tty: Prepare for destroying line discipline on hangup

tty file_operations (read/write/ioctl) wait for the ldisc reference
indefinitely (until ldisc lifetime events, such as hangup or TIOCSETD,
finish). Since hangup now destroys the ldisc and does not instance
another copy, file_operations must now be prepared to receive a NULL
ldisc reference from tty_ldisc_ref_wait():

CPU 0                                   CPU 1
-----                                   -----
(*f_op->read)() => tty_read()
                                        __tty_hangup()
                                        ...
                                        f_op = &hung_up_tty_fops;
                                        ...
                                        tty_ldisc_hangup()
                                           tty_ldisc_lock()
                                           tty_ldisc_kill()
                                              tty->ldisc = NULL
                                           tty_ldisc_unlock()
ld = tty_ldisc_ref_wait()
/* ld == NULL */

Instead, the action taken now is to return the same value as if the
tty had been hungup a moment earlier:

CPU 0                                   CPU 1
-----                                   -----
                                        __tty_hangup()
                                        ...
                                        f_op = &hung_up_tty_fops;
(*f_op->read)() => hung_up_tty_read()
return 0;
                                        ...
                                        tty_ldisc_hangup()
                                           tty_ldisc_lock()
                                           tty_ldisc_kill()
                                              tty->ldisc = NULL
                                           tty_ldisc_unlock()

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1068,6 +1068,8 @@ static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
 	/* We want to wait for the line discipline to sort out in this
 	   situation */
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return hung_up_tty_read(file, buf, count, ppos);
 	if (ld->ops->read)
 		i = ld->ops->read(tty, file, buf, count);
 	else
@@ -1242,6 +1244,8 @@ static ssize_t tty_write(struct file *file, const char __user *buf,
 	if (tty->ops->write_room == NULL)
 		tty_err(tty, "missing write_room method\n");
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return hung_up_tty_write(file, buf, count, ppos);
 	if (!ld->ops->write)
 		ret = -EIO;
 	else
@@ -2201,6 +2205,8 @@ static unsigned int tty_poll(struct file *filp, poll_table *wait)
 		return 0;
 
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return hung_up_tty_poll(filp, wait);
 	if (ld->ops->poll)
 		ret = ld->ops->poll(tty, filp, wait);
 	tty_ldisc_deref(ld);
@@ -2290,6 +2296,8 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
 		return -EFAULT;
 	tty_audit_tiocsti(tty, ch);
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return -EIO;
 	ld->ops->receive_buf(tty, &ch, &mbz, 1);
 	tty_ldisc_deref(ld);
 	return 0;
@@ -2682,6 +2690,8 @@ static int tiocgetd(struct tty_struct *tty, int __user *p)
 	int ret;
 
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return -EIO;
 	ret = put_user(ld->ops->num, p);
 	tty_ldisc_deref(ld);
 	return ret;
@@ -2979,6 +2989,8 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			return retval;
 	}
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return hung_up_tty_ioctl(file, cmd, arg);
 	retval = -EINVAL;
 	if (ld->ops->ioctl) {
 		retval = ld->ops->ioctl(tty, file, cmd, arg);
@@ -3007,6 +3019,8 @@ static long tty_compat_ioctl(struct file *file, unsigned int cmd,
 	}
 
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return hung_up_tty_compat_ioctl(file, cmd, arg);
 	if (ld->ops->compat_ioctl)
 		retval = ld->ops->compat_ioctl(tty, file, cmd, arg);
 	else

drivers/tty/tty_ldisc.c

@@ -262,8 +262,8 @@ const struct file_operations tty_ldiscs_proc_fops = {
  *	against a discipline change, such as an existing ldisc reference
  *	(which we check for)
  *
- *	Note: only callable from a file_operations routine (which
- *	guarantees tty->ldisc != NULL when the lock is acquired).
+ *	Note: a file_operations routine (read/poll/write) should use this
+ *	function to wait for any ldisc lifetime events to finish.
  */
 
 struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)

drivers/tty/vt/selection.c

@@ -347,6 +347,8 @@ int paste_selection(struct tty_struct *tty)
 	console_unlock();
 
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return -EIO;	/* ldisc was hung up */
 	tty_buffer_lock_exclusive(&vc->port);
 
 	add_wait_queue(&vc->paste_wait, &wait);