1 files changed, 69 insertions, 16 deletions

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index b0500a0a87b8..e4603b09863a 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -492,6 +492,41 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
 }
 
 /**
+ *      tty_ldisc_restore       -       helper for tty ldisc change
+ *      @tty: tty to recover
+ *      @old: previous ldisc
+ *
+ *      Restore the previous line discipline or N_TTY when a line discipline
+ *      change fails due to an open error
+ */
+
+static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
+{
+        struct tty_ldisc *new_ldisc;
+        int r;
+
+        /* There is an outstanding reference here so this is safe */
+        old = tty_ldisc_get(tty, old->ops->num);
+        WARN_ON(IS_ERR(old));
+        tty->ldisc = old;
+        tty_set_termios_ldisc(tty, old->ops->num);
+        if (tty_ldisc_open(tty, old) < 0) {
+                tty_ldisc_put(old);
+                /* This driver is always present */
+                new_ldisc = tty_ldisc_get(tty, N_TTY);
+                if (IS_ERR(new_ldisc))
+                        panic("n_tty: get");
+                tty->ldisc = new_ldisc;
+                tty_set_termios_ldisc(tty, N_TTY);
+                r = tty_ldisc_open(tty, new_ldisc);
+                if (r < 0)
+                        panic("Couldn't open N_TTY ldisc for "
+                              "%s --- error %d.",
+                              tty_name(tty), r);
+        }
+}
+
+/**
  *      tty_set_ldisc           -       set line discipline
  *      @tty: the terminal to set
  *      @ldisc: the line discipline
@@ -504,7 +539,12 @@ static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
 
 int tty_set_ldisc(struct tty_struct *tty, int disc)
 {
-        int retval, old_disc;
+        int retval;
+        struct tty_ldisc *old_ldisc, *new_ldisc;
+
+        new_ldisc = tty_ldisc_get(tty, disc);
+        if (IS_ERR(new_ldisc))
+                return PTR_ERR(new_ldisc);
 
         tty_lock(tty);
         retval = tty_ldisc_lock(tty, 5 * HZ);
@@ -517,8 +557,7 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
         }
 
         /* Check the no-op case */
-        old_disc = tty->ldisc->ops->num;
-        if (old_disc == disc)
+        if (tty->ldisc->ops->num == disc)
                 goto out;
 
         if (test_bit(TTY_HUPPED, &tty->flags)) {
@@ -527,25 +566,34 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
                 goto out;
         }
 
-        retval = tty_ldisc_reinit(tty, disc);
+        old_ldisc = tty->ldisc;
+
+        /* Shutdown the old discipline. */
+        tty_ldisc_close(tty, old_ldisc);
+
+        /* Now set up the new line discipline. */
+        tty->ldisc = new_ldisc;
+        tty_set_termios_ldisc(tty, disc);
+
+        retval = tty_ldisc_open(tty, new_ldisc);
         if (retval < 0) {
                 /* Back to the old one or N_TTY if we can't */
-                if (tty_ldisc_reinit(tty, old_disc) < 0) {
-                        pr_err("tty: TIOCSETD failed, reinitializing N_TTY\n");
-                        if (tty_ldisc_reinit(tty, N_TTY) < 0) {
-                                /* At this point we have tty->ldisc == NULL. */
-                                pr_err("tty: reinitializing N_TTY failed\n");
-                        }
-                }
+                tty_ldisc_put(new_ldisc);
+                tty_ldisc_restore(tty, old_ldisc);
         }
 
-        if (tty->ldisc && tty->ldisc->ops->num != old_disc &&
-            tty->ops->set_ldisc) {
+        if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
                 down_read(&tty->termios_rwsem);
                 tty->ops->set_ldisc(tty);
                 up_read(&tty->termios_rwsem);
         }
 
+        /* At this point we hold a reference to the new ldisc and a
+           reference to the old ldisc, or we hold two references to
+           the old ldisc (if it was restored as part of error cleanup
+           above). In either case, releasing a single reference from
+           the old ldisc is correct. */
+        new_ldisc = old_ldisc;
 out:
         tty_ldisc_unlock(tty);
 
@@ -553,6 +601,7 @@ out:
            already running */
         tty_buffer_restart_work(tty->port);
 err:
+        tty_ldisc_put(new_ldisc);       /* drop the extra reference */
         tty_unlock(tty);
         return retval;
 }
@@ -613,8 +662,10 @@ int tty_ldisc_reinit(struct tty_struct *tty, int disc)
         int retval;
 
         ld = tty_ldisc_get(tty, disc);
-        if (IS_ERR(ld))
+        if (IS_ERR(ld)) {
+                BUG_ON(disc == N_TTY);
                 return PTR_ERR(ld);
+        }
 
         if (tty->ldisc) {
                 tty_ldisc_close(tty, tty->ldisc);
@@ -626,8 +677,10 @@ int tty_ldisc_reinit(struct tty_struct *tty, int disc)
         tty_set_termios_ldisc(tty, disc);
         retval = tty_ldisc_open(tty, tty->ldisc);
         if (retval) {
-                tty_ldisc_put(tty->ldisc);
-                tty->ldisc = NULL;
+                if (!WARN_ON(disc == N_TTY)) {
+                        tty_ldisc_put(tty->ldisc);
+                        tty->ldisc = NULL;
+                }
         }
         return retval;
 }