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| author | Trent Piepho <tpiepho@gmail.com> | 2021-05-25 16:35:18 -0700 |
|---|---|---|
| committer | Sascha Hauer <s.hauer@pengutronix.de> | 2021-06-09 13:52:21 +0200 |
| commit | fa36f6ee6d763d2ac00adf3cc9ac985ac13a45e9 (patch) | |
| tree | 45995caada331c8ece57d74bf98709b8fd06b54e /lib | |
| parent | 8da059fca282c6b165e4d11fbda5753f83541c68 (diff) | |
| download | barebox-fa36f6ee6d763d2ac00adf3cc9ac985ac13a45e9.tar.gz barebox-fa36f6ee6d763d2ac00adf3cc9ac985ac13a45e9.tar.xz | |
lib/math/rational.c: Fix divide by zero
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.
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>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/math/rational.c | 15 |
1 files changed, 10 insertions, 5 deletions
diff --git a/lib/math/rational.c b/lib/math/rational.c index e5367e6a8a..8411e912fb 100644 --- a/lib/math/rational.c +++ b/lib/math/rational.c @@ -78,13 +78,18 @@ 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; } |