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

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Date:	Wed, 15 Feb 2012 17:06:52 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Alexey Dobriyan <adobriyan@...il.com>, Valdis.Kletnieks@...edu,
	Michal Marek <mmarek@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

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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