[ python-Bugs-1545668 ] gcc trunk (4.2) exposes a signed integer overflows

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Bugs item #1545668, was opened at 2006-08-23 20:14
Message generated for change (Comment added) made by nnorwitz
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Category: Python Interpreter Core
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Jack Howarth (jwhowarth)
>Assigned to: Tim Peters (tim_one)
Summary: gcc trunk (4.2) exposes a signed integer overflows

Initial Comment:
While building python 2.4.3 with the current gcc trunk (soon to be 4.2), 
I uncovered a signed integer overflows bug in Python with the help of 
one of the gcc developers. The bug I observed is documented in this 
gcc mailing list message...

http://gcc.gnu.org/ml/gcc/2006-08/msg00436.html

The gcc developer comments about its origin are in the messages...

http://gcc.gnu.org/ml/gcc/2006-08/msg00434.html
http://gcc.gnu.org/ml/gcc/2006-08/msg00442.html

which in short says...

It *is* a bug in python, here is the proof:
https://codespeak.net/viewvc/vendor/cpython/Python-r243/dist/src/
Objects/intobject.c?revision=25647&view=markup
Function

* i_divmod*(*register* *long* x, *register* *long* y,

the following lines:

/        /* (-sys.maxint-1)/-1 is the only overflow case. *//
	*if* (y == -1 && x < 0 && x == -x)
		*return* DIVMOD_OVERFLOW;

If overflow is barred then x==-x may happen only when x==0.
This conflicts with x<0, which means that the compiler may assume
that
  x<0 && x==-x
always yields false. This may allow the compiler to eliminate the whole 
if
statement. Hence, clearly python is at fault.

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>Comment By: Neal Norwitz (nnorwitz)
Date: 2006-08-24 07:54

Message:
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Assigning to Tim, he likes these problems. :-)
He recently fixed a similar problem in another piece of
code.  I'm going to try to grep and see if I can find more
occurrences of this.

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Comment By: Jack Jansen (jackjansen)
Date: 2006-08-24 07:37

Message:
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Hmm... intobject.c doesn't really have a "champion"...

Neal, I'm assigning this to you purely on the ground that you're the last person 
to have edited this file. Feel free to pass on (but not back:-).

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Comment By: Jack Howarth (jwhowarth)
Date: 2006-08-24 04:22

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The was a few other comments from the gcc developers on the proposed 
fix...

http://gcc.gnu.org/ml/gcc/2006-08/msg00448.html
http://gcc.gnu.org/ml/gcc/2006-08/msg00449.html

...so that since x is a long the more correct fix is...

--- Python-2.4.3/Objects/intobject.c.org        2006-08-24 
07:06:51.000000000 -0400
+++ Python-2.4.3/Objects/intobject.c    2006-08-24 07:08:06.000000000 
-0400
@@ -479,7 +479,7 @@
                return DIVMOD_ERROR;
        }
        /* (-sys.maxint-1)/-1 is the only overflow case. */
-       if (y == -1 && x < 0 && x == -x)
+       if (y == -1 && x < 0 && ((unsigned long)x) == -(unsigned long)x)
                return DIVMOD_OVERFLOW;
        xdivy = x / y;
        xmody = x - xdivy * y;

I have tested this form of the patch and it works as well. My main concern is 
that we get this fix in python 2.5 before release. Jack, could you reassign this 
to the person you think might be most appropriate out of the list of python 
developers? I really should be a pretty simple review for the patch.


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Comment By: Jack Jansen (jackjansen)
Date: 2006-08-24 02:14

Message:
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I don't think this is a mac-specific bug, and I'm also not really the right person 
to look into this...

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Comment By: Jack Howarth (jwhowarth)
Date: 2006-08-23 21:13

Message:
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As suggested by another gcc developer in...

http://gcc.gnu.org/ml/gcc/2006-08/msg00446.html

...the following patch eliminates the error when python is built with gcc 
trunk...

--- Python-2.4.3/Objects/intobject.c.org        2006-08-23 
23:49:33.000000000 -0400
+++ Python-2.4.3/Objects/intobject.c    2006-08-23 23:52:01.000000000 
-0400
@@ -479,7 +479,7 @@
                return DIVMOD_ERROR;
        }
        /* (-sys.maxint-1)/-1 is the only overflow case. */
-       if (y == -1 && x < 0 && x == -x)
+       if (y == -1 && x < 0 && ((unsigned)x) == -(unsigned)x)
                return DIVMOD_OVERFLOW;
        xdivy = x / y;
        xmody = x - xdivy * y;

This change allows python to completely pass its make check now when built 
with gcc trunk.

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