linux-kernel - Re: [PATCH v6 08/13] iio: afe: rescale: reduce risk of integer overflow

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Date:   Fri, 23 Jul 2021 23:17:47 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Liam Beguin <liambeguin@...il.com>, jic23@...nel.org,
        lars@...afoo.de, pmeerw@...erw.net
Cc:     linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
        devicetree@...r.kernel.org, robh+dt@...nel.org
Subject: Re: [PATCH v6 08/13] iio: afe: rescale: reduce risk of integer
 overflow

On 2021-07-21 05:06, Liam Beguin wrote:
> From: Liam Beguin <lvb@...hos.com>
> 
> Reduce the risk of integer overflow by doing the scale calculation with
> 64bit integers and looking for a Greatest Common Divider for both parts
> of the fractional value when required.
> 
> Signed-off-by: Liam Beguin <lvb@...hos.com>
> ---
>  drivers/iio/afe/iio-rescale.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> index 6f6a711ae3ae..35fa3b4e53e0 100644
> --- a/drivers/iio/afe/iio-rescale.c
> +++ b/drivers/iio/afe/iio-rescale.c
> @@ -21,12 +21,21 @@
>  int rescale_process_scale(struct rescale *rescale, int scale_type,
>  			  int *val, int *val2)
>  {
> -	unsigned long long tmp;
> +	s64 tmp, tmp2;
> +	u32 factor;
>  
>  	switch (scale_type) {
>  	case IIO_VAL_FRACTIONAL:
> -		*val *= rescale->numerator;
> -		*val2 *= rescale->denominator;
> +		if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
> +		    check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2)) {
> +			tmp = (s64)*val * rescale->numerator;
> +			tmp2 = (s64)*val2 * rescale->denominator;
> +			factor = gcd(tmp, tmp2);

Hi!

Reiterating that gcd() only works for unsigned operands, so this is broken for
negative values.

Cheers,
Peter

> +			tmp = div_s64(tmp, factor);
> +			tmp2 = div_s64(tmp2, factor);
> +		}
> +		*val = tmp;
> +		*val2 = tmp2;
>  		return scale_type;
>  	case IIO_VAL_INT:
>  		*val *= rescale->numerator;
>