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From:   Trent Piepho <tpiepho@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     andy@kernel.org, akpm@linux-foundation.org, oskar@scara.com,
        Daniel Latypov <dlatypov@google.com>,
        Trent Piepho <tpiepho@gmail.com>, Yiyuan Guo <yguoaz@gmail.com>
Subject: [PATCH v2 1/2] lib/math/rational.c: Fix divide by zero
Date:   Tue, 25 May 2021 07:42:49 -0700
Message-Id: <20210525144250.214670-1-tpiepho@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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>
---
 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