Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

Oleg Nesterov
2012-02-15T16:15:48Z

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.