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

[Sakari Ailus <sakari.ailus@xxxxxx>]

Hi Kieran,

On Tue, Jan 16, 2018 at 02:52:58PM +0000, Kieran Bingham wrote:
> From: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>
> 
> 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+renesas@xxxxxxxxxxxxxxxx>
> ---
>  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.
> +	 */

This would seem fine in the commit message, and it's essentially there
already. How about simply:

	/* Avoid re-registering a notifier. */
	
You should actually perform the check before initialising the notifier's
lists. Although things are likely in a quite bad shape already if this
happens.

> +	list_for_each_entry(n, &notifier_list, list) {
> +		if (n == notifier) {

if (WARN_ON(n == notifier)) {

And drop the error message below?

> +			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];
>  

-- 
Regards,

Sakari Ailus
e-mail: sakari.ailus@xxxxxx