linux-kernel - [PATCHv4 6/7] tty/ldsem: Add lockdep asserts for ldisc

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Message-Id: <20180912001702.18522-7-dima@arista.com>
Date:   Wed, 12 Sep 2018 01:17:00 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Nathan March <nathan@...net>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rong, Chen" <rong.a.chen@...el.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Subject: [PATCHv4 6/7] 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: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jiri Slaby <jslaby@...e.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Dmitry Safonov <dima@...sta.com>
---
 drivers/tty/tty_ldisc.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index fc4c97cae01e..bc0171f984a1 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -471,6 +471,7 @@ 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)
@@ -492,6 +493,7 @@ 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;
@@ -615,6 +617,7 @@ 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;
 	/*
@@ -662,6 +665,7 @@ 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);
@@ -760,6 +764,10 @@ 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);
@@ -825,6 +833,7 @@ 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;
-- 
2.13.6