Messages in this thread | | | Date | Wed, 15 Feb 2012 17:06:52 +0100 | From | Oleg Nesterov <> | Subject | Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree |
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Not a comment, but the question. I am just curious...
> +/* > + * We don't expose real in-memory order of objects for security > + * reasons, still the comparison results should be suitable for > + * sorting. Thus, we obfuscate kernel pointers values and compare > + * the production instead. > + */ > +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]; > +}
OK, but why do we need this per type? Just to add more obfuscation or there is another reason?
> +static __init int kcmp_cookies_init(void) > +{ > + int i; > + > + get_random_bytes(cookies, sizeof(cookies)); > + > + for (i = 0; i < KCMP_TYPES; i++) > + cookies[i][1] |= (~(~0UL >> 1) | 1);
I am puzzled, help ;) this is equal to
cookies[i][1] |= -LONG_MAX; or cookies[i][1] |= (LONG_MIN | 1);
for what? why do we want to set these 2 bits (MSB and LSB) ?
Oleg.
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