lkml.org 
[lkml]   [2018]   [Jan]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 8/8] platform: vivid-cec: fix potential integer overflow in vivid_cec_pin_adap_events

Quoting Hans Verkuil <hverkuil@xs4all.nl>:

> On 01/30/2018 09:51 AM, Gustavo A. R. Silva wrote:
>> Hi Hans,
>>
>> Quoting Hans Verkuil <hverkuil@xs4all.nl>:
>>
>>> Hi Gustavo,
>>>
>>> On 01/30/2018 01:33 AM, Gustavo A. R. Silva wrote:
>>>> Cast len to const u64 in order to avoid a potential integer
>>>> overflow. This variable is being used in a context that expects
>>>> an expression of type const u64.
>>>>
>>>> Addresses-Coverity-ID: 1454996 ("Unintentional integer overflow")
>>>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
>>>> ---
>>>> drivers/media/platform/vivid/vivid-cec.c | 2 +-
>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/media/platform/vivid/vivid-cec.c
>>>> b/drivers/media/platform/vivid/vivid-cec.c
>>>> index b55d278..30240ab 100644
>>>> --- a/drivers/media/platform/vivid/vivid-cec.c
>>>> +++ b/drivers/media/platform/vivid/vivid-cec.c
>>>> @@ -83,7 +83,7 @@ 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));
>>>> + (const u64)len * 10 * CEC_TIM_DATA_BIT_TOTAL));
>>>
>>> This makes no sense. Certainly the const part is pointless. And given that
>>> len is always <= 16 there definitely is no overflow.
>>>
>>
>> Yeah, I understand your point and I know there is no chance of an
>> overflow in this particular case.
>>
>>> I don't really want this cast in the code.
>>>
>>> Sorry,
>>>
>>
>> I'm working through all the Linux kernel Coverity reports, and I
>> thought of sending a patch for this because IMHO it doesn't hurt to
>> give the compiler complete information about the arithmetic in which
>> an expression is intended to be evaluated.
>>
>> I agree that the _const_ part is a bit odd. What do you think about
>> the cast to u64 alone?
>
> What happens if you do: ((u64)CEC_TIM_START_BIT_TOTAL +
>
> I think that forces everything else in the expression to be evaluated
> as u64.
>

Well, in this case the operator precedence takes place and the
expression len * 10 * CEC_TIM_DATA_BIT_TOTAL is computed first. So the
issue remains the same.

I can switch the expressions as follows:

(u64)len * 10 * CEC_TIM_DATA_BIT_TOTAL + CEC_TIM_START_BIT_TOTAL

and avoid the cast in the middle.

What do you think?

> It definitely needs a comment that this fixes a bogus Coverity report.
>

I actually added the following line to the message changelog:
Addresses-Coverity-ID: 1454996 ("Unintentional integer overflow")

Certainly, I've run across multiple false positives as in this case,
but I have also fixed many actual bugs thanks to the Coverity reports.
So I think in general it is valuable to take a look into these
reports, either if they spot actual bugs or promote code correctness.

Thanks
--
Gustavo






\
 
 \ /
  Last update: 2018-01-30 11:56    [W:0.044 / U:0.352 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site