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SubjectRe: [patch 2/4] [RFC] syscalls, x86: Add __NR_kcmp syscall v4
On Mon, Jan 23, 2012 at 10:48:57AM -0800, H. Peter Anvin wrote:
> On 01/23/2012 06:20 AM, Cyrill Gorcunov wrote:
> > +
> > +static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
> > +
> > +static long kptr_obfuscate(long v, int type)
> > +{
> > + return (v + cookies[type][0]) ^ cookies[type][1];
> > +}
> > +
>
> Arf... when I said to use xor I meant instead of the add, not instead of
> the multiply, so:
>
> return (v ^ cookies[type][0]) * cookies[type][1];
>
> Otherwise you have absolutely no source of diffusion at all (symmetric
> cryptography is about combinations of diffusion -- spreading the content
> -- and confusion -- scrambling individual bits of content.)
>
> + for (i = 0; i < KCMP_TYPES; i++) {
> + for (j = 0; j < 2; j++) {
> + get_random_bytes(&cookies[i][j],
> + sizeof(cookies[i][j]));
> + cookies[i][j] |= (~(~0UL >> 1) | 1);
> + }
> + }
>
> Only cookies[1] -- being used as a multiplicative constant -- needs the OR.

OK. Thanks. I'll update.

Cyrill


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