linux-kernel - [PATCH 3.16 019/305] media: v4l: event: Add subscription to list before calling "add" operation

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Message-ID: <lsq.1549201508.661508503@decadent.org.uk>
Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Sakari Ailus" <sakari.ailus@...ux.intel.com>,
        "Mauro Carvalho Chehab" <mchehab+samsung@...nel.org>,
        "Hans Verkuil" <hans.verkuil@...co.com>,
        "Dave Stevenson" <dave.stevenson@...pberrypi.org>
Subject: [PATCH 3.16 019/305] media: v4l: event: Add subscription to list
 before calling "add" operation

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Sakari Ailus <sakari.ailus@...ux.intel.com>

commit 92539d3eda2c090b382699bbb896d4b54e9bdece upstream.

Patch ad608fbcf166 changed how events were subscribed to address an issue
elsewhere. As a side effect of that change, the "add" callback was called
before the event subscription was added to the list of subscribed events,
causing the first event queued by the add callback (and possibly other
events arriving soon afterwards) to be lost.

Fix this by adding the subscription to the list before calling the "add"
callback, and clean up afterwards if that fails.

Fixes: ad608fbcf166 ("media: v4l: event: Prevent freeing event subscriptions while accessed")

Reported-by: Dave Stevenson <dave.stevenson@...pberrypi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
Tested-by: Dave Stevenson <dave.stevenson@...pberrypi.org>
Reviewed-by: Hans Verkuil <hans.verkuil@...co.com>
Tested-by: Hans Verkuil <hans.verkuil@...co.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/media/v4l2-core/v4l2-event.c | 43 ++++++++++++++++++++----------------
 1 file changed, 24 insertions(+), 19 deletions(-)

--- a/drivers/media/v4l2-core/v4l2-event.c
+++ b/drivers/media/v4l2-core/v4l2-event.c
@@ -194,6 +194,22 @@ int v4l2_event_pending(struct v4l2_fh *f
 }
 EXPORT_SYMBOL_GPL(v4l2_event_pending);
 
+static void __v4l2_event_unsubscribe(struct v4l2_subscribed_event *sev)
+{
+	struct v4l2_fh *fh = sev->fh;
+	unsigned int i;
+
+	lockdep_assert_held(&fh->subscribe_lock);
+	assert_spin_locked(&fh->vdev->fh_lock);
+
+	/* Remove any pending events for this subscription */
+	for (i = 0; i < sev->in_use; i++) {
+		list_del(&sev->events[sev_pos(sev, i)].list);
+		fh->navailable--;
+	}
+	list_del(&sev->list);
+}
+
 int v4l2_event_subscribe(struct v4l2_fh *fh,
 			 const struct v4l2_event_subscription *sub, unsigned elems,
 			 const struct v4l2_subscribed_event_ops *ops)
@@ -225,27 +241,23 @@ int v4l2_event_subscribe(struct v4l2_fh
 
 	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
 	found_ev = v4l2_event_subscribed(fh, sub->type, sub->id);
+	if (!found_ev)
+		list_add(&sev->list, &fh->subscribed);
 	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
 
 	if (found_ev) {
 		/* Already listening */
 		kfree(sev);
-		goto out_unlock;
-	}
-
-	if (sev->ops && sev->ops->add) {
+	} else if (sev->ops && sev->ops->add) {
 		ret = sev->ops->add(sev, elems);
 		if (ret) {
+			spin_lock_irqsave(&fh->vdev->fh_lock, flags);
+			__v4l2_event_unsubscribe(sev);
+			spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
 			kfree(sev);
-			goto out_unlock;
 		}
 	}
 
-	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
-	list_add(&sev->list, &fh->subscribed);
-	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
-
-out_unlock:
 	mutex_unlock(&fh->subscribe_lock);
 
 	return ret;
@@ -280,7 +292,6 @@ int v4l2_event_unsubscribe(struct v4l2_f
 {
 	struct v4l2_subscribed_event *sev;
 	unsigned long flags;
-	int i;
 
 	if (sub->type == V4L2_EVENT_ALL) {
 		v4l2_event_unsubscribe_all(fh);
@@ -292,14 +303,8 @@ int v4l2_event_unsubscribe(struct v4l2_f
 	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
 
 	sev = v4l2_event_subscribed(fh, sub->type, sub->id);
-	if (sev != NULL) {
-		/* Remove any pending events for this subscription */
-		for (i = 0; i < sev->in_use; i++) {
-			list_del(&sev->events[sev_pos(sev, i)].list);
-			fh->navailable--;
-		}
-		list_del(&sev->list);
-	}
+	if (sev != NULL)
+		__v4l2_event_unsubscribe(sev);
 
 	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);