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

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Date:   Tue, 30 Jan 2018 05:43:48 -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 8/8] platform: vivid-cec: fix potential integer overflow
 in vivid_cec_pin_adap_events


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

[...]

>>> 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
>
> What about:
>
> 10ULL * len * ...
>

Yeah, I like it.

>>
>> and avoid the cast in the middle.
>>
>> What do you think?
>
> My problem is that (u64)len suggests that there is some problem with len
> specifically, which isn't true.
>

That's a good point. Actually, I think the same applies for the rest  
of the patch series. Maybe it is a good idea to send a v2 of the whole  
patchset with that update.

>>
>>> 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")
>
> That needs to be in the source, otherwise someone will remove the
> cast (or ULL) at some time in the future since it isn't clear why
> it is done. And nobody reads commit logs from X years back :-)
>

You're right. I thought you were talking about the changelog.

And unless you think otherwise, I think there is no need for any  
additional code comment if the update you suggest is applied:

len * 10ULL * CEC_TIM_DATA_BIT_TOTAL

Thanks
--
Gustavo