Messages in this thread | | | Date | Tue, 30 Jan 2018 04:55:45 -0600 | From | "Gustavo A. R. Silva" <> | Subject | Re: [PATCH 8/8] platform: vivid-cec: fix potential integer overflow in vivid_cec_pin_adap_events |
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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
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