Messages in this thread | | | Date | Tue, 25 May 2021 19:00:28 +0300 | From | Andy Shevchenko <> | Subject | Re: [PATCH v2 1/2] lib/math/rational.c: Fix divide by zero |
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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@linux.intel.com>
> Fixes: 323dd2c3ed0 ("lib/math/rational.c: fix possible incorrect result from rational fractions helper") > Reported-by: Yiyuan Guo <yguoaz@gmail.com> > Signed-off-by: Trent Piepho <tpiepho@gmail.com> > --- > 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
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