Re: [PATCH v2] usb: gadget: aspeed_udc: avoid past-the-end iterator in dequeue

[Andrew Jeffery <andrew@codeconstruct.com.au>]

On Tue, 2026-05-19 at 16:02 +0800, Maoyi Xie wrote:
> ast_udc_ep_dequeue() declares the loop cursor `req` outside the
> list_for_each_entry(). After the loop it tests `&req->req != _req`
> to decide whether the request was found. If the queue holds no
> match, `req` is past-the-end. It then aliases
> container_of(&ep->queue, struct ast_udc_request, queue) via offset
> cancellation. Whether that synthetic address equals `_req` depends
> on heap layout. The function can return 0 without dequeueing
> anything.
> 
> Walk the list with a separate `iter`. Set `req` only when a
> request matches. After the loop, `req` is NULL if nothing
> matched.
> 
> Suggested-by: Alan Stern <stern@rowland.harvard.edu>
> Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
> ---
> v2: Switch the loop body to Alan Stern's shape: test inside
>     the if, assign `req`, break. Same behaviour as v1.
> v1: https://lore.kernel.org/linux-usb/20260518073403.1285339-1-maoyi.xie@ntu.edu.sg/
> 
>  drivers/usb/gadget/udc/aspeed_udc.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> --- a/drivers/usb/gadget/udc/aspeed_udc.c	2026-05-19 15:29:28.690931576 +0800
> +++ b/drivers/usb/gadget/udc/aspeed_udc.c	2026-05-19 15:29:59.482953528 +0800
> @@ -692,26 +692,30 @@
>  {
>  	struct ast_udc_ep *ep = to_ast_ep(_ep);
>  	struct ast_udc_dev *udc = ep->udc;
> -	struct ast_udc_request *req;
> +	struct ast_udc_request *req = NULL, *iter;
>  	unsigned long flags;
>  	int rc = 0;
>  
>  	spin_lock_irqsave(&udc->lock, flags);
>  
>  	/* make sure it's actually queued on this endpoint */
> -	list_for_each_entry(req, &ep->queue, queue) {
> -		if (&req->req == _req) {
> -			list_del_init(&req->queue);
> -			ast_udc_done(ep, req, -ESHUTDOWN);
> -			_req->status = -ECONNRESET;
> +	list_for_each_entry(iter, &ep->queue, queue) {
> +		if (&iter->req == _req) {
> +			req = iter;
>  			break;
>  		}
>  	}
>  
> -	/* dequeue request not found */
> -	if (&req->req != _req)
> +	if (!req) {
>  		rc = -EINVAL;
> +		goto out;
> +	}
> +
> +	list_del_init(&req->queue);
> +	ast_udc_done(ep, req, -ESHUTDOWN);
> +	_req->status = -ECONNRESET;
>  
> +out:
>  	spin_unlock_irqrestore(&udc->lock, flags);
>  
>  	return rc;

This is a bit of a bikeshed comment and doesn't solve making the code
similar to other cases, however: Golfing the diff a bit, perhaps we can
start from the assumption that there isn't a match, and require the
search disprove that. Then we don't have to test whether we saw
something after-the-fact, and we avoid the goto as proposed above.

Untested:

   diff --git a/drivers/usb/gadget/udc/aspeed_udc.c b/drivers/usb/gadget/udc/aspeed_udc.c
   index 7fc6696b7694..75f9c831b21a 100644
   --- a/drivers/usb/gadget/udc/aspeed_udc.c
   +++ b/drivers/usb/gadget/udc/aspeed_udc.c
   @@ -694,7 +694,7 @@ static int ast_udc_ep_dequeue(struct usb_ep *_ep, struct usb_request *_req)
           struct ast_udc_dev *udc = ep->udc;
           struct ast_udc_request *req;
           unsigned long flags;
   -       int rc = 0;
   +       int rc = -EINVAL;
   
           spin_lock_irqsave(&udc->lock, flags);
   
   @@ -704,14 +704,11 @@ static int ast_udc_ep_dequeue(struct usb_ep *_ep, struct usb_request *_req)
                           list_del_init(&req->queue);
                           ast_udc_done(ep, req, -ESHUTDOWN);
                           _req->status = -ECONNRESET;
   +                       rc = 0;
                           break;
                   }
           }
   
   -       /* dequeue request not found */
   -       if (&req->req != _req)
   -               rc = -EINVAL;
   -
           spin_unlock_irqrestore(&udc->lock, flags);
   
           return rc;