else if (iscntrl(c)) {
                                    if (L_ECHOCTL(tty))
                                          col += 2;
                              } else if (!is_continuation(c, tty))
                                    col++;
                              tail = (tail+1) & (N_TTY_BUF_SIZE-1);
                        }

                        /* should never happen */
                        if (tty->column > 0x80000000)
                              tty->column = 0; 

                        /* Now backup to that column. */
                        while (tty->column > col) {
                              /* Can't use opost here. */
                              put_char('\b', tty);
                              if (tty->column > 0)
                                    tty->column--;
                        }
                  } else {
                        if (iscntrl(c) && L_ECHOCTL(tty)) {
                              put_char('\b', tty);
                              put_char(' ', tty);
                              put_char('\b', tty);
                              if (tty->column > 0)
                                    tty->column--;
                        }
                        if (!iscntrl(c) || L_ECHOCTL(tty)) {
                              put_char('\b', tty);
                              put_char(' ', tty);
                              put_char('\b', tty);
                              if (tty->column > 0)
                                    tty->column--;
                        }
                  }
            }
            if (kill_type == ERASE)
                  break;
      }
      if (tty->read_head == tty->canon_head)
            finish_erasing(tty);
}

/**
 *    isig        -     handle the ISIG optio
 *    @sig: signal
 *    @tty: terminal
 *    @flush: force flush
 *
 *    Called when a signal is being sent due to terminal input. This
 *    may caus terminal flushing to take place according to the termios
 *    settings and character used. Called from the driver receive_buf
 *    path so serialized.
 */
 
static inline void isig(int sig, struct tty_struct *tty, int flush)
{
      if (tty->pgrp)
            kill_pgrp(tty->pgrp, sig, 1);
      if (flush || !L_NOFLSH(tty)) {
            n_tty_flush_buffer(tty);
            if (tty->driver->flush_buffer)
                  tty->driver->flush_buffer(tty);
      }
}

/**
 *    n_tty_receive_break     -     handle break
 *    @tty: terminal
 *
 *    An RS232 break event has been hit in the incoming bitstream. This
 *    can cause a variety of events depending upon the termios settings.
 *
 *    Called from the receive_buf path so single threaded.
 */
 
static inline void n_tty_receive_break(struct tty_struct *tty)
{
      if (I_IGNBRK(tty))
            return;
      if (I_BRKINT(tty)) {
            isig(SIGINT, tty, 1);
            return;
      }
      if (I_PARMRK(tty)) {
            put_tty_queue('\377', tty);
            put_tty_queue('\0', tty);
      }
      put_tty_queue('\0', tty);
      wake_up_interruptible(&tty->read_wait);
}

/**
 *    n_tty_receive_overrun   -     handle overrun reporting
 *    @tty: terminal
 *
 *    Data arrived faster than we could process it. While the tty
 *    driver has flagged this the bits that were missed are gone
 *    forever.
 *
 *    Called from the receive_buf path so single threaded. Does not
 *    need locking as num_overrun and overrun_time are function
 *    private.
 */
 
static inline void n_tty_receive_overrun(struct tty_struct *tty)
{
      char buf[64];

      tty->num_overrun++;
      if (time_before(tty->overrun_time, jiffies - HZ) ||
                  time_after(tty->overrun_time, jiffies)) {
            printk(KERN_WARNING "%s: %d input overrun(s)\n",
                  tty_name(tty, buf),
                  tty->num_overrun);
            tty->overrun_time = jiffies;
            tty->num_overrun = 0;
      }
}

/**
 *    n_tty_receive_parity_error    -     error notifier
 *    @tty: terminal device
 *    @c: character
 *
 *    Process a parity error and queue the right data to indicate
 *    the error case if necessary. Locking as per n_tty_receive_buf.
 */
static inline void n_tty_receive_parity_error(struct tty_struct *tty,
                                    unsigned char c)
{
      if (I_IGNPAR(tty)) {
            return;
      }
      if (I_PARMRK(tty)) {
            put_tty_queue('\377', tty);
            put_tty_queue('\0', tty);
            put_tty_queue(c, tty);
      } else      if (I_INPCK(tty))
            put_tty_queue('\0', tty);
      else
            put_tty_queue(c, tty);
      wake_up_interruptible(&tty->read_wait);
}

/**
 *    n_tty_receive_char      -     perform processing
 *    @tty: terminal device
 *    @c: character
 *
 *    Process an individual character of input received from the driver.
 *    This is serialized with respect to itself by the rules for the 
 *    driver above.
 */

static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
{
      unsigned long flags;

      if (tty->raw) {
            put_tty_queue(c, tty);
            return;
      }
      
      if (tty->stopped && !tty->flow_stopped &&
          I_IXON(tty) && I_IXANY(tty)) {
            start_tty(tty);
            return;
      }
      
      if (I_ISTRIP(tty))
            c &= 0x7f;
      if (I_IUCLC(tty) && L_IEXTEN(tty))
            c=tolower(c);

      if (tty->closing) {
            if (I_IXON(tty)) {
                  if (c == START_CHAR(tty))
                        start_tty(tty);
                  else if (c == STOP_CHAR(tty))
                        stop_tty(tty);
            }
            return;
      }

      /*
       * If the previous character was LNEXT, or we know that this
       * character is not one of the characters that we'll have to
       * handle specially, do shortcut processing to speed things
       * up.
       */
      if (!test_bit(c, tty->process_char_map) || tty->lnext) {
            finish_erasing(tty);
            tty->lnext = 0;
            if (L_ECHO(tty)) {
                  if (tty->read_cnt >= N_TTY_BUF_SIZE-1) {
                        put_char('\a', tty); /* beep if no space */
                        return;
                  }
                  /* Record the column of first canon char. */
                  if (tty->canon_head == tty->read_head)
                        tty->canon_column = tty->column;
                  echo_char(c, tty);
            }
            if (I_PARMRK(tty) && c == (unsigned char) '\377')
                  put_tty_queue(c, tty);
            put_tty_queue(c, tty);
            return;
      }
            
      if (c == '\r') {
            if (I_IGNCR(tty))
                  return;
            if (I_ICRNL(tty))
                  c = '\n';
      } else if (c == '\n' && I_INLCR(tty))
            c = '\r';
      if (I_IXON(tty)) {
            if (c == START_CHAR(tty)) {
                  start_tty(tty);
                  return;
            }
            if (c == STOP_CHAR(tty)) {
                  stop_tty(tty);
                  return;
            }
      }
      if (L_ISIG(tty)) {
            int signal;
            signal = SIGINT;
            if (c == INTR_CHAR(tty))
                  goto send_signal;
            signal = SIGQUIT;
            if (c == QUIT_CHAR(tty))
                  goto send_signal;
            signal = SIGTSTP;
            if (c == SUSP_CHAR(tty)) {
send_signal:
                  isig(signal, tty, 0);
                  return;
            }
      }
      if (tty->icanon) {
            if (c == ERASE_CHAR(tty) || c == KILL_CHAR(tty) ||
                (c == WERASE_CHAR(tty) && L_IEXTEN(tty))) {
                  eraser(c, tty);
                  return;
            }
            if (c == LNEXT_CHAR(tty) && L_IEXTEN(tty)) {
                  tty->lnext = 1;
                  if (L_ECHO(tty)) {
                        finish_erasing(tty);
                        if (L_ECHOCTL(tty)) {
                              put_char('^', tty);
                              put_char('\b', tty);
                        }
                  }
                  return;
            }
            if (c == REPRINT_CHAR(tty) && L_ECHO(tty) &&
                L_IEXTEN(tty)) {
                  unsigned long tail = tty->canon_head;

                  finish_erasing(tty);
                  echo_char(c, tty);
                  opost('\n', tty);
                  while (tail != tty->read_head) {
                        echo_char(tty->read_buf[tail], tty);
                        tail = (tail+1) & (N_TTY_BUF_SIZE-1);
                  }
                  return;
            }
            if (c == '\n') {
                  if (L_ECHO(tty) || L_ECHONL(tty)) {
                        if (tty->read_cnt >= N_TTY_BUF_SIZE-1)
                              put_char('\a', tty);
                        opost('\n', tty);
                  }
                  goto handle_newline;
            }
            if (c == EOF_CHAR(tty)) {
                    if (tty->canon_head != tty->read_head)
                          set_bit(TTY_PUSH, &tty->flags);
                  c = __DISABLED_CHAR;
                  goto handle_newline;
            }
            if ((c == EOL_CHAR(tty)) ||
                (c == EOL2_CHAR(tty) && L_IEXTEN(tty))) {
                  /*
                   * XXX are EOL_CHAR and EOL2_CHAR echoed?!?
                   */
                  if (L_ECHO(tty)) {
                        if (tty->read_cnt >= N_TTY_BUF_SIZE-1)
                              put_char('\a', tty);
                        /* Record the column of first canon char. */
                        if (tty->canon_head == tty->read_head)
                              tty->canon_column = tty->column;
                        echo_char(c, tty);
                  }
                  /*
                   * XXX does PARMRK doubling happen for
                   * EOL_CHAR and EOL2_CHAR?
                   */
                  if (I_PARMRK(tty) && c == (unsigned char) '\377')
                        put_tty_queue(c, tty);

            handle_newline:
                  spin_lock_irqsave(&tty->read_lock, flags);
                  set_bit(tty->read_head, tty->read_flags);
                  put_tty_queue_nolock(c, tty);
                  tty->canon_head = tty->read_head;
                  tty->canon_data++;
                  spin_unlock_irqrestore(&tty->read_lock, flags);
                  kill_fasync(&tty->fasync, SIGIO, POLL_IN);
                  if (waitqueue_active(&tty->read_wait))
                        wake_up_interruptible(&tty->read_wait);
                  return;
            }
      }
      
      finish_erasing(tty);
      if (L_ECHO(tty)) {
            if (tty->read_cnt >= N_TTY_BUF_SIZE-1) {
                  put_char('\a', tty); /* beep if no space */
                  return;
            }
            if (c == '\n')
                  opost('\n', tty);
            else {
                  /* Record the column of first canon char. */
                  if (tty->canon_head == tty->read_head)
                        tty->canon_column = tty->column;
                  echo_char(c, tty);
            }
      }

      if (I_PARMRK(tty) && c == (unsigned char) '\377')
            put_tty_queue(c, tty);

      put_tty_queue(c, tty);
}     


/**
 *    n_tty_write_wakeup      -     asynchronous I/O notifier
 *    @tty: tty device
 *
 *    Required for the ptys, serial driver etc. since processes
 *    that attach themselves to the master and rely on ASYNC
 *    IO must be woken up
 */

static void n_tty_write_wakeup(struct tty_struct *tty)
{
      if (tty->fasync)
      {
            set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
            kill_fasync(&tty->fasync, SIGIO, POLL_OUT);
      }
      return;
}

/**
 *    n_tty_receive_buf -     data receive
 *    @tty: terminal device
 *    @cp: buffer
 *    @fp: flag buffer
 *    @count: characters
 *
 *    Called by the terminal driver when a block of characters has
 *    been received. This function must be called from soft contexts
 *    not from interrupt context. The driver is responsible for making
 *    calls one at a time and in order (or using flush_to_ldisc)
 */
 
static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
                        char *fp, int count)
{
      const unsigned char *p;
      char *f, flags = TTY_NORMAL;
      int   i;
      char  buf[64];
      unsigned long cpuflags;

      if (!tty->read_buf)
            return;

      if (tty->real_raw) {
            spin_lock_irqsave(&tty->read_lock, cpuflags);
            i = min(N_TTY_BUF_SIZE - tty->read_cnt,
                  N_TTY_BUF_SIZE - tty->read_head);
            i = min(count, i);
            memcpy(tty->read_buf + tty->read_head, cp, i);
            tty->read_head = (tty->read_head + i) & (N_TTY_BUF_SIZE-1);
            tty->read_cnt += i;
            cp += i;
            count -= i;

            i = min(N_TTY_BUF_SIZE - tty->read_cnt,
                  N_TTY_BUF_SIZE - tty->read_head);
            i = min(count, i);
            memcpy(tty->read_buf + tty->read_head, cp, i);
            tty->read_head = (tty->read_head + i) & (N_TTY_BUF_SIZE-1);
            tty->read_cnt += i;
            spin_unlock_irqrestore(&tty->read_lock, cpuflags);
      } else {
            for (i=count, p = cp, f = fp; i; i--, p++) {
                  if (f)
                        flags = *f++;
                  switch (flags) {
                  case TTY_NORMAL:
                        n_tty_receive_char(tty, *p);
                        break;
                  case TTY_BREAK:
                        n_tty_receive_break(tty);
                        break;
                  case TTY_PARITY:
                  case TTY_FRAME:
                        n_tty_receive_parity_error(tty, *p);
                        break;
                  case TTY_OVERRUN:
                        n_tty_receive_overrun(tty);
                        break;
                  default:
                        printk("%s: unknown flag %d\n",
                               tty_name(tty, buf), flags);
                        break;
                  }
            }
            if (tty->driver->flush_chars)
                  tty->driver->flush_chars(tty);
      }

      n_tty_set_room(tty);

      if (!tty->icanon && (tty->read_cnt >= tty->minimum_to_wake)) {
            kill_fasync(&tty->fasync, SIGIO, POLL_IN);
            if (waitqueue_active(&tty->read_wait))
                  wake_up_interruptible(&tty->read_wait);
      }

      /*
       * Check the remaining room for the input canonicalization
       * mode.  We don't want to throttle the driver if we're in
       * canonical mode and don't have a newline yet!
       */
      if (tty->receive_room < TTY_THRESHOLD_THROTTLE) {
            /* check TTY_THROTTLED first so it indicates our state */
            if (!test_and_set_bit(TTY_THROTTLED, &tty->flags) &&
                tty->driver->throttle)
                  tty->driver->throttle(tty);
      }
}

int is_ignored(int sig)
{
      return (sigismember(&current->blocked, sig) ||
              current->sighand->action[sig-1].sa.sa_handler == SIG_IGN);
}

/**
 *    n_tty_set_termios -     termios data changed
 *    @tty: terminal
 *    @old: previous data
 *
 *    Called by the tty layer when the user changes termios flags so
 *    that the line discipline can plan ahead. This function cannot sleep
 *    and is protected from re-entry by the tty layer. The user is 
 *    guaranteed that this function will not be re-entered or in progress
 *    when the ldisc is closed.
 */
 
static void n_tty_set_termios(struct tty_struct *tty, struct ktermios * old)
{
      if (!tty)
            return;
      
      tty->icanon = (L_ICANON(tty) != 0);
      if (test_bit(TTY_HW_COOK_IN, &tty->flags)) {
            tty->raw = 1;
            tty->real_raw = 1;
            n_tty_set_room(tty);
            return;
      }
      if (I_ISTRIP(tty) || I_IUCLC(tty) || I_IGNCR(tty) ||
          I_ICRNL(tty) || I_INLCR(tty) || L_ICANON(tty) ||
          I_IXON(tty) || L_ISIG(tty) || L_ECHO(tty) ||
          I_PARMRK(tty)) {
            memset(tty->process_char_map, 0, 256/8);

            if (I_IGNCR(tty) || I_ICRNL(tty))
                  set_bit('\r', tty->process_char_map);
            if (I_INLCR(tty))
                  set_bit('\n', tty->process_char_map);

            if (L_ICANON(tty)) {
                  set_bit(ERASE_CHAR(tty), tty->process_char_map);
                  set_bit(KILL_CHAR(tty), tty->process_char_map);
                  set_bit(EOF_CHAR(tty), tty->process_char_map);
                  set_bit('\n', tty->process_char_map);
                  set_bit(EOL_CHAR(tty), tty->process_char_map);
                  if (L_IEXTEN(tty)) {
                        set_bit(WERASE_CHAR(tty),
                              tty->process_char_map);
                        set_bit(LNEXT_CHAR(tty),
                              tty->process_char_map);
                        set_bit(EOL2_CHAR(tty),
                              tty->process_char_map);
                        if (L_ECHO(tty))
                              set_bit(REPRINT_CHAR(tty),
                                    tty->process_char_map);
                  }
            }
            if (I_IXON(tty)) {
                  set_bit(START_CHAR(tty), tty->process_char_map);
                  set_bit(STOP_CHAR(tty), tty->process_char_map);
            }
            if (L_ISIG(tty)) {
                  set_bit(INTR_CHAR(tty), tty->process_char_map);
                  set_bit(QUIT_CHAR(tty), tty->process_char_map);
                  set_bit(SUSP_CHAR(tty), tty->process_char_map);
            }
            clear_bit(__DISABLED_CHAR, tty->process_char_map);
            tty->raw = 0;
            tty->real_raw = 0;
      } else {
            tty->raw = 1;
            if ((I_IGNBRK(tty) || (!I_BRKINT(tty) && !I_PARMRK(tty))) &&
                (I_IGNPAR(tty) || !I_INPCK(tty)) &&
                (tty->driver->flags & TTY_DRIVER_REAL_RAW))
                  tty->real_raw = 1;
            else
                  tty->real_raw = 0;
      }
      n_tty_set_room(tty);
}

/**
 *    n_tty_close       -     close the ldisc for this tty
 *    @tty: device
 *
 *    Called from the terminal layer when this line discipline is 
 *    being shut down, either because of a close or becsuse of a 
 *    discipline change. The function will not be called while other
 *    ldisc methods are in progress.
 */
 
static void n_tty_close(struct tty_struct *tty)
{
      n_tty_flush_buffer(tty);
      if (tty->read_buf) {
            free_buf(tty->read_buf);
            tty->read_buf = NULL;
      }
}

/**
 *    n_tty_open        -     open an ldisc
 *    @tty: terminal to open
 *
 *    Called when this line discipline is being attached to the 
 *    terminal device. Can sleep. Called serialized so that no
 *    other events will occur in parallel. No further open will occur
 *    until a close.
 */

static int n_tty_open(struct tty_struct *tty)
{
      if (!tty)
            return -EINVAL;

      /* This one is ugly. Currently a malloc failure here can panic */
      if (!tty->read_buf) {
            tty->read_buf = alloc_buf();
            if (!tty->read_buf)
                  return -ENOMEM;
      }
      memset(tty->read_buf, 0, N_TTY_BUF_SIZE);
      reset_buffer_flags(tty);
      tty->column = 0;
      n_tty_set_termios(tty, NULL);
      tty->minimum_to_wake = 1;
      tty->closing = 0;
      return 0;
}

static inline int input_available_p(struct tty_struct *tty, int amt)
{
      if (tty->icanon) {
            if (tty->canon_data)
                  return 1;
      } else if (tty->read_cnt >= (amt ? amt : 1))
            return 1;

      return 0;
}

/**
 *    copy_from_read_buf      -     copy read data directly
 *    @tty: terminal device
 *    @b: user data
 *    @nr: size of data
 *
 *    Helper function to speed up read_chan.  It is only called when
 *    ICANON is off; it copies characters straight from the tty queue to
 *    user space directly.  It can be profitably called twice; once to
 *    drain the space from the tail pointer to the (physical) end of the
 *    buffer, and once to drain the space from the (physical) beginning of
 *    the buffer to head pointer.
 *
 *    Called under the tty->atomic_read_lock sem
 *
 */
 
static int copy_from_read_buf(struct tty_struct *tty,
                              unsigned char __user **b,
                              size_t *nr)

{
      int retval;
      size_t n;
      unsigned long flags;

      retval = 0;
      spin_lock_irqsave(&tty->read_lock, flags);
      n = min(tty->read_cnt, N_TTY_BUF_SIZE - tty->read_tail);
      n = min(*nr, n);
      spin_unlock_irqrestore(&tty->read_lock, flags);
      if (n) {
            retval = copy_to_user(*b, &tty->read_buf[tty->read_tail], n);
            n -= retval;
            tty_audit_add_data(tty, &tty->read_buf[tty->read_tail], n);
            spin_lock_irqsave(&tty->read_lock, flags);
            tty->read_tail = (tty->read_tail + n) & (N_TTY_BUF_SIZE-1);
            tty->read_cnt -= n;
            spin_unlock_irqrestore(&tty->read_lock, flags);
            *b += n;
            *nr -= n;
      }
      return retval;
}

extern ssize_t redirected_tty_write(struct file *,const char *,size_t,loff_t *);

/**
 *    job_control       -     check job control
 *    @tty: tty
 *    @file: file handle
 *
 *    Perform job control management checks on this file/tty descriptor
 *    and if appropriate send any needed signals and return a negative 
 *    error code if action should be taken.
 */
 
static int job_control(struct tty_struct *tty, struct file *file)
{
      /* Job control check -- must be done at start and after
         every sleep (POSIX.1 7.1.1.4). */
      /* NOTE: not yet done after every sleep pending a thorough
         check of the logic of this change. -- jlc */
      /* don't stop on /dev/console */
      if (file->f_op->write != redirected_tty_write &&
          current->signal->tty == tty) {
            if (!tty->pgrp)
                  printk("read_chan: no tty->pgrp!\n");
            else if (task_pgrp(current) != tty->pgrp) {
                  if (is_ignored(SIGTTIN) ||
                      is_current_pgrp_orphaned())
                        return -EIO;
                  kill_pgrp(task_pgrp(current), SIGTTIN, 1);
                  set_thread_flag(TIF_SIGPENDING);
                  return -ERESTARTSYS;
            }
      }
      return 0;
}
 

/**
 *    read_chan         -     read function for tty
 *    @tty: tty device
 *    @file: file object
 *    @buf: userspace buffer pointer
 *    @nr: size of I/O
 *
 *    Perform reads for the line discipline. We are guaranteed that the
 *    line discipline will not be closed under us but we may get multiple
 *    parallel readers and must handle this ourselves. We may also get
 *    a hangup. Always called in user context, may sleep.
 *
 *    This code must be sure never to sleep through a hangup.
 */
 
static ssize_t read_chan(struct tty_struct *tty, struct file *file,
                   unsigned char __user *buf, size_t nr)
{
      unsigned char __user *b = buf;
      DECLARE_WAITQUEUE(wait, current);
      int c;
      int minimum, time;
      ssize_t retval = 0;
      ssize_t size;
      long timeout;
      unsigned long flags;

do_it_again:

      if (!tty->read_buf) {
            printk("n_tty_read_chan: called with read_buf == NULL?!?\n");
            return -EIO;
      }

      c = job_control(tty, file);
      if(c < 0)
            return c;
      
      minimum = time = 0;
      timeout = MAX_SCHEDULE_TIMEOUT;
      if (!tty->icanon) {
            time = (HZ / 10) * TIME_CHAR(tty);
            minimum = MIN_CHAR(tty);
            if (minimum) {
                  if (time)
                        tty->minimum_to_wake = 1;
                  else if (!waitqueue_active(&tty->read_wait) ||
                         (tty->minimum_to_wake > minimum))
                        tty->minimum_to_wake = minimum;
            } else {
                  timeout = 0;
                  if (time) {
                        timeout = time;
                        time = 0;
                  }
                  tty->minimum_to_wake = minimum = 1;
            }
      }

      /*
       *    Internal serialization of reads.
       */
      if (file->f_flags & O_NONBLOCK) {
            if (!mutex_trylock(&tty->atomic_read_lock))
                  return -EAGAIN;
      }
      else {
            if (mutex_lock_interruptible(&tty->atomic_read_lock))
                  return -ERESTARTSYS;
      }

      add_wait_queue(&tty->read_wait, &wait);
      while (nr) {
            /* First test for status change. */
            if (tty->packet && tty->link->ctrl_status) {
                  unsigned char cs;
                  if (b != buf)
                        break;
                  cs = tty->link->ctrl_status;
                  tty->link->ctrl_status = 0;
                  if (tty_put_user(tty, cs, b++)) {
                        retval = -EFAULT;
                        b--;
                        break;
                  }
                  nr--;
                  break;
            }
            /* This statement must be first before checking for input
               so that any interrupt will set the state back to
               TASK_RUNNING. */
            set_current_state(TASK_INTERRUPTIBLE);
            
            if (((minimum - (b - buf)) < tty->minimum_to_wake) &&
                ((minimum - (b - buf)) >= 1))
                  tty->minimum_to_wake = (minimum - (b - buf));
            
            if (!input_available_p(tty, 0)) {
                  if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
                        retval = -EIO;
                        break;
                  }
                  if (tty_hung_up_p(file))
                        break;
                  if (!timeout)
                        break;
                  if (file->f_flags & O_NONBLOCK) {
                        retval = -EAGAIN;
                        break;
                  }
                  if (signal_pending(current)) {
                        retval = -ERESTARTSYS;
                        break;
                  }
                  n_tty_set_room(tty);
                  timeout = schedule_timeout(timeout);
                  continue;
            }
            __set_current_state(TASK_RUNNING);

            /* Deal with packet mode. */
            if (tty->packet && b == buf) {
                  if (tty_put_user(tty, TIOCPKT_DATA, b++)) {
                        retval = -EFAULT;
                        b--;
                        break;
                  }
                  nr--;
            }

            if (tty->icanon) {
                  /* N.B. avoid overrun if nr == 0 */
                  while (nr && tty->read_cnt) {
                        int eol;

                        eol = test_and_clear_bit(tty->read_tail,
                                    tty->read_flags);
                        c = tty->read_buf[tty->read_tail];
                        spin_lock_irqsave(&tty->read_lock, flags);
                        tty->read_tail = ((tty->read_tail+1) &
                                      (N_TTY_BUF_SIZE-1));
                        tty->read_cnt--;
                        if (eol) {
                              /* this test should be redundant:
                               * we shouldn't be reading data if
                               * canon_data is 0
                               */
                              if (--tty->canon_data < 0)
                                    tty->canon_data = 0;
                        }
                        spin_unlock_irqrestore(&tty->read_lock, flags);

                        if (!eol || (c != __DISABLED_CHAR)) {
                              if (tty_put_user(tty, c, b++)) {
                                    retval = -EFAULT;
                                    b--;
                                    break;
                              }
                              nr--;
                        }
                        if (eol) {
                              tty_audit_push(tty);
                              break;
                        }
                  }
                  if (retval)
                        break;
            } else {
                  int uncopied;
                  uncopied = copy_from_read_buf(tty, &b, &nr);
                  uncopied += copy_from_read_buf(tty, &b, &nr);
                  if (uncopied) {
                        retval = -EFAULT;
                        break;
                  }
            }

            /* If there is enough space in the read buffer now, let the
             * low-level driver know. We use n_tty_chars_in_buffer() to
             * check the buffer, as it now knows about canonical mode.
             * Otherwise, if the driver is throttled and the line is
             * longer than TTY_THRESHOLD_UNTHROTTLE in canonical mode,
             * we won't get any more characters.
             */
            if (n_tty_chars_in_buffer(tty) <= TTY_THRESHOLD_UNTHROTTLE) {
                  n_tty_set_room(tty);
                  check_unthrottle(tty);
            }

            if (b - buf >= minimum)
                  break;
            if (time)
                  timeout = time;
      }
      mutex_unlock(&tty->atomic_read_lock);
      remove_wait_queue(&tty->read_wait, &wait);

      if (!waitqueue_active(&tty->read_wait))
            tty->minimum_to_wake = minimum;

      __set_current_state(TASK_RUNNING);
      size = b - buf;
      if (size) {
            retval = size;
            if (nr)
                        clear_bit(TTY_PUSH, &tty->flags);
      } else if (test_and_clear_bit(TTY_PUSH, &tty->flags))
             goto do_it_again;

      n_tty_set_room(tty);

      return retval;
}

/**
 *    write_chan        -     write function for tty
 *    @tty: tty device
 *    @file: file object
 *    @buf: userspace buffer pointer
 *    @nr: size of I/O
 *
 *    Write function of the terminal device. This is serialized with
 *    respect to other write callers but not to termios changes, reads
 *    and other such events. We must be careful with N_TTY as the receive
 *    code will echo characters, thus calling driver write methods.
 *
 *    This code must be sure never to sleep through a hangup.
 */
 
static ssize_t write_chan(struct tty_struct * tty, struct file * file,
                    const unsigned char * buf, size_t nr)
{
      const unsigned char *b = buf;
      DECLARE_WAITQUEUE(wait, current);
      int c;
      ssize_t retval = 0;

      /* Job control check -- must be done at start (POSIX.1 7.1.1.4). */
      if (L_TOSTOP(tty) && file->f_op->write != redirected_tty_write) {
            retval = tty_check_change(tty);
            if (retval)
                  return retval;
      }

      add_wait_queue(&tty->write_wait, &wait);
      while (1) {
            set_current_state(TASK_INTERRUPTIBLE);
            if (signal_pending(current)) {
                  retval = -ERESTARTSYS;
                  break;
            }
            if (tty_hung_up_p(file) || (tty->link && !tty->link->count)) {
                  retval = -EIO;
                  break;
            }
            if (O_OPOST(tty) && !(test_bit(TTY_HW_COOK_OUT, &tty->flags))) {
                  while (nr > 0) {
                        ssize_t num = opost_block(tty, b, nr);
                        if (num < 0) {
                              if (num == -EAGAIN)
                                    break;
                              retval = num;
                              goto break_out;
                        }
                        b += num;
                        nr -= num;
                        if (nr == 0)
                              break;
                        c = *b;
                        if (opost(c, tty) < 0)
                              break;
                        b++; nr--;
                  }
                  if (tty->driver->flush_chars)
                        tty->driver->flush_chars(tty);
            } else {
                  while (nr > 0) {
                        c = tty->driver->write(tty, b, nr);
                        if (c < 0) {
                              retval = c;
                              goto break_out;
                        }
                        if (!c)
                              break;
                        b += c;
                        nr -= c;
                  }
            }
            if (!nr)
                  break;
            if (file->f_flags & O_NONBLOCK) {
                  retval = -EAGAIN;
                  break;
            }
            schedule();
      }
break_out:
      __set_current_state(TASK_RUNNING);
      remove_wait_queue(&tty->write_wait, &wait);
      return (b - buf) ? b - buf : retval;
}

/**
 *    normal_poll       -     poll method for N_TTY
 *    @tty: terminal device
 *    @file: file accessing it
 *    @wait: poll table
 *
 *    Called when the line discipline is asked to poll() for data or
 *    for special events. This code is not serialized with respect to
 *    other events save open/close.
 *
 *    This code must be sure never to sleep through a hangup.
 *    Called without the kernel lock held - fine
 *
 *    FIXME: if someone changes the VMIN or discipline settings for the
 *    terminal while another process is in poll() the poll does not
 *    recompute the new limits. Possibly set_termios should issue
 *    a read wakeup to fix this bug.
 */
 
static unsigned int normal_poll(struct tty_struct * tty, struct file * file, poll_table *wait)
{
      unsigned int mask = 0;

      poll_wait(file, &tty->read_wait, wait);
      poll_wait(file, &tty->write_wait, wait);
      if (input_available_p(tty, TIME_CHAR(tty) ? 0 : MIN_CHAR(tty)))
            mask |= POLLIN | POLLRDNORM;
      if (tty->packet && tty->link->ctrl_status)
            mask |= POLLPRI | POLLIN | POLLRDNORM;
      if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
            mask |= POLLHUP;
      if (tty_hung_up_p(file))
            mask |= POLLHUP;
      if (!(mask & (POLLHUP | POLLIN | POLLRDNORM))) {
            if (MIN_CHAR(tty) && !TIME_CHAR(tty))
                  tty->minimum_to_wake = MIN_CHAR(tty);
            else
                  tty->minimum_to_wake = 1;
      }
      if (!tty_is_writelocked(tty) &&
                  tty->driver->chars_in_buffer(tty) < WAKEUP_CHARS &&
                  tty->driver->write_room(tty) > 0)
            mask |= POLLOUT | POLLWRNORM;
      return mask;
}

struct tty_ldisc tty_ldisc_N_TTY = {
      .magic           = TTY_LDISC_MAGIC,
      .name            = "n_tty",
      .open            = n_tty_open,
      .close           = n_tty_close,
      .flush_buffer    = n_tty_flush_buffer,
      .chars_in_buffer = n_tty_chars_in_buffer,
      .read            = read_chan,
      .write           = write_chan,
      .ioctl           = n_tty_ioctl,
      .set_termios     = n_tty_set_termios,
      .poll            = normal_poll,
      .receive_buf     = n_tty_receive_buf,
      .write_wakeup    = n_tty_write_wakeup
};


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