Patch "lib/math/rational.c: fix divide by zero" has been added to the 5.13-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    lib/math/rational.c: fix divide by zero

to the 5.13-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     lib-math-rational.c-fix-divide-by-zero.patch
and it can be found in the queue-5.13 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit d62584700509ec16de9b98d0df3f0df210d05852
Author: Trent Piepho <tpiepho@xxxxxxxxx>
Date:   Wed Jun 30 18:55:49 2021 -0700

    lib/math/rational.c: fix divide by zero
    
    [ Upstream commit 65a0d3c14685663ba111038a35db70f559e39336 ]
    
    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.
    
    Link: https://lkml.kernel.org/r/20210525144250.214670-1-tpiepho@xxxxxxxxx
    Fixes: 323dd2c3ed0 ("lib/math/rational.c: fix possible incorrect result from rational fractions helper")
    Signed-off-by: Trent Piepho <tpiepho@xxxxxxxxx>
    Reported-by: Yiyuan Guo <yguoaz@xxxxxxxxx>
    Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
    Cc: Oskar Schirmer <oskar@xxxxxxxxx>
    Cc: Daniel Latypov <dlatypov@xxxxxxxxxx>
    Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

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;
 			}