[PATCH 3.16 018/366] media: dvb_frontend: fix locking issues at dvb_frontend_get_event()

Ben Hutchings
2018-11-11T20:42:08Z

From: Mauro Carvalho Chehab <mchehab@s-opensource.com>

commit 76d81243a487c09619822ef8e7201a756e58a87d upstream.

As warned by smatch:
drivers/media/dvb-core/dvb_frontend.c:314 dvb_frontend_get_event() warn: inconsistent returns 'sem:&fepriv->sem'.
Locked on: line 288
line 295
line 306
line 314
Unlocked on: line 303

The lock implementation for get event is wrong, as, if an
interrupt occurs, down_interruptible() will fail, and the
routine will call up() twice when userspace calls the ioctl
again.

The bad code is there since when Linux migrated to git, in
2005.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/media/dvb-core/dvb_frontend.c | 23 +++++++++++++++--------
1 file changed, 15 insertions(+), 8 deletions(-)

--- a/drivers/media/dvb-core/dvb_frontend.c
+++ b/drivers/media/dvb-core/dvb_frontend.c
@@ -229,8 +229,20 @@ static void dvb_frontend_add_event(struc
wake_up_interruptible (&events->wait_queue);
}

+static int dvb_frontend_test_event(struct dvb_frontend_private *fepriv,
+ struct dvb_fe_events *events)
+{
+ int ret;
+
+ up(&fepriv->sem);
+ ret = events->eventw != events->eventr;
+ down(&fepriv->sem);
+
+ return ret;
+}
+
static int dvb_frontend_get_event(struct dvb_frontend *fe,
- struct dvb_frontend_event *event, int flags)
+ struct dvb_frontend_event *event, int flags)
{
struct dvb_frontend_private *fepriv = fe->frontend_priv;
struct dvb_fe_events *events = &fepriv->events;
@@ -248,13 +260,8 @@ static int dvb_frontend_get_event(struct
if (flags & O_NONBLOCK)
return -EWOULDBLOCK;

- up(&fepriv->sem);
-
- ret = wait_event_interruptible (events->wait_queue,
- events->eventw != events->eventr);
-
- if (down_interruptible (&fepriv->sem))
- return -ERESTARTSYS;
+ ret = wait_event_interruptible(events->wait_queue,
+ dvb_frontend_test_event(fepriv, events));

if (ret < 0)
return ret;