tty/ldsem: Add lockdep asserts for ldisc_sem

Make sure under CONFIG_LOCKDEP that each change to line discipline
is done with held write semaphor.
Otherwise potential reader will have a good time dereferencing
incomplete/uninitialized ldisc.

An exception here is tty_ldisc_open(), as it's called without ldisc_sem
locked by tty_init_dev() => tty_ldisc_setup() for the tty->link.

It seem valid as tty_init_dev() will call tty_driver_install_tty()
which will find ops->install(). Install will establish tty->link in
pty_common_install(), just after allocation of slave tty with
alloc_tty_struct(). So, no one should have a reference to slave pty yet.

Cc: Jiri Slaby <jslaby@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 9434d20..45eda69 100644 (file)
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -478,6 +478,7 @@ static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
 
 static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
 {
+       lockdep_assert_held_exclusive(&tty->ldisc_sem);
        WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
        clear_bit(TTY_LDISC_OPEN, &tty->flags);
        if (ld->ops->close)
@@ -499,6 +500,7 @@ static int tty_ldisc_failto(struct tty_struct *tty, int ld)
        struct tty_ldisc *disc = tty_ldisc_get(tty, ld);
        int r;
 
+       lockdep_assert_held_exclusive(&tty->ldisc_sem);
        if (IS_ERR(disc))
                return PTR_ERR(disc);
        tty->ldisc = disc;
@@ -622,6 +624,7 @@ EXPORT_SYMBOL_GPL(tty_set_ldisc);
  */
 static void tty_ldisc_kill(struct tty_struct *tty)
 {
+       lockdep_assert_held_exclusive(&tty->ldisc_sem);
        if (!tty->ldisc)
                return;
        /*
@@ -669,6 +672,7 @@ int tty_ldisc_reinit(struct tty_struct *tty, int disc)
        struct tty_ldisc *ld;
        int retval;
 
+       lockdep_assert_held_exclusive(&tty->ldisc_sem);
        ld = tty_ldisc_get(tty, disc);
        if (IS_ERR(ld)) {
                BUG_ON(disc == N_TTY);
@@ -767,6 +771,10 @@ int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
                return retval;
 
        if (o_tty) {
+               /*
+                * Called without o_tty->ldisc_sem held, as o_tty has been
+                * just allocated and no one has a reference to it.
+                */
                retval = tty_ldisc_open(o_tty, o_tty->ldisc);
                if (retval) {
                        tty_ldisc_close(tty, tty->ldisc);
@@ -832,6 +840,7 @@ int tty_ldisc_init(struct tty_struct *tty)
  */
 void tty_ldisc_deinit(struct tty_struct *tty)
 {
+       /* no ldisc_sem, tty is being destroyed */
        if (tty->ldisc)
                tty_ldisc_put(tty->ldisc);
        tty->ldisc = NULL;