# 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