Messages in this thread Patch in this message | | | Date | Wed, 11 Apr 2012 02:48:09 +0400 | From | Cyrill Gorcunov <> | Subject | Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree |
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On Tue, Apr 10, 2012 at 03:39:53PM -0700, H. Peter Anvin wrote: > On 04/10/2012 03:37 PM, Cyrill Gorcunov wrote: > > + * bits even more (the oddity is important here, it allow > > + * us to have meaningful production even if multiplicants > > + * are big numbers). > > I would be more clear: > > "the odd multiplier guarantees that the product is unique ever after the > high bits are truncated, since any odd number is relative prime to 2^n". >
Yeah, updated, thanks. Andrew, fold it up please.
--- Subject: Add a comment on kcmp obfuscation method
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org> CC: "H. Peter Anvin" <hpa@zytor.com> --- kernel/kcmp.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
Index: linux-2.6.git/kernel/kcmp.c =================================================================== --- linux-2.6.git.orig/kernel/kcmp.c +++ linux-2.6.git/kernel/kcmp.c @@ -17,6 +17,17 @@ * reasons, still the comparison results should be suitable for * sorting. Thus, we obfuscate kernel pointers values and compare * the production instead. + * + * The obfuscation is done in two steps. First -- we use xor on + * kernel pointer with random value, which puts pointer into + * a new position in reordered space. Second -- we multiply + * the xor production with big odd random number to permute + * bits even more (the odd multiplier guarantees that the product + * is unique ever after the high bits are truncated, since any odd + * number is relative prime to 2^n). + * + * Note also the obfuscation itself is invisible to user-space + * and if needed it can be changed to any suitable scheme. */ static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
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