On Sun, 2018-07-01 at 18:02 +0200, Greg Kroah-Hartman wrote:
> 4.4-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxxx>
>
> 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.
[...]
> --- a/drivers/media/dvb-core/dvb_frontend.c
> +++ b/drivers/media/dvb-core/dvb_frontend.c
[...]
> +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;
> +}
This is broken. You must not wait inside a wait condition. (It would
be nice if this rule was explicitly documented.)
Why not leave the condition as it was and only change the
down_interruptible() to down()?
Ben.
> 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;
> @@ -249,13 +261,8 @@ 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;
>
>
>
--
Ben Hutchings, Software Developer Codethink Ltd
https://www.codethink.co.uk/ Dale House, 35 Dale Street
Manchester, M1 2HF, United Kingdom
