linux-kernel - Re: [PATCH v3 4/8] i2c: ov9650: use 64-bit arithmetic instead of 32-bit

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Date:   Thu, 15 Feb 2018 10:12:46 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        Sakari Ailus <sakari.ailus@....fi>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/8] i2c: ov9650: use 64-bit arithmetic instead of
 32-bit



On 02/15/2018 07:52 AM, Hans Verkuil wrote:
> On 08/02/18 17:39, Gustavo A. R. Silva wrote:
>> Hi Sakari,
>>
>> On 02/07/2018 03:59 PM, Sakari Ailus wrote:
>>> Hi Gustavo,
>>>
>>> On Tue, Feb 06, 2018 at 10:47:50AM -0600, Gustavo A. R. Silva wrote:
>>>> Add suffix ULL to constants 10000 and 1000000 in order to give the
>>>> compiler complete information about the proper arithmetic to use.
>>>> Notice that these constants are used in contexts that expect
>>>> expressions of type u64 (64 bits, unsigned).
>>>>
>>>> The following expressions:
>>>>
>>>> (u64)(fi->interval.numerator * 10000)
>>>> (u64)(iv->interval.numerator * 10000)
>>>> fiv->interval.numerator * 1000000 / fiv->interval.denominator
>>>>
>>>> are currently being evaluated using 32-bit arithmetic.
>>>>
>>>> Notice that those casts to u64 for the first two expressions are only
>>>> effective after such expressions are evaluated using 32-bit arithmetic,
>>>> which leads to potential integer overflows. So based on those casts, it
>>>> seems that the original intention of the code is to actually use 64-bit
>>>> arithmetic instead of 32-bit.
>>>>
>>>> Also, notice that once the suffix ULL is added to the constants, the
>>>> outer casts to u64 are no longer needed.
>>>>
>>>> Addresses-Coverity-ID: 1324146 ("Unintentional integer overflow")
>>>> Fixes: 84a15ded76ec ("[media] V4L: Add driver for OV9650/52 image sensors")
>>>> Fixes: 79211c8ed19c ("remove abs64()")
>>>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>>>> ---
>>>> Changes in v2:
>>>>    - Update subject and changelog to better reflect the proposed code changes.
>>>>    - Add suffix ULL to constants instead of casting variables.
>>>>    - Remove unnecessary casts to u64 as part of the code change.
>>>>    - Extend the same code change to other similar expressions.
>>>>
>>>> Changes in v3:
>>>>    - None.
>>>>
>>>>    drivers/media/i2c/ov9650.c | 9 +++++----
>>>>    1 file changed, 5 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/media/i2c/ov9650.c b/drivers/media/i2c/ov9650.c
>>>> index e519f27..e716e98 100644
>>>> --- a/drivers/media/i2c/ov9650.c
>>>> +++ b/drivers/media/i2c/ov9650.c
>>>> @@ -1130,7 +1130,7 @@ static int __ov965x_set_frame_interval(struct ov965x *ov965x,
>>>>        if (fi->interval.denominator == 0)
>>>>            return -EINVAL;
>>>>    -    req_int = (u64)(fi->interval.numerator * 10000) /
>>>> +    req_int = fi->interval.numerator * 10000ULL /
>>>>            fi->interval.denominator;
>>>
>>> This has been addressed by your earlier patch "i2c: ov9650: fix potential integer overflow in
>>> __ov965x_set_frame_interval" I tweaked a little. It's not in media tree
>>> master yet.
>>>
>>
>> Yeah. Actually this patch is supposed to be an improved version of the one you mention. That is why this is version 3.
>>
>> Also, I wonder if the same issue you mention below regarding 32-bit ARM applies in this case too?
>>
>>>>          for (i = 0; i < ARRAY_SIZE(ov965x_intervals); i++) {
>>>> @@ -1139,7 +1139,7 @@ static int __ov965x_set_frame_interval(struct ov965x *ov965x,
>>>>            if (mbus_fmt->width != iv->size.width ||
>>>>                mbus_fmt->height != iv->size.height)
>>>>                continue;
>>>> -        err = abs((u64)(iv->interval.numerator * 10000) /
>>>> +        err = abs(iv->interval.numerator * 10000ULL /
>>>
>>> This and the chunk below won't work on e.g. 32-bit ARM. do_div(), please.
>>>
>>
>> Thanks for pointing this out.
>>
>>>>                    iv->interval.denominator - req_int);
>>>>            if (err < min_err) {
>>>>                fiv = iv;
>>>> @@ -1148,8 +1148,9 @@ static int __ov965x_set_frame_interval(struct ov965x *ov965x,
>>>>        }
>>>>        ov965x->fiv = fiv;
>>>>    -    v4l2_dbg(1, debug, &ov965x->sd, "Changed frame interval to %u us\n",
>>>> -         fiv->interval.numerator * 1000000 / fiv->interval.denominator);
>>>> +    v4l2_dbg(1, debug, &ov965x->sd, "Changed frame interval to %llu us\n",
>>>> +         fiv->interval.numerator * 1000000ULL /
>>>> +         fiv->interval.denominator);
>>
>> I wonder if do_div should be used for the code above?
> 
> Yes, do_div should be used.
>

I got it.

Thanks, Hans.
--
Gustavo