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 14:52:33 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>,
        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 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.

Hans

> 
> I appreciate your feedback.
> 
> Thank you!