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

Quoting Hans Verkuil <hverkuil@xs4all.nl>:

> On 02/05/18 22:54, Gustavo A. R. Silva wrote:
>> Hi Hans,
>>
>> Quoting Hans Verkuil <hverkuil@xs4all.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@embeddedor.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





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