[PATCH] v4l: async: Protect against double notifier regstrations

[Kieran Bingham]

From: Kieran Bingham <kieran.bingham+rene...@ideasonboard.com>

It can be easy to attempt to register the same notifier twice
in mis-handled error cases such as working with -EPROBE_DEFER.

This results in odd kernel crashes where the notifier_list becomes
corrupted due to adding the same entry twice.

Protect against this so that a developer has some sense of the pending
failure.

Signed-off-by: Kieran Bingham <kieran.bingham+rene...@ideasonboard.com>
---
 drivers/media/v4l2-core/v4l2-async.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/media/v4l2-core/v4l2-async.c 
b/drivers/media/v4l2-core/v4l2-async.c
index 2b08d03b251d..e8476f0755ca 100644
--- a/drivers/media/v4l2-core/v4l2-async.c
+++ b/drivers/media/v4l2-core/v4l2-async.c
@@ -374,6 +374,7 @@ static int __v4l2_async_notifier_register(struct 
v4l2_async_notifier *notifier)
        struct device *dev =
                notifier->v4l2_dev ? notifier->v4l2_dev->dev : NULL;
        struct v4l2_async_subdev *asd;
+       struct v4l2_async_notifier *n;
        int ret;
        int i;
 
@@ -385,6 +386,19 @@ static int __v4l2_async_notifier_register(struct 
v4l2_async_notifier *notifier)
 
        mutex_lock(&list_lock);
 
+       /*
+        * Registering the same notifier can occur if a driver incorrectly
+        * handles a -EPROBE_DEFER for example, and will break in a
+        * confusing fashion with linked-list corruption.
+        */
+       list_for_each_entry(n, &notifier_list, list) {
+               if (n == notifier) {
+                       dev_err(dev, "Notifier has already been registered\n");
+                       ret = -EEXIST;
+                       goto err_unlock;
+               }
+       }
+
        for (i = 0; i < notifier->num_subdevs; i++) {
                asd = notifier->subdevs[i];
 
-- 
2.7.4