linux-kernel - A divide by zero bug in lib/math/rational.c (with triggering input)

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Message-ID: <CAM7=BFoktwgy=T0GK6Mpmp2gYToCUs=CrM29MRWw8O7TPypQ8w@mail.gmail.com>
Date:   Fri, 21 May 2021 13:09:13 +0800
From:   Yiyuan guo <yguoaz@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     andy@...nel.org, tpiepho@...il.com, akpm@...ux-foundation.org,
        oskar@...ra.com, Yiyuan guo <yguoaz@...il.com>
Subject: A divide by zero bug in lib/math/rational.c (with triggering input)

In the file lib/math/rational.c, the function
rational_best_approximation has the following
code:

void rational_best_approximation(
    unsigned long given_numerator, unsigned long given_denominator,
    unsigned long max_numerator, unsigned long max_denominator,
    unsigned long *best_numerator, unsigned long *best_denominator) {
   ...
   if ((n2 > max_numerator) || (d2 > max_denominator)) {
            unsigned long t = min((max_numerator - n0) / n1,
                          (max_denominator - d0) / d1);
   ...
}

d1 may be equal to zero when performing the division, leading to a
divide by zero problem.

One input  to trigger the divide by zero bug is:
rational_best_approximation(31415, 100, (1 << 8) - 1, (1 << 5) - 1, &n, &d)