// SPDX-License-Identifier: GPL-1.0+

  /*
   * 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.

   *

   * 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 n_tty_write 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 <linux/uaccess.h>

  #include <linux/module.h>

  #include <linux/ratelimit.h>

  #include <linux/vmalloc.h>

  /*
   * Until this number of characters is queued in the xmit buffer, select will
   * return "we have room for writes".
   */

  #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

  /*
   * Special byte codes used in the echo buffer to represent operations
   * or special handling of characters.  Bytes in the echo buffer that
   * are not part of such special blocks are treated as normal character
   * codes.
   */
  #define ECHO_OP_START 0xff
  #define ECHO_OP_MOVE_BACK_COL 0x80
  #define ECHO_OP_SET_CANON_COL 0x81
  #define ECHO_OP_ERASE_TAB 0x82

  #define ECHO_COMMIT_WATERMARK	256
  #define ECHO_BLOCK		256
  #define ECHO_DISCARD_WATERMARK	N_TTY_BUF_SIZE - (ECHO_BLOCK + 32)

  #undef N_TTY_TRACE
  #ifdef N_TTY_TRACE
  # define n_tty_trace(f, args...)	trace_printk(f, ##args)
  #else

  # define n_tty_trace(f, args...)	no_printk(f, ##args)

  #endif

  struct n_tty_data {

  	/* producer-published */
  	size_t read_head;

  	size_t commit_head;

  	size_t canon_head;

  	size_t echo_head;
  	size_t echo_commit;

  	size_t echo_mark;

  	DECLARE_BITMAP(char_map, 256);

  
  	/* private to n_tty_receive_overrun (single-threaded) */

  	unsigned long overrun_time;
  	int num_overrun;

  	/* non-atomic */
  	bool no_room;

  	/* must hold exclusive termios_rwsem to reset these */

  	unsigned char lnext:1, erasing:1, raw:1, real_raw:1, icanon:1;

  	unsigned char push:1;

  	/* shared by producer and consumer */

  	char read_buf[N_TTY_BUF_SIZE];

  	DECLARE_BITMAP(read_flags, N_TTY_BUF_SIZE);

  	unsigned char echo_buf[N_TTY_BUF_SIZE];

  	/* consumer-published */
  	size_t read_tail;

  	size_t line_start;

  	/* protected by output lock */
  	unsigned int column;

  	unsigned int canon_column;

  	size_t echo_tail;

  
  	struct mutex atomic_read_lock;
  	struct mutex output_lock;

  };

  #define MASK(x) ((x) & (N_TTY_BUF_SIZE - 1))

  static inline size_t read_cnt(struct n_tty_data *ldata)
  {

  	return ldata->read_head - ldata->read_tail;

  }

  static inline unsigned char read_buf(struct n_tty_data *ldata, size_t i)
  {
  	return ldata->read_buf[i & (N_TTY_BUF_SIZE - 1)];
  }
  
  static inline unsigned char *read_buf_addr(struct n_tty_data *ldata, size_t i)
  {
  	return &ldata->read_buf[i & (N_TTY_BUF_SIZE - 1)];
  }

  static inline unsigned char echo_buf(struct n_tty_data *ldata, size_t i)
  {

  	smp_rmb(); /* Matches smp_wmb() in add_echo_byte(). */

  	return ldata->echo_buf[i & (N_TTY_BUF_SIZE - 1)];
  }
  
  static inline unsigned char *echo_buf_addr(struct n_tty_data *ldata, size_t i)
  {
  	return &ldata->echo_buf[i & (N_TTY_BUF_SIZE - 1)];
  }

  /* If we are not echoing the data, perhaps this is a secret so erase it */
  static void zero_buffer(struct tty_struct *tty, u8 *buffer, int size)
  {
  	bool icanon = !!L_ICANON(tty);
  	bool no_echo = !L_ECHO(tty);
  
  	if (icanon && no_echo)
  		memset(buffer, 0x00, size);
  }

  static int tty_copy_to_user(struct tty_struct *tty, void __user *to,
  			    size_t tail, size_t n)

  {
  	struct n_tty_data *ldata = tty->disc_data;

  	size_t size = N_TTY_BUF_SIZE - tail;

  	void *from = read_buf_addr(ldata, tail);

  	int uncopied;
  
  	if (n > size) {

  		tty_audit_add_data(tty, from, size);

  		uncopied = copy_to_user(to, from, size);

  		zero_buffer(tty, from, size - uncopied);

  		if (uncopied)
  			return uncopied;
  		to += size;
  		n -= size;
  		from = ldata->read_buf;
  	}

  	tty_audit_add_data(tty, from, n);

  	uncopied = copy_to_user(to, from, n);
  	zero_buffer(tty, from, n - uncopied);
  	return uncopied;

  }

  /**

   *	n_tty_kick_worker - start input worker (if required)

   *	@tty: terminal
   *

   *	Re-schedules the flip buffer work if it may have stopped

   *

   *	Caller holds exclusive termios_rwsem
   *	   or
   *	n_tty_read()/consumer path:
   *		holds non-exclusive termios_rwsem

   */

  static void n_tty_kick_worker(struct tty_struct *tty)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	/* Did the input worker stop? Restart it */
  	if (unlikely(ldata->no_room)) {

  		ldata->no_room = 0;

  		WARN_RATELIMIT(tty->port->itty == NULL,

  				"scheduling with invalid itty
  ");

  		/* see if ldisc has been killed - if so, this means that
  		 * even though the ldisc has been halted and ->buf.work
  		 * cancelled, ->buf.work is about to be rescheduled
  		 */
  		WARN_RATELIMIT(test_bit(TTY_LDISC_HALTED, &tty->flags),
  			       "scheduling buffer work for halted ldisc
  ");

  		tty_buffer_restart_work(tty->port);

  	}

  }

  static ssize_t chars_in_buffer(struct tty_struct *tty)
  {
  	struct n_tty_data *ldata = tty->disc_data;
  	ssize_t n = 0;
  
  	if (!ldata->icanon)

  		n = ldata->commit_head - ldata->read_tail;

  	else
  		n = ldata->canon_head - ldata->read_tail;
  	return n;
  }

  /**
   *	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)
  {

  	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
  	kill_fasync(&tty->fasync, SIGIO, POLL_OUT);

  }

  static void n_tty_check_throttle(struct tty_struct *tty)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	/*
  	 * 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 (ldata->icanon && ldata->canon_head == ldata->read_tail)
  		return;

  	while (1) {
  		int throttled;
  		tty_set_flow_change(tty, TTY_THROTTLE_SAFE);

  		if (N_TTY_BUF_SIZE - read_cnt(ldata) >= TTY_THRESHOLD_THROTTLE)

  			break;
  		throttled = tty_throttle_safe(tty);
  		if (!throttled)
  			break;
  	}
  	__tty_set_flow_change(tty, 0);
  }

  static void n_tty_check_unthrottle(struct tty_struct *tty)

  {

  	if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {

  		if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
  			return;

  		n_tty_kick_worker(tty);

  		tty_wakeup(tty->link);

  		return;
  	}

  	/* If there is enough space in the read buffer now, let the
  	 * low-level driver know. We use 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.
  	 */
  
  	while (1) {
  		int unthrottled;
  		tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
  		if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
  			break;

  		n_tty_kick_worker(tty);

  		unthrottled = tty_unthrottle_safe(tty);
  		if (!unthrottled)
  			break;
  	}
  	__tty_set_flow_change(tty, 0);
  }

  /**
   *	put_tty_queue		-	add character to tty
   *	@c: character

   *	@ldata: n_tty data

   *

   *	Add a character to the tty read_buf queue.
   *
   *	n_tty_receive_buf()/producer path:
   *		caller holds non-exclusive termios_rwsem

   */

  static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)

  {

  	*read_buf_addr(ldata, ldata->read_head) = c;
  	ldata->read_head++;

  }
  
  /**

   *	reset_buffer_flags	-	reset buffer state
   *	@tty: terminal to reset
   *

   *	Reset the read buffer counters and clear the flags.
   *	Called from n_tty_open() and n_tty_flush_buffer().

   *

   *	Locking: caller holds exclusive termios_rwsem
   *		 (or locking is not required)

   */

  static void reset_buffer_flags(struct n_tty_data *ldata)

  {

  	ldata->read_head = ldata->canon_head = ldata->read_tail = 0;

  	ldata->commit_head = 0;

  	ldata->line_start = 0;

  	ldata->erasing = 0;

  	bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);

  	ldata->push = 0;

  }

  static void n_tty_packet_mode_flush(struct tty_struct *tty)
  {
  	unsigned long flags;

  	if (tty->link->packet) {

  		spin_lock_irqsave(&tty->ctrl_lock, flags);

  		tty->ctrl_status |= TIOCPKT_FLUSHREAD;

  		spin_unlock_irqrestore(&tty->ctrl_lock, flags);

  		wake_up_interruptible(&tty->link->read_wait);

  	}

  }

  /**
   *	n_tty_flush_buffer	-	clean input queue
   *	@tty:	terminal device
   *

   *	Flush the input buffer. Called 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).

   *

   *	Holds termios_rwsem to exclude producer/consumer while
   *	buffer indices are reset.
   *
   *	Locking: ctrl_lock, exclusive termios_rwsem

   */

  
  static void n_tty_flush_buffer(struct tty_struct *tty)

  {

  	down_write(&tty->termios_rwsem);

  	reset_buffer_flags(tty->disc_data);

  	n_tty_kick_worker(tty);

  	if (tty->link)
  		n_tty_packet_mode_flush(tty);

  	up_write(&tty->termios_rwsem);

  }

  /**

   *	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);
  }
  
  /**

   *	do_output_char			-	output one character

   *	@c: character (or partial unicode symbol)
   *	@tty: terminal device

   *	@space: space available in tty driver write buffer

   *

   *	This is a helper function that handles one output character
   *	(including special characters like TAB, CR, LF, etc.),

   *	doing OPOST processing and putting the results in the
   *	tty driver's write buffer.

   *
   *	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.

   *

   *	Returns the number of bytes of buffer space used or -1 if
   *	no space left.
   *
   *	Locking: should be called under the output_lock to protect
   *		 the column state and space left in the buffer

   */

  static int do_output_char(unsigned char c, struct tty_struct *tty, int space)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	int	spaces;

  	if (!space)
  		return -1;

  	switch (c) {
  	case '
  ':
  		if (O_ONLRET(tty))

  			ldata->column = 0;

  		if (O_ONLCR(tty)) {
  			if (space < 2)
  				return -1;

  			ldata->canon_column = ldata->column = 0;

  			tty->ops->write(tty, "\r
  ", 2);

  			return 2;
  		}

  		ldata->canon_column = ldata->column;

  		break;
  	case '\r':

  		if (O_ONOCR(tty) && ldata->column == 0)

  			return 0;
  		if (O_OCRNL(tty)) {
  			c = '
  ';
  			if (O_ONLRET(tty))

  				ldata->canon_column = ldata->column = 0;

  			break;

  		}

  		ldata->canon_column = ldata->column = 0;

  		break;
  	case '\t':

  		spaces = 8 - (ldata->column & 7);

  		if (O_TABDLY(tty) == XTABS) {
  			if (space < spaces)
  				return -1;

  			ldata->column += spaces;

  			tty->ops->write(tty, "        ", spaces);
  			return spaces;

  		}

  		ldata->column += spaces;

  		break;
  	case '\b':

  		if (ldata->column > 0)
  			ldata->column--;

  		break;
  	default:

  		if (!iscntrl(c)) {
  			if (O_OLCUC(tty))
  				c = toupper(c);
  			if (!is_continuation(c, tty))

  				ldata->column++;

  		}

  		break;

  	}

  	tty_put_char(tty, c);

  	return 1;
  }
  
  /**
   *	process_output			-	output post processor
   *	@c: character (or partial unicode symbol)
   *	@tty: terminal device
   *

   *	Output one character with OPOST processing.
   *	Returns -1 when the output device is full and the character
   *	must be retried.

   *
   *	Locking: output_lock to protect column state and space left
   *		 (also, this is called from n_tty_write under the
   *		  tty layer write lock)
   */
  
  static int process_output(unsigned char c, struct tty_struct *tty)
  {

  	struct n_tty_data *ldata = tty->disc_data;

  	int	space, retval;

  	mutex_lock(&ldata->output_lock);

  
  	space = tty_write_room(tty);
  	retval = do_output_char(c, tty, space);

  	mutex_unlock(&ldata->output_lock);

  	if (retval < 0)
  		return -1;
  	else
  		return 0;

  }
  
  /**

   *	process_output_block		-	block post processor

   *	@tty: terminal device

   *	@buf: character buffer
   *	@nr: number of bytes to output
   *
   *	Output a block of characters with OPOST processing.
   *	Returns the number of characters output.

   *
   *	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.
   *

   *	Locking: output_lock to protect column state and space left
   *		 (also, this is called from n_tty_write under the
   *		  tty layer write lock)

   */

  static ssize_t process_output_block(struct tty_struct *tty,
  				    const unsigned char *buf, unsigned int nr)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	int	space;

  	int	i;

  	const unsigned char *cp;

  	mutex_lock(&ldata->output_lock);

  	space = tty_write_room(tty);

  	if (space <= 0) {

  		mutex_unlock(&ldata->output_lock);

  		return space;

  	}

  	if (nr > space)
  		nr = space;
  
  	for (i = 0, cp = buf; i < nr; i++, cp++) {

  		unsigned char c = *cp;
  
  		switch (c) {

  		case '
  ':
  			if (O_ONLRET(tty))

  				ldata->column = 0;

  			if (O_ONLCR(tty))
  				goto break_out;

  			ldata->canon_column = ldata->column;

  			break;
  		case '\r':

  			if (O_ONOCR(tty) && ldata->column == 0)

  				goto break_out;
  			if (O_OCRNL(tty))
  				goto break_out;

  			ldata->canon_column = ldata->column = 0;

  			break;
  		case '\t':
  			goto break_out;
  		case '\b':

  			if (ldata->column > 0)
  				ldata->column--;

  			break;
  		default:

  			if (!iscntrl(c)) {
  				if (O_OLCUC(tty))
  					goto break_out;
  				if (!is_continuation(c, tty))

  					ldata->column++;

  			}

  			break;
  		}
  	}
  break_out:

  	i = tty->ops->write(tty, buf, i);

  	mutex_unlock(&ldata->output_lock);

  	return i;
  }

  /**
   *	process_echoes	-	write pending echo characters
   *	@tty: terminal device
   *
   *	Write previously buffered echo (and other ldisc-generated)
   *	characters to the tty.
   *
   *	Characters generated by the ldisc (including echoes) need to
   *	be buffered because the driver's write buffer can fill during
   *	heavy program output.  Echoing straight to the driver will
   *	often fail under these conditions, causing lost characters and
   *	resulting mismatches of ldisc state information.
   *
   *	Since the ldisc state must represent the characters actually sent
   *	to the driver at the time of the write, operations like certain
   *	changes in column state are also saved in the buffer and executed
   *	here.
   *
   *	A circular fifo buffer is used so that the most recent characters
   *	are prioritized.  Also, when control characters are echoed with a
   *	prefixed "^", the pair is treated atomically and thus not separated.
   *

   *	Locking: callers must hold output_lock

   */

  static size_t __process_echoes(struct tty_struct *tty)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	int	space, old_space;

  	size_t tail;

  	unsigned char c;

  	old_space = space = tty_write_room(tty);

  	tail = ldata->echo_tail;

  	while (MASK(ldata->echo_commit) != MASK(tail)) {

  		c = echo_buf(ldata, tail);

  		if (c == ECHO_OP_START) {
  			unsigned char op;

  			int no_space_left = 0;
  
  			/*

  			 * Since add_echo_byte() is called without holding
  			 * output_lock, we might see only portion of multi-byte
  			 * operation.
  			 */
  			if (MASK(ldata->echo_commit) == MASK(tail + 1))
  				goto not_yet_stored;
  			/*

  			 * If the buffer byte is the start of a multi-byte
  			 * operation, get the next byte, which is either the
  			 * op code or a control character value.
  			 */

  			op = echo_buf(ldata, tail + 1);

  			switch (op) {

  			case ECHO_OP_ERASE_TAB: {

  				unsigned int num_chars, num_bs;

  				if (MASK(ldata->echo_commit) == MASK(tail + 2))
  					goto not_yet_stored;

  				num_chars = echo_buf(ldata, tail + 2);

  
  				/*
  				 * Determine how many columns to go back
  				 * in order to erase the tab.
  				 * This depends on the number of columns
  				 * used by other characters within the tab
  				 * area.  If this (modulo 8) count is from
  				 * the start of input rather than from a
  				 * previous tab, we offset by canon column.
  				 * Otherwise, tab spacing is normal.
  				 */
  				if (!(num_chars & 0x80))

  					num_chars += ldata->canon_column;

  				num_bs = 8 - (num_chars & 7);
  
  				if (num_bs > space) {
  					no_space_left = 1;
  					break;
  				}
  				space -= num_bs;
  				while (num_bs--) {
  					tty_put_char(tty, '\b');

  					if (ldata->column > 0)
  						ldata->column--;

  				}

  				tail += 3;

  				break;

  			}

  			case ECHO_OP_SET_CANON_COL:

  				ldata->canon_column = ldata->column;

  				tail += 2;

  				break;
  
  			case ECHO_OP_MOVE_BACK_COL:

  				if (ldata->column > 0)
  					ldata->column--;

  				tail += 2;

  				break;
  
  			case ECHO_OP_START:
  				/* This is an escaped echo op start code */
  				if (!space) {
  					no_space_left = 1;
  					break;
  				}
  				tty_put_char(tty, ECHO_OP_START);

  				ldata->column++;

  				space--;

  				tail += 2;

  				break;
  
  			default:

  				/*

  				 * If the op is not a special byte code,
  				 * it is a ctrl char tagged to be echoed
  				 * as "^X" (where X is the letter
  				 * representing the control char).
  				 * Note that we must ensure there is
  				 * enough space for the whole ctrl pair.
  				 *

  				 */

  				if (space < 2) {
  					no_space_left = 1;
  					break;
  				}
  				tty_put_char(tty, '^');
  				tty_put_char(tty, op ^ 0100);

  				ldata->column += 2;

  				space -= 2;

  				tail += 2;

  			}
  
  			if (no_space_left)
  				break;
  		} else {

  			if (O_OPOST(tty)) {

  				int retval = do_output_char(c, tty, space);
  				if (retval < 0)
  					break;
  				space -= retval;
  			} else {
  				if (!space)
  					break;
  				tty_put_char(tty, c);
  				space -= 1;
  			}

  			tail += 1;

  		}

  	}

  	/* If the echo buffer is nearly full (so that the possibility exists
  	 * of echo overrun before the next commit), then discard enough
  	 * data at the tail to prevent a subsequent overrun */

  	while (ldata->echo_commit > tail &&
  	       ldata->echo_commit - tail >= ECHO_DISCARD_WATERMARK) {

  		if (echo_buf(ldata, tail) == ECHO_OP_START) {

  			if (echo_buf(ldata, tail + 1) == ECHO_OP_ERASE_TAB)

  				tail += 3;
  			else
  				tail += 2;
  		} else
  			tail++;
  	}

   not_yet_stored:

  	ldata->echo_tail = tail;

  	return old_space - space;

  }
  
  static void commit_echoes(struct tty_struct *tty)
  {
  	struct n_tty_data *ldata = tty->disc_data;

  	size_t nr, old, echoed;

  	size_t head;

  	mutex_lock(&ldata->output_lock);

  	head = ldata->echo_head;

  	ldata->echo_mark = head;

  	old = ldata->echo_commit - ldata->echo_tail;
  
  	/* Process committed echoes if the accumulated # of bytes
  	 * is over the threshold (and try again each time another
  	 * block is accumulated) */
  	nr = head - ldata->echo_tail;

  	if (nr < ECHO_COMMIT_WATERMARK ||
  	    (nr % ECHO_BLOCK > old % ECHO_BLOCK)) {
  		mutex_unlock(&ldata->output_lock);

  		return;

  	}

  	ldata->echo_commit = head;

  	echoed = __process_echoes(tty);

  	mutex_unlock(&ldata->output_lock);

  	if (echoed && tty->ops->flush_chars)

  		tty->ops->flush_chars(tty);
  }

  static void process_echoes(struct tty_struct *tty)

  {
  	struct n_tty_data *ldata = tty->disc_data;

  	size_t echoed;

  	if (ldata->echo_mark == ldata->echo_tail)

  		return;
  
  	mutex_lock(&ldata->output_lock);

  	ldata->echo_commit = ldata->echo_mark;

  	echoed = __process_echoes(tty);

  	mutex_unlock(&ldata->output_lock);

  	if (echoed && tty->ops->flush_chars)

  		tty->ops->flush_chars(tty);

  }

  /* NB: echo_mark and echo_head should be equivalent here */

  static void flush_echoes(struct tty_struct *tty)
  {
  	struct n_tty_data *ldata = tty->disc_data;

  	if ((!L_ECHO(tty) && !L_ECHONL(tty)) ||
  	    ldata->echo_commit == ldata->echo_head)

  		return;
  
  	mutex_lock(&ldata->output_lock);
  	ldata->echo_commit = ldata->echo_head;
  	__process_echoes(tty);
  	mutex_unlock(&ldata->output_lock);
  }

  /**
   *	add_echo_byte	-	add a byte to the echo buffer
   *	@c: unicode byte to echo

   *	@ldata: n_tty data

   *
   *	Add a character or operation byte to the echo buffer.

   */

  static inline void add_echo_byte(unsigned char c, struct n_tty_data *ldata)

  {

  	*echo_buf_addr(ldata, ldata->echo_head) = c;
  	smp_wmb(); /* Matches smp_rmb() in echo_buf(). */
  	ldata->echo_head++;

  }
  
  /**
   *	echo_move_back_col	-	add operation to move back a column

   *	@ldata: n_tty data

   *
   *	Add an operation to the echo buffer to move back one column.

   */

  static void echo_move_back_col(struct n_tty_data *ldata)

  {

  	add_echo_byte(ECHO_OP_START, ldata);
  	add_echo_byte(ECHO_OP_MOVE_BACK_COL, ldata);

  }
  
  /**
   *	echo_set_canon_col	-	add operation to set the canon column

   *	@ldata: n_tty data

   *
   *	Add an operation to the echo buffer to set the canon column
   *	to the current column.

   */

  static void echo_set_canon_col(struct n_tty_data *ldata)

  {

  	add_echo_byte(ECHO_OP_START, ldata);
  	add_echo_byte(ECHO_OP_SET_CANON_COL, ldata);

  }
  
  /**
   *	echo_erase_tab	-	add operation to erase a tab
   *	@num_chars: number of character columns already used
   *	@after_tab: true if num_chars starts after a previous tab

   *	@ldata: n_tty data

   *
   *	Add an operation to the echo buffer to erase a tab.
   *
   *	Called by the eraser function, which knows how many character
   *	columns have been used since either a previous tab or the start
   *	of input.  This information will be used later, along with
   *	canon column (if applicable), to go back the correct number
   *	of columns.

   */
  
  static void echo_erase_tab(unsigned int num_chars, int after_tab,

  			   struct n_tty_data *ldata)

  {

  	add_echo_byte(ECHO_OP_START, ldata);
  	add_echo_byte(ECHO_OP_ERASE_TAB, ldata);

  
  	/* We only need to know this modulo 8 (tab spacing) */
  	num_chars &= 7;
  
  	/* Set the high bit as a flag if num_chars is after a previous tab */
  	if (after_tab)
  		num_chars |= 0x80;

  	add_echo_byte(num_chars, ldata);

  }
  
  /**
   *	echo_char_raw	-	echo a character raw
   *	@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.
   *
   *	This variant does not treat control characters specially.

   */

  static void echo_char_raw(unsigned char c, struct n_tty_data *ldata)

  {

  	if (c == ECHO_OP_START) {

  		add_echo_byte(ECHO_OP_START, ldata);
  		add_echo_byte(ECHO_OP_START, ldata);

  	} else {

  		add_echo_byte(c, ldata);

  	}

  }

  
  /**

   *	echo_char	-	echo a character

   *	@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.

   *

   *	This variant tags control characters to be echoed as "^X"
   *	(where X is the letter representing the control char).

   */
  
  static void echo_char(unsigned char c, struct tty_struct *tty)
  {

  	struct n_tty_data *ldata = tty->disc_data;

  	if (c == ECHO_OP_START) {

  		add_echo_byte(ECHO_OP_START, ldata);
  		add_echo_byte(ECHO_OP_START, ldata);

  	} else {

  		if (L_ECHOCTL(tty) && iscntrl(c) && c != '\t')

  			add_echo_byte(ECHO_OP_START, ldata);
  		add_echo_byte(c, ldata);

  	}

  }

  /**

   *	finish_erasing		-	complete erase

   *	@ldata: n_tty data

   */

  static inline void finish_erasing(struct n_tty_data *ldata)

  {

  	if (ldata->erasing) {

  		echo_char_raw('/', ldata);

  		ldata->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.

   *

   *	n_tty_receive_buf()/producer path:
   *		caller holds non-exclusive termios_rwsem

   */

  static void eraser(unsigned char c, struct tty_struct *tty)
  {

  	struct n_tty_data *ldata = tty->disc_data;

  	enum { ERASE, WERASE, KILL } kill_type;

  	size_t head;
  	size_t cnt;
  	int seen_alnums;

  	if (ldata->read_head == ldata->canon_head) {

  		/* process_output('\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)) {

  			ldata->read_head = ldata->canon_head;

  			return;
  		}
  		if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {

  			ldata->read_head = ldata->canon_head;

  			finish_erasing(ldata);

  			echo_char(KILL_CHAR(tty), tty);
  			/* Add a newline if ECHOK is on and ECHOKE is off. */
  			if (L_ECHOK(tty))

  				echo_char_raw('
  ', ldata);

  			return;
  		}
  		kill_type = KILL;
  	}
  
  	seen_alnums = 0;

  	while (MASK(ldata->read_head) != MASK(ldata->canon_head)) {

  		head = ldata->read_head;

  
  		/* erase a single possibly multibyte character */
  		do {

  			head--;
  			c = read_buf(ldata, head);

  		} while (is_continuation(c, tty) &&
  			 MASK(head) != MASK(ldata->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 = ldata->read_head - head;

  		ldata->read_head = head;

  		if (L_ECHO(tty)) {
  			if (L_ECHOPRT(tty)) {

  				if (!ldata->erasing) {

  					echo_char_raw('\\', ldata);

  					ldata->erasing = 1;

  				}
  				/* if cnt > 1, output a multi-byte character */
  				echo_char(c, tty);
  				while (--cnt > 0) {

  					head++;
  					echo_char_raw(read_buf(ldata, head), ldata);

  					echo_move_back_col(ldata);

  				}
  			} else if (kill_type == ERASE && !L_ECHOE(tty)) {
  				echo_char(ERASE_CHAR(tty), tty);
  			} else if (c == '\t') {

  				unsigned int num_chars = 0;
  				int after_tab = 0;

  				size_t tail = ldata->read_head;

  
  				/*
  				 * Count the columns used for characters
  				 * since the start of input or after a
  				 * previous tab.
  				 * This info is used to go back the correct
  				 * number of columns.
  				 */

  				while (MASK(tail) != MASK(ldata->canon_head)) {

  					tail--;
  					c = read_buf(ldata, tail);

  					if (c == '\t') {
  						after_tab = 1;
  						break;

  					} else if (iscntrl(c)) {

  						if (L_ECHOCTL(tty))

  							num_chars += 2;
  					} else if (!is_continuation(c, tty)) {
  						num_chars++;
  					}

  				}

  				echo_erase_tab(num_chars, after_tab, ldata);

  			} else {
  				if (iscntrl(c) && L_ECHOCTL(tty)) {

  					echo_char_raw('\b', ldata);
  					echo_char_raw(' ', ldata);
  					echo_char_raw('\b', ldata);

  				}
  				if (!iscntrl(c) || L_ECHOCTL(tty)) {

  					echo_char_raw('\b', ldata);
  					echo_char_raw(' ', ldata);
  					echo_char_raw('\b', ldata);

  				}
  			}
  		}
  		if (kill_type == ERASE)
  			break;
  	}

  	if (ldata->read_head == ldata->canon_head && L_ECHO(tty))

  		finish_erasing(ldata);

  }
  
  /**
   *	isig		-	handle the ISIG optio
   *	@sig: signal
   *	@tty: terminal

   *

   *	Called when a signal is being sent due to terminal input.
   *	Called from the driver receive_buf path so serialized.

   *

   *	Performs input and output flush if !NOFLSH. In this context, the echo
   *	buffer is 'output'. The signal is processed first to alert any current
   *	readers or writers to discontinue and exit their i/o loops.
   *

   *	Locking: ctrl_lock

   */

  static void __isig(int sig, struct tty_struct *tty)

  {

  	struct pid *tty_pgrp = tty_get_pgrp(tty);
  	if (tty_pgrp) {
  		kill_pgrp(tty_pgrp, sig, 1);
  		put_pid(tty_pgrp);

  	}

  }

  static void isig(int sig, struct tty_struct *tty)
  {
  	struct n_tty_data *ldata = tty->disc_data;
  
  	if (L_NOFLSH(tty)) {
  		/* signal only */
  		__isig(sig, tty);
  
  	} else { /* signal and flush */

  		up_read(&tty->termios_rwsem);
  		down_write(&tty->termios_rwsem);

  		__isig(sig, tty);

  		/* clear echo buffer */
  		mutex_lock(&ldata->output_lock);
  		ldata->echo_head = ldata->echo_tail = 0;
  		ldata->echo_mark = ldata->echo_commit = 0;
  		mutex_unlock(&ldata->output_lock);
  
  		/* clear output buffer */
  		tty_driver_flush_buffer(tty);
  
  		/* clear input buffer */
  		reset_buffer_flags(tty->disc_data);
  
  		/* notify pty master of flush */
  		if (tty->link)
  			n_tty_packet_mode_flush(tty);
  
  		up_write(&tty->termios_rwsem);
  		down_read(&tty->termios_rwsem);
  	}

  }
  
  /**
   *	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.
   *

   *	n_tty_receive_buf()/producer path:
   *		caller holds non-exclusive termios_rwsem

   *
   *	Note: may get exclusive termios_rwsem if flushing input buffer

   */

  static void n_tty_receive_break(struct tty_struct *tty)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	if (I_IGNBRK(tty))
  		return;
  	if (I_BRKINT(tty)) {

  		isig(SIGINT, tty);

  		return;
  	}
  	if (I_PARMRK(tty)) {

  		put_tty_queue('\377', ldata);
  		put_tty_queue('\0', ldata);

  	}

  	put_tty_queue('\0', ldata);

  }
  
  /**
   *	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 void n_tty_receive_overrun(struct tty_struct *tty)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	ldata->num_overrun++;
  	if (time_after(jiffies, ldata->overrun_time + HZ) ||
  			time_after(ldata->overrun_time, jiffies)) {

  		tty_warn(tty, "%d input overrun(s)
  ", ldata->num_overrun);

  		ldata->overrun_time = jiffies;
  		ldata->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.
   *
   *	n_tty_receive_buf()/producer path:
   *		caller holds non-exclusive termios_rwsem

   */

  static void n_tty_receive_parity_error(struct tty_struct *tty, unsigned char c)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	if (I_INPCK(tty)) {
  		if (I_IGNPAR(tty))
  			return;
  		if (I_PARMRK(tty)) {
  			put_tty_queue('\377', ldata);
  			put_tty_queue('\0', ldata);
  			put_tty_queue(c, ldata);
  		} else
  			put_tty_queue('\0', ldata);
  	} else

  		put_tty_queue(c, ldata);

  }

  static void
  n_tty_receive_signal_char(struct tty_struct *tty, int signal, unsigned char c)
  {

  	isig(signal, tty);

  	if (I_IXON(tty))
  		start_tty(tty);
  	if (L_ECHO(tty)) {
  		echo_char(c, tty);
  		commit_echoes(tty);

  	} else
  		process_echoes(tty);

  	return;
  }

  /**
   *	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.

   *
   *	n_tty_receive_buf()/producer path:
   *		caller holds non-exclusive termios_rwsem
   *		publishes canon_head if canonical mode is active

   *
   *	Returns 1 if LNEXT was received, else returns 0

   */

  static int

  n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)

  {

  	struct n_tty_data *ldata = tty->disc_data;

  	if (I_IXON(tty)) {
  		if (c == START_CHAR(tty)) {
  			start_tty(tty);

  			process_echoes(tty);

  			return 0;

  		}
  		if (c == STOP_CHAR(tty)) {
  			stop_tty(tty);

  			return 0;

  		}
  	}

  	if (L_ISIG(tty)) {

  		if (c == INTR_CHAR(tty)) {
  			n_tty_receive_signal_char(tty, SIGINT, c);

  			return 0;

  		} else if (c == QUIT_CHAR(tty)) {
  			n_tty_receive_signal_char(tty, SIGQUIT, c);

  			return 0;

  		} else if (c == SUSP_CHAR(tty)) {
  			n_tty_receive_signal_char(tty, SIGTSTP, c);

  			return 0;

  		}
  	}

  	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) && I_IXANY(tty)) {
  		start_tty(tty);
  		process_echoes(tty);
  	}

  	if (c == '\r') {
  		if (I_IGNCR(tty))

  			return 0;

  		if (I_ICRNL(tty))
  			c = '
  ';
  	} else if (c == '
  ' && I_INLCR(tty))
  		c = '\r';

  	if (ldata->icanon) {

  		if (c == ERASE_CHAR(tty) || c == KILL_CHAR(tty) ||
  		    (c == WERASE_CHAR(tty) && L_IEXTEN(tty))) {
  			eraser(c, tty);

  			commit_echoes(tty);

  			return 0;

  		}
  		if (c == LNEXT_CHAR(tty) && L_IEXTEN(tty)) {

  			ldata->lnext = 1;

  			if (L_ECHO(tty)) {

  				finish_erasing(ldata);

  				if (L_ECHOCTL(tty)) {

  					echo_char_raw('^', ldata);
  					echo_char_raw('\b', ldata);

  					commit_echoes(tty);

  				}
  			}

  			return 1;

  		}

  		if (c == REPRINT_CHAR(tty) && L_ECHO(tty) && L_IEXTEN(tty)) {

  			size_t tail = ldata->canon_head;

  			finish_erasing(ldata);

  			echo_char(c, tty);

  			echo_char_raw('
  ', ldata);

  			while (MASK(tail) != MASK(ldata->read_head)) {

  				echo_char(read_buf(ldata, tail), tty);
  				tail++;

  			}

  			commit_echoes(tty);

  			return 0;

  		}
  		if (c == '
  ') {

  			if (L_ECHO(tty) || L_ECHONL(tty)) {

  				echo_char_raw('
  ', ldata);

  				commit_echoes(tty);

  			}
  			goto handle_newline;
  		}
  		if (c == EOF_CHAR(tty)) {

  			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)) {

  				/* Record the column of first canon char. */

  				if (ldata->canon_head == ldata->read_head)

  					echo_set_canon_col(ldata);

  				echo_char(c, tty);

  				commit_echoes(tty);

  			}
  			/*
  			 * XXX does PARMRK doubling happen for
  			 * EOL_CHAR and EOL2_CHAR?
  			 */

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

  				put_tty_queue(c, ldata);

  handle_newline:

  			set_bit(ldata->read_head & (N_TTY_BUF_SIZE - 1), ldata->read_flags);

  			put_tty_queue(c, ldata);

  			smp_store_release(&ldata->canon_head, ldata->read_head);

  			kill_fasync(&tty->fasync, SIGIO, POLL_IN);

  			wake_up_interruptible_poll(&tty->read_wait, EPOLLIN);

  			return 0;

  		}
  	}

  	if (L_ECHO(tty)) {

  		finish_erasing(ldata);

  		if (c == '
  ')

  			echo_char_raw('
  ', ldata);

  		else {
  			/* Record the column of first canon char. */

  			if (ldata->canon_head == ldata->read_head)

  				echo_set_canon_col(ldata);

  			echo_char(c, tty);
  		}

  		commit_echoes(tty);

  	}

  	/* PARMRK doubling check */
  	if (c == (unsigned char) '\377' && I_PARMRK(tty))

  		put_tty_queue(c, ldata);

  	put_tty_queue(c, ldata);

  	return 0;

  }

  static inline void
  n_tty_receive_char_inline(struct tty_struct *tty, unsigned char c)

  {
  	struct n_tty_data *ldata = tty->disc_data;

  	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) && I_IXANY(tty)) {
  		start_tty(tty);
  		process_echoes(tty);
  	}
  	if (L_ECHO(tty)) {
  		finish_erasing(ldata);
  		/* Record the column of first canon char. */
  		if (ldata->canon_head == ldata->read_head)
  			echo_set_canon_col(ldata);
  		echo_char(c, tty);
  		commit_echoes(tty);

  	}

  	/* PARMRK doubling check */
  	if (c == (unsigned char) '\377' && I_PARMRK(tty))

  		put_tty_queue(c, ldata);
  	put_tty_queue(c, ldata);
  }

  static void n_tty_receive_char(struct tty_struct *tty, unsigned char c)

  {
  	n_tty_receive_char_inline(tty, c);

  }

  static inline void

  n_tty_receive_char_fast(struct tty_struct *tty, unsigned char c)
  {
  	struct n_tty_data *ldata = tty->disc_data;

  	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) && I_IXANY(tty)) {
  		start_tty(tty);
  		process_echoes(tty);

  	}

  	if (L_ECHO(tty)) {
  		finish_erasing(ldata);
  		/* Record the column of first canon char. */
  		if (ldata->canon_head == ldata->read_head)
  			echo_set_canon_col(ldata);
  		echo_char(c, tty);
  		commit_echoes(tty);
  	}
  	put_tty_queue(c, ldata);