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Message-Id: <9C232B22-5B93-463A-AE1B-581F9C30390C@apple.com>
Date: Fri, 3 Dec 2010 11:05:53 -0800
From: Geoff Keating <geoffk@...le.com>
To: oss-security@...ts.openwall.com
Cc: "Chad R. Dougherty" <crd@...t.org>,
David Svoboda <svoboda@...t.org>
Subject: Re: Interesting behavior with struct initiailization
On 03/12/2010, at 6:44 AM, Robert Seacord wrote:
> With respect to this specific problem:
>
>> then the compiler is free to change the padding bytes after 'x.b' to whatever it likes, because you changed 'x.a', even though you might >
>> think you cleared them and the compiler would have no reason to make this change. In practice this might manifest in the case of
>
>> memset (&x, 0, sizeof(x));
>> x.a = 1; x.b = 2; x.c = 3;
>
>> by the compiler optimising out the 'memset' as a dead store.
>
> CERT proposed #5 memset_s() to clear memory, without fear of removal (see http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1358.pdf).
Even if the memset is not removed, a compiler could implement 'x.b = 2' by
- setting the low byte of a 32-bit register to 2, leaving the high bytes unchanged
- storing all 32 bits of the register into memory
which would store nonzero data in the high bytes, possibly containing sensitive information.
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