linux-kernel - Re: [PATCH] usb: gadget: net2272: assert for a valid dma request

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Date:   Sun, 30 Aug 2020 14:35:41 -0700
From:   Tom Rix <trix@...hat.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     balbi@...nel.org, gregkh@...uxfoundation.org,
        natechancellor@...il.com, ndesaulniers@...gle.com,
        christophe.jaillet@...adoo.fr, peter.chen@....com, b-liu@...com,
        chunfeng.yun@...iatek.com, novikov@...ras.ru, yanaijie@...wei.com,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: gadget: net2272: assert for a valid dma request


On 8/30/20 2:22 PM, Alan Stern wrote:
> On Sun, Aug 30, 2020 at 11:36:46AM -0700, trix@...hat.com wrote:
>> From: Tom Rix <trix@...hat.com>
>>
>> clang static analysis flags this representive problem
>>
>> net2272.c:1541:8: warning: Dereference of null pointer
>>     if ((req->req.length % ep->ep.maxpacket != 0) ||
>>          ^~~~~~~~~~~~~~~
>> This is mostly not a problem.
>>
>> In net2272_handle_dma(), even though every path through
>> the routine dereferences req, it is set to NULL when the
>> ep->queue is empty.  If the eq->queue was ever empty this
>> would have oops.
>>
>> So the else statement should not be needed.  If it is,
>> flag it as a BUG.
>>
>> Signed-off-by: Tom Rix <trix@...hat.com>
> This patch really seems to be overkill.  In particular, Linus Torvalds 
> feels very strongly that people should not add BUG or BUG_ON calls 
> except in extreme situations -- which this isn't.
>
>> ---
>>  drivers/usb/gadget/udc/net2272.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/usb/gadget/udc/net2272.c b/drivers/usb/gadget/udc/net2272.c
>> index 44d1ea2307bb..8301723a8b1a 100644
>> --- a/drivers/usb/gadget/udc/net2272.c
>> +++ b/drivers/usb/gadget/udc/net2272.c
>> @@ -1506,17 +1506,16 @@ static int net2272_stop(struct usb_gadget *_gadget)
>>  static void
>>  net2272_handle_dma(struct net2272_ep *ep)
>>  {
>> -	struct net2272_request *req;
>> +	struct net2272_request *req = NULL;
>>  	unsigned len;
>>  	int status;
>>  
>>  	if (!list_empty(&ep->queue))
>>  		req = list_entry(ep->queue.next,
>>  				struct net2272_request, queue);
>> -	else
>> -		req = NULL;
>>  
>>  	dev_vdbg(ep->dev->dev, "handle_dma %s req %p\n", ep->ep.name, req);
>> +	BUG_ON(!req);
> There's no point in adding this.  If the function goes on to dereference 
> a NULL pointer, you'll get the same effect in any case -- an oops.
>
> If you want to make the point that req had better not be NULL, just get 
> rid of the "if" test entirely.  You could even change the list_entry to 
> list_first_entry.

Since nothing is really going to change, drop this patch.

Tom

> Alan Stern
>