Re: Undefined behavior in conv-integer.i.c?

[Mark H Weaver]

Hi,

Miroslav Lichvar <address@hidden> writes:

> I was looking at a problem with guile-1.8.8 when compiled with
> gcc-6.0. Two of the tests from the test suite were failing with
> strange "out of range" errors [1]. After some investigation I think
> the bug is that the code in libguile/conv-integer.i.c relies on
> overflow of signed integers in the following code (starting on line
> 77), specifically -TYPE_MIN being less than zero. Adding -fwrapv to
> CFLAGS worked as a workaround for me.
>
>           if (mpz_sgn (SCM_I_BIG_MPZ (val)) >= 0)
>             {
>               if (n < 0)
>                 goto out_of_range;
>             }
>           else
>             {
>               n = -n;
>               if (n >= 0)
>                 goto out_of_range;
>             }

Thanks for bringing this to our attention.  I've attached a preliminary
patch to address these issues on the 'stable-2.0' branch.

> Looking at the current guile code, conv-integer.i.c is identical to
> what it was in 1.8.8, but interestingly the tests didn't fail for me.
> Maybe something else is preventing gcc from using the optimization?

The build system of recent Guile 2.0.x automatically adds -fwrapv to
CFLAGS where supported.  However, I hope to remove -fwrapv in the
future, when we gain confidence that no code in Guile depends on it.

> I'm not sure what would be the best way to fix it. Maybe n should
> really be unsigned and compared to the maximum values, but what would
> be the absolute value of TYPE_MIN if it should work also with other
> integer representations than two's complement?

My approach was to compare (abs_n - 1) to -(TYPE_MIN + 1) in the case
where n is negative.

    Thanks,
      Mark


>From be75e1713caa7b66aedc837a226091f78dfcb8fe Mon Sep 17 00:00:00 2001
From: Mark H Weaver <address@hidden>
Date: Wed, 24 Feb 2016 02:17:43 -0500
Subject: [PATCH] Avoid signed integer overflows in numeric conversions.

* libguile/conv-integer.i.c: Avoid signed overflow.
* libguile/numbers.c (scm_is_signed_integer): Avoid signed overflow.
---
 libguile/conv-integer.i.c | 15 ++++++++++-----
 libguile/numbers.c        | 15 ++++++++++-----
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/libguile/conv-integer.i.c b/libguile/conv-integer.i.c
index 4cf887c..fe2d363 100644
--- a/libguile/conv-integer.i.c
+++ b/libguile/conv-integer.i.c
@@ -64,25 +64,30 @@ SCM_TO_TYPE_PROTO (SCM val)
        }
       else
        {
-         scm_t_intmax n;
+         scm_t_uintmax abs_n;
+         TYPE n;
          size_t count;
 
          if (mpz_sizeinbase (SCM_I_BIG_MPZ (val), 2)
              > CHAR_BIT*sizeof (scm_t_uintmax))
            goto out_of_range;
          
-         mpz_export (&n, &count, 1, sizeof (scm_t_uintmax), 0, 0,
+         mpz_export (&abs_n, &count, 1, sizeof (scm_t_uintmax), 0, 0,
                      SCM_I_BIG_MPZ (val));
 
          if (mpz_sgn (SCM_I_BIG_MPZ (val)) >= 0)
            {
-             if (n < 0)
+             if (abs_n <= TYPE_MAX)
+               n = abs_n;
+             else
                goto out_of_range;
            }
          else
            {
-             n = -n;
-             if (n >= 0)
+             /* Carefully avoid signed integer overflow. */
+             if (TYPE_MIN < 0 && abs_n - 1 <= -(TYPE_MIN + 1))
+               n = -1 - (TYPE)(abs_n - 1);
+             else
                goto out_of_range;
            }
 
diff --git a/libguile/numbers.c b/libguile/numbers.c
index 1e3fc30..5aaac64 100644
--- a/libguile/numbers.c
+++ b/libguile/numbers.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1995-2015 Free Software Foundation, Inc.
+/* Copyright (C) 1995-2016 Free Software Foundation, Inc.
  *
  * Portions Copyright 1990, 1991, 1992, 1993 by AT&T Bell Laboratories
  * and Bellcore.  See scm_divide.
@@ -9622,6 +9622,7 @@ scm_is_signed_integer (SCM val, scm_t_intmax min, 
scm_t_intmax max)
        }
       else
        {
+         scm_t_uintmax abs_n;
          scm_t_intmax n;
          size_t count;
 
@@ -9629,18 +9630,22 @@ scm_is_signed_integer (SCM val, scm_t_intmax min, 
scm_t_intmax max)
              > CHAR_BIT*sizeof (scm_t_uintmax))
            return 0;
          
-         mpz_export (&n, &count, 1, sizeof (scm_t_uintmax), 0, 0,
+         mpz_export (&abs_n, &count, 1, sizeof (scm_t_uintmax), 0, 0,
                      SCM_I_BIG_MPZ (val));
 
          if (mpz_sgn (SCM_I_BIG_MPZ (val)) >= 0)
            {
-             if (n < 0)
+             if (abs_n <= max)
+               n = abs_n;
+             else
                return 0;
            }
          else
            {
-             n = -n;
-             if (n >= 0)
+             /* Carefully avoid signed integer overflow. */
+             if (min < 0 && abs_n - 1 <= -(min + 1))
+               n = -1 - (scm_t_intmax)(abs_n - 1);
+             else
                return 0;
            }
 
-- 
2.6.3