oss-security - Re: gcc 4.2 optimizations and integer overflow checks

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Date: Mon, 7 Apr 2008 18:29:56 +0200
From: Marcus Meissner <meissner@...e.de>
To: oss-security@...ts.openwall.com
Subject: Re: gcc 4.2 optimizations and integer overflow checks

On Mon, Apr 07, 2008 at 12:12:34PM -0400, Josh Bressers wrote:
> Has anyone started to look at this yet:
> http://www.kb.cert.org/vuls/id/162289
> 
> I suspect this isn't going to be trivial to detect.  Ideas are welcome.
> 
> Thanks.

Please note this section:

"Note: this issue does not strictly constitute a vulnerability in gcc
 itself. The behavior that gcc exhibits in this case is permitted by the
 ISO/IEC 9899:1999 C specification (§6.5.6p8). "

The gcc folks have discussed this to death already and in the discussion
the code received a -Wstrict-overflow option.

	> cat xx.c
	int f(int a, int b) {
		return a < a+b;
	}

	int g(int a) {
		return f(a,1<<30);
	}
	> LANG=C projects/gcc/BIN/bin/gcc -O2 -c xx.c -Wstrict-overflow -Wall 
	xx.c: In function 'g':
	xx.c:2: warning: assuming signed overflow does not occur when assuming that (X + c) >= X is always true

Ciao, Marcus

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