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	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 // 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>
 #include "tty.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;
 
     /* # of chars looked ahead (to find software flow control chars) */
     size_t lookahead_count;
 
     /* 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 void tty_copy(struct tty_struct *tty, void *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);
 
     if (n > size) {
         tty_audit_add_data(tty, from, size);
         memcpy(to, from, size);
         zero_buffer(tty, from, size);
         to += size;
         n -= size;
         from = ldata->read_buf;
     }
 
     tty_audit_add_data(tty, from, n);
     memcpy(to, from, n);
     zero_buffer(tty, from, n);
 }
 
 /**
  * n_tty_kick_worker - start input worker (if required)
  * @tty: terminal
  *
  * Re-schedules the flip buffer work if it may have stopped.
  *
  * Locking:
  *  * 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\n");
         /* 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\n");
         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.
  *
  * Locking:
  *  * 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
  * @ldata: line disc data 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;
 
     ldata->lookahead_count = 0;
 }
 
 static void n_tty_packet_mode_flush(struct tty_struct *tty)
 {
     unsigned long flags;
 
     if (tty->link->ctrl.packet) {
         spin_lock_irqsave(&tty->ctrl.lock, flags);
         tty->ctrl.pktstatus |= 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
  * @tty: terminal device
  *
  * 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 '\n':
         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\n", 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 = '\n';
             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.
  *
  * 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.
  *
  * Returns: the number of characters output.
  *
  * 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 '\n':
             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
  * @ldata: line disc data
  *
  * Echo user input back onto the screen. This must be called only when
  * L_ECHO(tty) is true. Called from the &tty_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 &tty_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.
  *
  * Locking: 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('\n', 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);
 }
 
 
 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);
     }
 }
 
 /**
  * isig         -   handle the ISIG optio
  * @sig: signal
  * @tty: terminal
  *
  * Called when a signal is being sent due to terminal input. Called from the
  * &tty_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 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.
  *
  * Locking: 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)\n", 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.
  *
  * Locking: 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);
 }
 
 static bool n_tty_is_char_flow_ctrl(struct tty_struct *tty, unsigned char c)
 {
     return c == START_CHAR(tty) || c == STOP_CHAR(tty);
 }
 
 /**
  * n_tty_receive_char_flow_ctrl - receive flow control chars
  * @tty: terminal device
  * @c: character
  * @lookahead_done: lookahead has processed this character already
  *
  * Receive and process flow control character actions.
  *
  * In case lookahead for flow control chars already handled the character in
  * advance to the normal receive, the actions are skipped during normal
  * receive.
  *
  * Returns true if @c is consumed as flow-control character, the character
  * must not be treated as normal character.
  */
 static bool n_tty_receive_char_flow_ctrl(struct tty_struct *tty, unsigned char c,
                      bool lookahead_done)
 {
     if (!n_tty_is_char_flow_ctrl(tty, c))
         return false;
 
     if (lookahead_done)
         return true;
 
     if (c == START_CHAR(tty)) {
         start_tty(tty);
         process_echoes(tty);
         return true;
     }
 
     /* STOP_CHAR */
     stop_tty(tty);
     return true;
 }
 
 static void n_tty_receive_char_special(struct tty_struct *tty, unsigned char c,
                        bool lookahead_done)
 {
     struct n_tty_data *ldata = tty->disc_data;
 
     if (I_IXON(tty) && n_tty_receive_char_flow_ctrl(tty, c, lookahead_done))
         return;
 
     if (L_ISIG(tty)) {
         if (c == INTR_CHAR(tty)) {
             n_tty_receive_signal_char(tty, SIGINT, c);
             return;
         } else if (c == QUIT_CHAR(tty)) {
             n_tty_receive_signal_char(tty, SIGQUIT, c);
             return;
         } else if (c == SUSP_CHAR(tty)) {
             n_tty_receive_signal_char(tty, SIGTSTP, c);
             return;
         }
     }
 
     if (tty->flow.stopped && !tty->flow.tco_stopped && I_IXON(tty) && I_IXANY(tty)) {
         start_tty(tty);
         process_echoes(tty);
     }
 
     if (c == '\r') {
         if (I_IGNCR(tty))
             return;
         if (I_ICRNL(tty))
             c = '\n';
     } else if (c == '\n' && 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;
         }
         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;
         }
         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('\n', ldata);
             while (MASK(tail) != MASK(ldata->read_head)) {
                 echo_char(read_buf(ldata, tail), tty);
                 tail++;
             }
             commit_echoes(tty);
             return;
         }
         if (c == '\n') {
             if (L_ECHO(tty) || L_ECHONL(tty)) {
                 echo_char_raw('\n', 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 | EPOLLRDNORM);
             return;
         }
     }
 
     if (L_ECHO(tty)) {
         finish_erasing(ldata);
         if (c == '\n')
             echo_char_raw('\n', 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);
 }
 
 /**
  * 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.
  *
  * Locking: n_tty_receive_buf()/producer path:
  *  caller holds non-exclusive %termios_rwsem
  *  publishes canon_head if canonical mode is active
  */
 static void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
 {
     struct n_tty_data *ldata = tty->disc_data;
 
     if (tty->flow.stopped && !tty->flow.tco_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_closing(struct tty_struct *tty, unsigned char c,
                        bool lookahead_done)
 {
     if (I_ISTRIP(tty))
         c &= 0x7f;
     if (I_IUCLC(tty) && L_IEXTEN(tty))
         c = tolower(c);
 
     if (I_IXON(tty)) {
         if (!n_tty_receive_char_flow_ctrl(tty, c, lookahead_done) &&
             tty->flow.stopped && !tty->flow.tco_stopped && I_IXANY(tty) &&
             c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) &&
             c != SUSP_CHAR(tty)) {
             start_tty(tty);
             process_echoes(tty);
         }
     }
 }
 
 static void
 n_tty_receive_char_flagged(struct tty_struct *tty, unsigned char c, char flag)
 {
     switch (flag) {
     case TTY_BREAK:
         n_tty_receive_break(tty);
         break;
     case TTY_PARITY:
     case TTY_FRAME:
         n_tty_receive_parity_error(tty, c);
         break;
     case TTY_OVERRUN:
         n_tty_receive_overrun(tty);
         break;
     default:
         tty_err(tty, "unknown flag %d\n", flag);
         break;
     }
 }
 
 static void
 n_tty_receive_char_lnext(struct tty_struct *tty, unsigned char c, char flag)
 {
     struct n_tty_data *ldata = tty->disc_data;
 
     ldata->lnext = 0;
     if (likely(flag == TTY_NORMAL)) {
         if (I_ISTRIP(tty))
             c &= 0x7f;
         if (I_IUCLC(tty) && L_IEXTEN(tty))
             c = tolower(c);
         n_tty_receive_char(tty, c);
     } else
         n_tty_receive_char_flagged(tty, c, flag);
 }
 
 /* Caller must ensure count > 0 */
 static void n_tty_lookahead_flow_ctrl(struct tty_struct *tty, const unsigned char *cp,
                       const unsigned char *fp, unsigned int count)
 {
     struct n_tty_data *ldata = tty->disc_data;
     unsigned char flag = TTY_NORMAL;
 
     ldata->lookahead_count += count;
 
     if (!I_IXON(tty))
         return;
 
     while (count--) {
         if (fp)
             flag = *fp++;
         if (likely(flag == TTY_NORMAL))
             n_tty_receive_char_flow_ctrl(tty, *cp, false);
         cp++;
     }
 }
 
 static void
 n_tty_receive_buf_real_raw(struct tty_struct *tty, const unsigned char *cp,
                const char *fp, int count)
 {
     struct n_tty_data *ldata = tty->disc_data;
     size_t n, head;
 
     head = ldata->read_head & (N_TTY_BUF_SIZE - 1);
     n = min_t(size_t, count, N_TTY_BUF_SIZE - head);
     memcpy(read_buf_addr(ldata, head), cp, n);
     ldata->read_head += n;
     cp += n;
     count -= n;
 
     head = ldata->read_head & (N_TTY_BUF_SIZE - 1);
     n = min_t(size_t, count, N_TTY_BUF_SIZE - head);
     memcpy(read_buf_addr(ldata, head), cp, n);
     ldata->read_head += n;
 }
 
 static void
 n_tty_receive_buf_raw(struct tty_struct *tty, const unsigned char *cp,
               const char *fp, int count)
 {
     struct n_tty_data *ldata = tty->disc_data;
     char flag = TTY_NORMAL;
 
     while (count--) {
         if (fp)
             flag = *fp++;
         if (likely(flag == TTY_NORMAL))
             put_tty_queue(*cp++, ldata);
         else
             n_tty_receive_char_flagged(tty, *cp++, flag);
     }
 }
 
 static void
 n_tty_receive_buf_closing(struct tty_struct *tty, const unsigned char *cp,
               const char *fp, int count, bool lookahead_done)
 {
     char flag = TTY_NORMAL;
 
     while (count--) {
         if (fp)
             flag = *fp++;
         if (likely(flag == TTY_NORMAL))
             n_tty_receive_char_closing(tty, *cp++, lookahead_done);
     }
 }
 
 static void n_tty_receive_buf_standard(struct tty_struct *tty,
         const unsigned char *cp, const char *fp, int count, bool lookahead_done)
 {
     struct n_tty_data *ldata = tty->disc_data;
     char flag = TTY_NORMAL;
 
     while (count--) {
         unsigned char c = *cp++;
 
         if (fp)
             flag = *fp++;
 
         if (ldata->lnext) {
             n_tty_receive_char_lnext(tty, c, flag);
             continue;
         }
 
         if (unlikely(flag != TTY_NORMAL)) {
             n_tty_receive_char_flagged(tty, c, flag);
             continue;
         }
 
         if (I_ISTRIP(tty))
             c &= 0x7f;
         if (I_IUCLC(tty) && L_IEXTEN(tty))
             c = tolower(c);
         if (L_EXTPROC(tty)) {
             put_tty_queue(c, ldata);
             continue;
         }
 
         if (test_bit(c, ldata->char_map))
             n_tty_receive_char_special(tty, c, lookahead_done);
         else
             n_tty_receive_char(tty, c);
     }
 }
 
 static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
               const char *fp, int count)
 {
     struct n_tty_data *ldata = tty->disc_data;
     bool preops = I_ISTRIP(tty) || (I_IUCLC(tty) && L_IEXTEN(tty));
     size_t la_count = min_t(size_t, ldata->lookahead_count, count);
 
     if (ldata->real_raw)
         n_tty_receive_buf_real_raw(tty, cp, fp, count);
     else if (ldata->raw || (L_EXTPROC(tty) && !preops))
         n_tty_receive_buf_raw(tty, cp, fp, count);
     else if (tty->closing && !L_EXTPROC(tty)) {
         if (la_count > 0)
             n_tty_receive_buf_closing(tty, cp, fp, la_count, true);
         if (count > la_count)
             n_tty_receive_buf_closing(tty, cp, fp, count - la_count, false);
     } else {
         if (la_count > 0)
             n_tty_receive_buf_standard(tty, cp, fp, la_count, true);
         if (count > la_count)
             n_tty_receive_buf_standard(tty, cp, fp, count - la_count, false);
 
         flush_echoes(tty);
         if (tty->ops->flush_chars)
             tty->ops->flush_chars(tty);
     }
 
     ldata->lookahead_count -= la_count;
 
     if (ldata->icanon && !L_EXTPROC(tty))
         return;
 
     /* publish read_head to consumer */
     smp_store_release(&ldata->commit_head, ldata->read_head);
 
     if (read_cnt(ldata)) {
         kill_fasync(&tty->fasync, SIGIO, POLL_IN);
         wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
     }
 }
 
 /**
  * n_tty_receive_buf_common -   process input
  * @tty: device to receive input
  * @cp: input chars
  * @fp: flags for each char (if %NULL, all chars are %TTY_NORMAL)
  * @count: number of input chars in @cp
  * @flow: enable flow control
  *
  * 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()).
  *
  * Returns: the # of input chars from @cp which were processed.
  *
  * In canonical mode, the maximum line length is 4096 chars (including the line
  * termination char); lines longer than 4096 chars are truncated. After 4095
  * chars, input data is still processed but not stored. Overflow processing
  * ensures the tty can always receive more input until at least one line can be
  * read.
  *
  * In non-canonical mode, the read buffer will only accept 4095 chars; this
  * provides the necessary space for a newline char if the input mode is
  * switched to canonical.
  *
  * Note it is possible for the read buffer to _contain_ 4096 chars in
  * non-canonical mode: the read buffer could already contain the maximum canon
  * line of 4096 chars when the mode is switched to non-canonical.
  *
  * Locking: n_tty_receive_buf()/producer path:
  *  claims non-exclusive %termios_rwsem
  *  publishes commit_head or canon_head
  */
 static int
 n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
              const char *fp, int count, int flow)
 {
     struct n_tty_data *ldata = tty->disc_data;
     int room, n, rcvd = 0, overflow;
 
     down_read(&tty->termios_rwsem);
 
     do {
         /*
          * When PARMRK is set, each input char may take up to 3 chars
          * in the read buf; reduce the buffer space avail by 3x
          *
          * 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.
          *
          * paired with store in *_copy_from_read_buf() -- guarantees
          * the consumer has loaded the data in read_buf up to the new
          * read_tail (so this producer will not overwrite unread data)
          */
         size_t tail = smp_load_acquire(&ldata->read_tail);
 
         room = N_TTY_BUF_SIZE - (ldata->read_head - tail);
         if (I_PARMRK(tty))
             room = (room + 2) / 3;
         room--;
         if (room <= 0) {
             overflow = ldata->icanon && ldata->canon_head == tail;
             if (overflow && room < 0)
                 ldata->read_head--;
             room = overflow;
             ldata->no_room = flow && !room;
         } else
             overflow = 0;
 
         n = min(count, room);
         if (!n)
             break;
 
         /* ignore parity errors if handling overflow */
         if (!overflow || !fp || *fp != TTY_PARITY)
             __receive_buf(tty, cp, fp, n);
 
         cp += n;
         if (fp)
             fp += n;
         count -= n;
         rcvd += n;
     } while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));
 
     tty->receive_room = room;
 
     /* Unthrottle if handling overflow on pty */
     if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {
         if (overflow) {
             tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
             tty_unthrottle_safe(tty);
             __tty_set_flow_change(tty, 0);
         }
     } else
         n_tty_check_throttle(tty);
 
     up_read(&tty->termios_rwsem);
 
     return rcvd;
 }
 
 static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
                   const char *fp, int count)
 {
     n_tty_receive_buf_common(tty, cp, fp, count, 0);
 }
 
 static int n_tty_receive_buf2(struct tty_struct *tty, const unsigned char *cp,
                   const char *fp, int count)
 {
     return n_tty_receive_buf_common(tty, cp, fp, count, 1);
 }
 
 /**
  * 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.
  *
  * Locking: Caller holds @tty->termios_rwsem
  */
 static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
 {
     struct n_tty_data *ldata = tty->disc_data;
 
     if (!old || (old->c_lflag ^ tty->termios.c_lflag) & (ICANON | EXTPROC)) {
         bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
         ldata->line_start = ldata->read_tail;
         if (!L_ICANON(tty) || !read_cnt(ldata)) {
             ldata->canon_head = ldata->read_tail;
             ldata->push = 0;
         } else {
             set_bit((ldata->read_head - 1) & (N_TTY_BUF_SIZE - 1),
                 ldata->read_flags);
             ldata->canon_head = ldata->read_head;
             ldata->push = 1;
         }
         ldata->commit_head = ldata->read_head;
         ldata->erasing = 0;
         ldata->lnext = 0;
     }
 
     ldata->icanon = (L_ICANON(tty) != 0);
 
     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)) {
         bitmap_zero(ldata->char_map, 256);
 
         if (I_IGNCR(tty) || I_ICRNL(tty))
             set_bit('\r', ldata->char_map);
         if (I_INLCR(tty))
             set_bit('\n', ldata->char_map);
 
         if (L_ICANON(tty)) {
             set_bit(ERASE_CHAR(tty), ldata->char_map);
             set_bit(KILL_CHAR(tty), ldata->char_map);
             set_bit(EOF_CHAR(tty), ldata->char_map);
             set_bit('\n', ldata->char_map);
             set_bit(EOL_CHAR(tty), ldata->char_map);
             if (L_IEXTEN(tty)) {
                 set_bit(WERASE_CHAR(tty), ldata->char_map);
                 set_bit(LNEXT_CHAR(tty), ldata->char_map);
                 set_bit(EOL2_CHAR(tty), ldata->char_map);
                 if (L_ECHO(tty))
                     set_bit(REPRINT_CHAR(tty),
                         ldata->char_map);
             }
         }
         if (I_IXON(tty)) {
             set_bit(START_CHAR(tty), ldata->char_map);
             set_bit(STOP_CHAR(tty), ldata->char_map);
         }
         if (L_ISIG(tty)) {
             set_bit(INTR_CHAR(tty), ldata->char_map);
             set_bit(QUIT_CHAR(tty), ldata->char_map);
             set_bit(SUSP_CHAR(tty), ldata->char_map);
         }
         clear_bit(__DISABLED_CHAR, ldata->char_map);
         ldata->raw = 0;
         ldata->real_raw = 0;
     } else {
         ldata->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))
             ldata->real_raw = 1;
         else
             ldata->real_raw = 0;
     }
     /*
      * Fix tty hang when I_IXON(tty) is cleared, but the tty
      * been stopped by STOP_CHAR(tty) before it.
      */
     if (!I_IXON(tty) && old && (old->c_iflag & IXON) && !tty->flow.tco_stopped) {
         start_tty(tty);
         process_echoes(tty);
     }
 
     /* The termios change make the tty ready for I/O */
     wake_up_interruptible(&tty->write_wait);
     wake_up_interruptible(&tty->read_wait);
 }
 
 /**
  * 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)
 {
     struct n_tty_data *ldata = tty->disc_data;
 
     if (tty->link)
         n_tty_packet_mode_flush(tty);
 
     down_write(&tty->termios_rwsem);
     vfree(ldata);
     tty->disc_data = NULL;
     up_write(&tty->termios_rwsem);
 }
 
 /**
  * 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)
 {
     struct n_tty_data *ldata;
 
     /* Currently a malloc failure here can panic */
     ldata = vzalloc(sizeof(*ldata));
     if (!ldata)
         return -ENOMEM;
 
     ldata->overrun_time = jiffies;
     mutex_init(&ldata->atomic_read_lock);
     mutex_init(&ldata->output_lock);
 
     tty->disc_data = ldata;
     tty->closing = 0;
     /* indicate buffer work may resume */
     clear_bit(TTY_LDISC_HALTED, &tty->flags);
     n_tty_set_termios(tty, NULL);
     tty_unthrottle(tty);
     return 0;
 }
 
 static inline int input_available_p(struct tty_struct *tty, int poll)
 {
     struct n_tty_data *ldata = tty->disc_data;
     int amt = poll && !TIME_CHAR(tty) && MIN_CHAR(tty) ? MIN_CHAR(tty) : 1;
 
     if (ldata->icanon && !L_EXTPROC(tty))
         return ldata->canon_head != ldata->read_tail;
     else
         return ldata->commit_head - ldata->read_tail >= amt;
 }
 
 /**
  * copy_from_read_buf   -   copy read data directly
  * @tty: terminal device
  * @kbp: data
  * @nr: size of data
  *
  * Helper function to speed up n_tty_read(). It is only called when %ICANON is
  * off; it copies characters straight from the tty queue.
  *
  * Returns: true if it successfully copied data, but there is still more data
  * to be had.
  *
  * Locking:
  *  * called under the @ldata->atomic_read_lock sem
  *  * n_tty_read()/consumer path:
  *      caller holds non-exclusive %termios_rwsem;
  *      read_tail published
  */
 static bool copy_from_read_buf(struct tty_struct *tty,
                       unsigned char **kbp,
                       size_t *nr)
 
 {
     struct n_tty_data *ldata = tty->disc_data;
     size_t n;
     bool is_eof;
     size_t head = smp_load_acquire(&ldata->commit_head);
     size_t tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1);
 
     n = min(head - ldata->read_tail, N_TTY_BUF_SIZE - tail);
     n = min(*nr, n);
     if (n) {
         unsigned char *from = read_buf_addr(ldata, tail);
         memcpy(*kbp, from, n);
         is_eof = n == 1 && *from == EOF_CHAR(tty);
         tty_audit_add_data(tty, from, n);
         zero_buffer(tty, from, n);
         smp_store_release(&ldata->read_tail, ldata->read_tail + n);
         /* Turn single EOF into zero-length read */
         if (L_EXTPROC(tty) && ldata->icanon && is_eof &&
             (head == ldata->read_tail))
             return false;
         *kbp += n;
         *nr -= n;
 
         /* If we have more to copy, let the caller know */
         return head != ldata->read_tail;
     }
     return false;
 }
 
 /**
  * canon_copy_from_read_buf -   copy read data in canonical mode
  * @tty: terminal device
  * @kbp: data
  * @nr: size of data
  *
  * Helper function for n_tty_read(). It is only called when %ICANON is on; it
  * copies one line of input up to and including the line-delimiting character
  * into the result buffer.
  *
  * Note: When termios is changed from non-canonical to canonical mode and the
  * read buffer contains data, n_tty_set_termios() simulates an EOF push (as if
  * C-d were input) _without_ the %DISABLED_CHAR in the buffer. This causes data
  * already processed as input to be immediately available as input although a
  * newline has not been received.
  *
  * Locking:
  *  * called under the %atomic_read_lock mutex
  *  * n_tty_read()/consumer path:
  *  caller holds non-exclusive %termios_rwsem;
  *  read_tail published
  */
 static bool canon_copy_from_read_buf(struct tty_struct *tty,
                      unsigned char **kbp,
                      size_t *nr)
 {
     struct n_tty_data *ldata = tty->disc_data;
     size_t n, size, more, c;
     size_t eol;
     size_t tail, canon_head;
     int found = 0;
 
     /* N.B. avoid overrun if nr == 0 */
     if (!*nr)
         return false;
 
     canon_head = smp_load_acquire(&ldata->canon_head);
     n = min(*nr, canon_head - ldata->read_tail);
 
     tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1);
     size = min_t(size_t, tail + n, N_TTY_BUF_SIZE);
 
     n_tty_trace("%s: nr:%zu tail:%zu n:%zu size:%zu\n",
             __func__, *nr, tail, n, size);
 
     eol = find_next_bit(ldata->read_flags, size, tail);
     more = n - (size - tail);
     if (eol == N_TTY_BUF_SIZE && more) {
         /* scan wrapped without finding set bit */
         eol = find_first_bit(ldata->read_flags, more);
         found = eol != more;
     } else
         found = eol != size;
 
     n = eol - tail;
     if (n > N_TTY_BUF_SIZE)
         n += N_TTY_BUF_SIZE;
     c = n + found;
 
     if (!found || read_buf(ldata, eol) != __DISABLED_CHAR)
         n = c;
 
     n_tty_trace("%s: eol:%zu found:%d n:%zu c:%zu tail:%zu more:%zu\n",
             __func__, eol, found, n, c, tail, more);
 
     tty_copy(tty, *kbp, tail, n);
     *kbp += n;
     *nr -= n;
 
     if (found)
         clear_bit(eol, ldata->read_flags);
     smp_store_release(&ldata->read_tail, ldata->read_tail + c);
 
     if (found) {
         if (!ldata->push)
             ldata->line_start = ldata->read_tail;
         else
             ldata->push = 0;
         tty_audit_push();
         return false;
     }
 
     /* No EOL found - do a continuation retry if there is more data */
     return ldata->read_tail != canon_head;
 }
 
 /*
  * If we finished a read at the exact location of an
  * EOF (special EOL character that's a __DISABLED_CHAR)
  * in the stream, silently eat the EOF.
  */
 static void canon_skip_eof(struct tty_struct *tty)
 {
     struct n_tty_data *ldata = tty->disc_data;
     size_t tail, canon_head;
 
     canon_head = smp_load_acquire(&ldata->canon_head);
     tail = ldata->read_tail;
 
     // No data?
     if (tail == canon_head)
         return;
 
     // See if the tail position is EOF in the circular buffer
     tail &= (N_TTY_BUF_SIZE - 1);
     if (!test_bit(tail, ldata->read_flags))
         return;
     if (read_buf(ldata, tail) != __DISABLED_CHAR)
         return;
 
     // Clear the EOL bit, skip the EOF char.
     clear_bit(tail, ldata->read_flags);
     smp_store_release(&ldata->read_tail, ldata->read_tail + 1);
 }
 
 /**
  * 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.
  *
  * Locking:
  *  * redirected write test is safe
  *  * current->signal->tty check is safe
  *  * ctrl.lock to safely reference @tty->ctrl.pgrp
  */
 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_iter == redirected_tty_write)
         return 0;
 
     return __tty_check_change(tty, SIGTTIN);
 }
 
 
 /**
  * n_tty_read       -   read function for tty
  * @tty: tty device
  * @file: file object
  * @kbuf: kernelspace buffer pointer
  * @nr: size of I/O
  * @cookie: if non-%NULL, this is a continuation read
  * @offset: where to continue reading from (unused in n_tty)
  *
  * 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.
  *
  * Locking: n_tty_read()/consumer path:
  *  claims non-exclusive termios_rwsem;
  *  publishes read_tail
  */
 static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
               unsigned char *kbuf, size_t nr,
               void **cookie, unsigned long offset)
 {
     struct n_tty_data *ldata = tty->disc_data;
     unsigned char *kb = kbuf;
     DEFINE_WAIT_FUNC(wait, woken_wake_function);
     int c;
     int minimum, time;
     ssize_t retval = 0;
     long timeout;
     bool packet;
     size_t tail;
 
     /*
      * Is this a continuation of a read started earler?
      *
      * If so, we still hold the atomic_read_lock and the
      * termios_rwsem, and can just continue to copy data.
      */
     if (*cookie) {
         if (ldata->icanon && !L_EXTPROC(tty)) {
             /*
              * If we have filled the user buffer, see
              * if we should skip an EOF character before
              * releasing the lock and returning done.
              */
             if (!nr)
                 canon_skip_eof(tty);
             else if (canon_copy_from_read_buf(tty, &kb, &nr))
                 return kb - kbuf;
         } else {
             if (copy_from_read_buf(tty, &kb, &nr))
                 return kb - kbuf;
         }
 
         /* No more data - release locks and stop retries */
         n_tty_kick_worker(tty);
         n_tty_check_unthrottle(tty);
         up_read(&tty->termios_rwsem);
         mutex_unlock(&ldata->atomic_read_lock);
         *cookie = NULL;
         return kb - kbuf;
     }
 
     c = job_control(tty, file);
     if (c < 0)
         return c;
 
     /*
      *  Internal serialization of reads.
      */
     if (file->f_flags & O_NONBLOCK) {
         if (!mutex_trylock(&ldata->atomic_read_lock))
             return -EAGAIN;
     } else {
         if (mutex_lock_interruptible(&ldata->atomic_read_lock))
             return -ERESTARTSYS;
     }
 
     down_read(&tty->termios_rwsem);
 
     minimum = time = 0;
     timeout = MAX_SCHEDULE_TIMEOUT;
     if (!ldata->icanon) {
         minimum = MIN_CHAR(tty);
         if (minimum) {
             time = (HZ / 10) * TIME_CHAR(tty);
         } else {
             timeout = (HZ / 10) * TIME_CHAR(tty);
             minimum = 1;
         }
     }
 
     packet = tty->ctrl.packet;
     tail = ldata->read_tail;
 
     add_wait_queue(&tty->read_wait, &wait);
     while (nr) {
         /* First test for status change. */
         if (packet && tty->link->ctrl.pktstatus) {
             unsigned char cs;
             if (kb != kbuf)
                 break;
             spin_lock_irq(&tty->link->ctrl.lock);
             cs = tty->link->ctrl.pktstatus;
             tty->link->ctrl.pktstatus = 0;
             spin_unlock_irq(&tty->link->ctrl.lock);
             *kb++ = cs;
             nr--;
             break;
         }
 
         if (!input_available_p(tty, 0)) {
             up_read(&tty->termios_rwsem);
             tty_buffer_flush_work(tty->port);
             down_read(&tty->termios_rwsem);
             if (!input_available_p(tty, 0)) {
                 if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
                     retval = -EIO;
                     break;
                 }
                 if (tty_hung_up_p(file))
                     break;
                 /*
                  * Abort readers for ttys which never actually
                  * get hung up.  See __tty_hangup().
                  */
                 if (test_bit(TTY_HUPPING, &tty->flags))
                     break;
                 if (!timeout)
                     break;
                 if (tty_io_nonblock(tty, file)) {
                     retval = -EAGAIN;
                     break;
                 }
                 if (signal_pending(current)) {
                     retval = -ERESTARTSYS;
                     break;
                 }
                 up_read(&tty->termios_rwsem);
 
                 timeout = wait_woken(&wait, TASK_INTERRUPTIBLE,
                         timeout);
 
                 down_read(&tty->termios_rwsem);
                 continue;
             }
         }
 
         if (ldata->icanon && !L_EXTPROC(tty)) {
             if (canon_copy_from_read_buf(tty, &kb, &nr))
                 goto more_to_be_read;
         } else {
             /* Deal with packet mode. */
             if (packet && kb == kbuf) {
                 *kb++ = TIOCPKT_DATA;
                 nr--;
             }
 
             /*
              * Copy data, and if there is more to be had
              * and we have nothing more to wait for, then
              * let's mark us for retries.
              *
              * NOTE! We return here with both the termios_sem
              * and atomic_read_lock still held, the retries
              * will release them when done.
              */
             if (copy_from_read_buf(tty, &kb, &nr) && kb - kbuf >= minimum) {
 more_to_be_read:
                 remove_wait_queue(&tty->read_wait, &wait);
                 *cookie = cookie;
                 return kb - kbuf;
             }
         }
 
         n_tty_check_unthrottle(tty);
 
         if (kb - kbuf >= minimum)
             break;
         if (time)
             timeout = time;
     }
     if (tail != ldata->read_tail)
         n_tty_kick_worker(tty);
     up_read(&tty->termios_rwsem);
 
     remove_wait_queue(&tty->read_wait, &wait);
     mutex_unlock(&ldata->atomic_read_lock);
 
     if (kb - kbuf)
         retval = kb - kbuf;
 
     return retval;
 }
 
 /**
  * n_tty_write      -   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.
  * Since the receive code will echo characters, thus calling driver write
  * methods, the %output_lock is used in the output processing functions called
  * here as well as in the echo processing function to protect the column state
  * and space left in the buffer.
  *
  * This code must be sure never to sleep through a hangup.
  *
  * Locking: output_lock to protect column state and space left
  *   (note that the process_output*() functions take this lock themselves)
  */
 
 static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
                const unsigned char *buf, size_t nr)
 {
     const unsigned char *b = buf;
     DEFINE_WAIT_FUNC(wait, woken_wake_function);
     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_iter != redirected_tty_write) {
         retval = tty_check_change(tty);
         if (retval)
             return retval;
     }
 
     down_read(&tty->termios_rwsem);
 
     /* Write out any echoed characters that are still pending */
     process_echoes(tty);
 
     add_wait_queue(&tty->write_wait, &wait);
     while (1) {
         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)) {
             while (nr > 0) {
                 ssize_t num = process_output_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 (process_output(c, tty) < 0)
                     break;
                 b++; nr--;
             }
             if (tty->ops->flush_chars)
                 tty->ops->flush_chars(tty);
         } else {
             struct n_tty_data *ldata = tty->disc_data;
 
             while (nr > 0) {
                 mutex_lock(&ldata->output_lock);
                 c = tty->ops->write(tty, b, nr);
                 mutex_unlock(&ldata->output_lock);
                 if (c < 0) {
                     retval = c;
                     goto break_out;
                 }
                 if (!c)
                     break;
                 b += c;
                 nr -= c;
             }
         }
         if (!nr)
             break;
         if (tty_io_nonblock(tty, file)) {
             retval = -EAGAIN;
             break;
         }
         up_read(&tty->termios_rwsem);
 
         wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
 
         down_read(&tty->termios_rwsem);
     }
 break_out:
     remove_wait_queue(&tty->write_wait, &wait);
     if (nr && tty->fasync)
         set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
     up_read(&tty->termios_rwsem);
     return (b - buf) ? b - buf : retval;
 }
 
 /**
  * n_tty_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.
  *
  * Locking: called without the kernel lock held -- fine.
  */
 static __poll_t n_tty_poll(struct tty_struct *tty, struct file *file,
                             poll_table *wait)
 {
     __poll_t mask = 0;
 
     poll_wait(file, &tty->read_wait, wait);
     poll_wait(file, &tty->write_wait, wait);
     if (input_available_p(tty, 1))
         mask |= EPOLLIN | EPOLLRDNORM;
     else {
         tty_buffer_flush_work(tty->port);
         if (input_available_p(tty, 1))
             mask |= EPOLLIN | EPOLLRDNORM;
     }
     if (tty->ctrl.packet && tty->link->ctrl.pktstatus)
         mask |= EPOLLPRI | EPOLLIN | EPOLLRDNORM;
     if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
         mask |= EPOLLHUP;
     if (tty_hung_up_p(file))
         mask |= EPOLLHUP;
     if (tty->ops->write && !tty_is_writelocked(tty) &&
             tty_chars_in_buffer(tty) < WAKEUP_CHARS &&
             tty_write_room(tty) > 0)
         mask |= EPOLLOUT | EPOLLWRNORM;
     return mask;
 }
 
 static unsigned long inq_canon(struct n_tty_data *ldata)
 {
     size_t nr, head, tail;
 
     if (ldata->canon_head == ldata->read_tail)
         return 0;
     head = ldata->canon_head;
     tail = ldata->read_tail;
     nr = head - tail;
     /* Skip EOF-chars.. */
     while (MASK(head) != MASK(tail)) {
         if (test_bit(tail & (N_TTY_BUF_SIZE - 1), ldata->read_flags) &&
             read_buf(ldata, tail) == __DISABLED_CHAR)
             nr--;
         tail++;
     }
     return nr;
 }
 
 static int n_tty_ioctl(struct tty_struct *tty, unsigned int cmd,
                unsigned long arg)
 {
     struct n_tty_data *ldata = tty->disc_data;
     int retval;
 
     switch (cmd) {
     case TIOCOUTQ:
         return put_user(tty_chars_in_buffer(tty), (int __user *) arg);
     case TIOCINQ:
         down_write(&tty->termios_rwsem);
         if (L_ICANON(tty) && !L_EXTPROC(tty))
             retval = inq_canon(ldata);
         else
             retval = read_cnt(ldata);
         up_write(&tty->termios_rwsem);
         return put_user(retval, (unsigned int __user *) arg);
     default:
         return n_tty_ioctl_helper(tty, cmd, arg);
     }
 }
 
 static struct tty_ldisc_ops n_tty_ops = {
     .owner       = THIS_MODULE,
     .num         = N_TTY,
     .name            = "n_tty",
     .open            = n_tty_open,
     .close           = n_tty_close,
     .flush_buffer    = n_tty_flush_buffer,
     .read            = n_tty_read,
     .write           = n_tty_write,
     .ioctl           = n_tty_ioctl,
     .set_termios     = n_tty_set_termios,
     .poll            = n_tty_poll,
     .receive_buf     = n_tty_receive_buf,
     .write_wakeup    = n_tty_write_wakeup,
     .receive_buf2    = n_tty_receive_buf2,
     .lookahead_buf   = n_tty_lookahead_flow_ctrl,
 };
 
 /**
  *  n_tty_inherit_ops   -   inherit N_TTY methods
  *  @ops: struct tty_ldisc_ops where to save N_TTY methods
  *
  *  Enables a 'subclass' line discipline to 'inherit' N_TTY methods.
  */
 
 void n_tty_inherit_ops(struct tty_ldisc_ops *ops)
 {
     *ops = n_tty_ops;
     ops->owner = NULL;
 }
 EXPORT_SYMBOL_GPL(n_tty_inherit_ops);
 
 void __init n_tty_init(void)
 {
     tty_register_ldisc(&n_tty_ops);
 }

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