tty: Handle NULL tty->ldisc

BugLink: http://bugs.launchpad.net/bugs/1709126

In preparation of destroying line discipline on hangup, fix
ldisc core operations to properly handle when the tty's ldisc is
NULL.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit a570a49a)
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Marcelo Cerri <marcelo.cerri@canonical.com>
Acked-by: Benjamin M Romer <benjamin.romer@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/tty/tty_ldisc.c

@@ -262,7 +262,8 @@ const struct file_operations tty_ldiscs_proc_fops = {
 struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
 {
 	ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
-	WARN_ON(!tty->ldisc);
+	if (!tty->ldisc)
+		ldsem_up_read(&tty->ldisc_sem);
 	return tty->ldisc;
 }
 EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);
@@ -455,7 +456,7 @@ static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
 		if (ret)
 			clear_bit(TTY_LDISC_OPEN, &tty->flags);
 
-		tty_ldisc_debug(tty, "%p: opened\n", tty->ldisc);
+		tty_ldisc_debug(tty, "%p: opened\n", ld);
 		return ret;
 	}
 	return 0;
@@ -476,7 +477,7 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
 	clear_bit(TTY_LDISC_OPEN, &tty->flags);
 	if (ld->ops->close)
 		ld->ops->close(tty);
-	tty_ldisc_debug(tty, "%p: closed\n", tty->ldisc);
+	tty_ldisc_debug(tty, "%p: closed\n", ld);
 }
 
 /**
@@ -539,6 +540,11 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 	if (retval)
 		goto err;
 
+	if (!tty->ldisc) {
+		retval = -EIO;
+		goto out;
+	}
+
 	/* Check the no-op case */
 	if (tty->ldisc->ops->num == ldisc)
 		goto out;
@@ -654,7 +660,7 @@ void tty_ldisc_hangup(struct tty_struct *tty)
 	int reset = tty->driver->flags & TTY_DRIVER_RESET_TERMIOS;
 	int err = 0;
 
-	tty_ldisc_debug(tty, "%p: closing\n", tty->ldisc);
+	tty_ldisc_debug(tty, "%p: hangup\n", tty->ldisc);
 
 	ld = tty_ldisc_ref(tty);
 	if (ld != NULL) {
@@ -738,6 +744,8 @@ int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
 
 static void tty_ldisc_kill(struct tty_struct *tty)
 {
+	if (!tty->ldisc)
+		return;
 	/*
 	 * Now kill off the ldisc
 	 */