# Re: [PATCH v2 1/2] lib/math/rational.c: Fix divide by zero

From: Andy Shevchenko
Date: Tue May 25 2021 - 12:01:05 EST

On Tue, May 25, 2021 at 07:42:49AM -0700, Trent Piepho wrote:
> If the input is out of the range of the allowed values, either larger
> than the largest value or closer to zero than the smallest non-zero
> allowed value, then a division by zero would occur.
>
> In the case of input too large, the division by zero will occur on the
> first iteration. The best result (largest allowed value) will be found
> by always choosing the semi-convergent and excluding the denominator
> based limit when finding it.
>
> In the case of the input too small, the division by zero will occur on
> the second iteration. The numerator based semi-convergent should not be
> calculated to avoid the division by zero. But the semi-convergent vs
> previous convergent test is still needed, which effectively chooses
> between 0 (the previous convergent) vs the smallest allowed fraction
> (best semi-convergent) as the result.

LGTM, thanks!
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

> Fixes: 323dd2c3ed0 ("lib/math/rational.c: fix possible incorrect result from rational fractions helper")
> Reported-by: Yiyuan Guo <yguoaz@xxxxxxxxx>
> Signed-off-by: Trent Piepho <tpiepho@xxxxxxxxx>
> ---
> lib/math/rational.c | 16 +++++++++++-----
> 1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/lib/math/rational.c b/lib/math/rational.c
> index 9781d521963d..c0ab51d8fbb9 100644
> --- a/lib/math/rational.c
> +++ b/lib/math/rational.c
> @@ -12,6 +12,7 @@
> #include <linux/compiler.h>
> #include <linux/export.h>
> #include <linux/minmax.h>
> +#include <linux/limits.h>
>
> /*
> * calculate best rational approximation for a given fraction
> @@ -78,13 +79,18 @@ void rational_best_approximation(
> * found below as 't'.
> */
> if ((n2 > max_numerator) || (d2 > max_denominator)) {
> - unsigned long t = min((max_numerator - n0) / n1,
> - (max_denominator - d0) / d1);
> + unsigned long t = ULONG_MAX;
>
> - /* This tests if the semi-convergent is closer
> - * than the previous convergent.
> + if (d1)
> + t = (max_denominator - d0) / d1;
> + if (n1)
> + t = min(t, (max_numerator - n0) / n1);
> +
> + /* This tests if the semi-convergent is closer than the previous
> + * convergent. If d1 is zero there is no previous convergent as this
> + * is the 1st iteration, so always choose the semi-convergent.
> */
> - if (2u * t > a || (2u * t == a && d0 * dp > d1 * d)) {
> + if (!d1 || 2u * t > a || (2u * t == a && d0 * dp > d1 * d)) {
> n1 = n0 + t * n1;
> d1 = d0 + t * d1;
> }
> --
> 2.26.2
>

--
With Best Regards,
Andy Shevchenko