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SubjectRe: [PATCH v3 4/8] i2c: ov9650: use 64-bit arithmetic instead of 32-bit
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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@embeddedor.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?

I appreciate your feedback.

Thank you!
--
Gustavo

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