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

[Gustavo A. R. Silva]

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@xxxxxxxxxxxxxx>
> > ---
> > 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