linux-kernel - Re: [PATCH v2] usb: raw-gadget: fix raw_event_queue

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Date:   Tue, 7 Apr 2020 17:29:36 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Felipe Balbi <balbi@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH v2] usb: raw-gadget: fix raw_event_queue_fetch locking

On Tue, Apr 07, 2020 at 04:14:50PM +0200, Andrey Konovalov wrote:
> @@ -89,11 +90,18 @@ static struct usb_raw_event *raw_event_queue_fetch(
>  	 * there's at least one event queued by decrementing the semaphore,
>  	 * and then take the lock to protect queue struct fields.
>  	 */
> -	if (down_interruptible(&queue->sema))
> -		return NULL;
> +	ret = down_interruptible(&queue->sema);
> +	if (ret)
> +		return ERR_PTR(ret);
>  	spin_lock_irqsave(&queue->lock, flags);
> -	if (WARN_ON(!queue->size))
> +	/*
> +	 * queue->size must have the same value as queue->sema counter (before
> +	 * the down_interruptible() call above), so this check is a fail-safe.
> +	 */
> +	if (WARN_ON(!queue->size)) {
> +		spin_unlock_irqrestore(&queue->lock, flags);
>  		return NULL;

I'm sorry for not noticing this earlier.  When a function returns both
error pointers and NULL then NULL is supposed to a special case of
success.  For example:

	my_struct_pointer = get_optional_feature();

If there is a memory allocation failure then my_struct_pointer is
-ENOMEM and we fail.  But say the optional feature is disabled, then
we can't return a valid pointer, but it's also working as designed so
it's not an error.  In that case we return NULL.  The surrounding code
should be written to allow NULL pointers.

So I don't think returning NULL here is correct.

regards,
dan carpenter