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

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Message-ID: <20180207215944.quwowjy52dclk7uc@valkosipuli.retiisi.org.uk>
Date:   Wed, 7 Feb 2018 23:59:46 +0200
From:   Sakari Ailus <sakari.ailus@....fi>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Subject: Re: [PATCH v3 4/8] i2c: ov9650: use 64-bit arithmetic instead of
 32-bit

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.

>  
>  	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.

>  			    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);
>  
>  	return 0;
>  }

-- 
Regards,

Sakari Ailus
e-mail: sakari.ailus@....fi