Re: [PATCH 8/8] platform: vivid-cec: fix potential integer overflow in vivid_cec_pin_adap_events

[Gustavo A. R. Silva]

Quoting Hans Verkuil <hverkuil@xxxxxxxxx>:

> On 01/30/2018 09:51 AM, Gustavo A. R. Silva wrote:
> > Hi Hans,
> >
> > Quoting Hans Verkuil <hverkuil@xxxxxxxxx>:
> >
> > > 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@xxxxxxxxxxxxxx>
> > > > ---
> > > >  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