linux-kernel - Re: [PATCH v2 8/8] platform: vivid-cec: use 64-bit arithmetic instead of 32-bit

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Message-ID: <20180206103542.Horde.31PkL6hVZU4E1wZePDF8kId@gator4166.hostgator.com>
Date:   Tue, 06 Feb 2018 10:35:42 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 8/8] platform: vivid-cec: use 64-bit arithmetic
 instead of 32-bit


Quoting Hans Verkuil <hverkuil@...all.nl>:

> On 02/05/18 22:54, Gustavo A. R. Silva wrote:
>> Hi Hans,
>>
>> Quoting Hans Verkuil <hverkuil@...all.nl>:
>>
>>> On 02/05/2018 09:36 PM, Gustavo A. R. Silva wrote:
>>>> Add suffix ULL to constant 10 in order to give the compiler complete
>>>> information about the proper arithmetic to use. Notice that this
>>>> constant is used in a context that expects an expression of type
>>>> u64 (64 bits, unsigned).
>>>>
>>>> The expression len * 10 * CEC_TIM_DATA_BIT_TOTAL is currently being
>>>> evaluated using 32-bit arithmetic.
>>>>
>>>> Also, remove unnecessary parentheses and add a code comment to make it
>>>> clear what is the reason of the code change.
>>>>
>>>> Addresses-Coverity-ID: 1454996
>>>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>>>> ---
>>>> Changes in v2:
>>>>  - Update subject and changelog to better reflect the proposed  
>>>> code changes.
>>>>  - Add suffix ULL to constant instead of casting a variable.
>>>>  - Remove unncessary parentheses.
>>>
>>> unncessary -> unnecessary
>>>
>>
>> Thanks for this.
>>
>>>>  - Add code comment.
>>>>
>>>>  drivers/media/platform/vivid/vivid-cec.c | 11 +++++++++--
>>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/media/platform/vivid/vivid-cec.c
>>>> b/drivers/media/platform/vivid/vivid-cec.c
>>>> index b55d278..614787b 100644
>>>> --- a/drivers/media/platform/vivid/vivid-cec.c
>>>> +++ b/drivers/media/platform/vivid/vivid-cec.c
>>>> @@ -82,8 +82,15 @@ static void vivid_cec_pin_adap_events(struct
>>>> cec_adapter *adap, ktime_t ts,
>>>>
>>>>  	if (adap == NULL)
>>>>  		return;
>>>> -	ts = ktime_sub_us(ts, (CEC_TIM_START_BIT_TOTAL +
>>>> -			       len * 10 * CEC_TIM_DATA_BIT_TOTAL));
>>>> +
>>>> +	/*
>>>> +	 * Suffix ULL on constant 10 makes the expression
>>>> +	 * CEC_TIM_START_BIT_TOTAL + 10ULL * len * CEC_TIM_DATA_BIT_TOTAL
>>>> +	 * be evaluated using 64-bit unsigned arithmetic (u64), which
>>>> +	 * is what ktime_sub_us expects as second argument.
>>>> +	 */
>>>
>>> That's not really the comment that I was looking for. It still doesn't
>>> explain *why* this is needed at all. How about something like this:
>>>
>>
>> In MHO the reason for the change is simply the discrepancy between the
>> arithmetic expected by
>> the function ktime_sub_us and the arithmetic in which the expression
>> is being evaluated. And this
>> has nothing to do with any particular tool.
>
> Hmm, you have a point.
>
> OK, I've looked at the other patches in this patch series as well, and
> the only thing I would like to see changed is the 'Addresses-Coverity-ID'
> line in the patches: patch 4 says:
>
> Addresses-Coverity-ID: 1324146 ("Unintentional integer overflow")
>
> but that's the only one that mentions the specific coverity error.
> It would be nice if that can be added to the other patches as well so
> we have a record of the actual coverity error.
>

OK. I'll send v3 of the whole patch series shortly.

Thank you!

>>
>>> /*
>>>  * Add the ULL suffix to the constant 10 to work around a false Coverity
>>>  * "Unintentional integer overflow" warning. Coverity isn't smart enough
>>>  * to understand that len is always <= 16, so there is no chance of an
>>>  * integer overflow.
>>>  */
>>>
>>
>> :P
>>
>> In my opinion it is not a good idea to tie the code to a particular tool.
>> There are only three appearances of the word 'Coverity' in the whole
>> code base, and, honestly I don't want to add more.
>>
>> So I think I will document this issue as a FP in the Coverity platform.
>
> FP?
>

False Positive.

> Regards,
>
> 	Hans

--
Gustavo