linux-kernel - Re: [RFC] syscalls, x86: Add __NR

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Date:	Fri, 20 Jan 2012 06:51:31 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>, david@...g.hm,
	Alexey Dobriyan <adobriyan@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrey Vagin <avagin@...nvz.org>, Ingo Molnar <mingo@...e.hu>,
	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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Valdis.Kletnieks@...edu
Subject: Re: [RFC] syscalls, x86: Add __NR_kcmp syscall

On 01/20/2012 01:02 AM, Cyrill Gorcunov wrote:
> On Fri, Jan 20, 2012 at 12:40:08PM +0400, Cyrill Gorcunov wrote:
> ...
>> +
>> +static __init int kcmp_cookie_init(void)
>> +{
>> +	int i, j;
>> +
>> +	for (i = 0; i < KCMP_TYPES; i++) {
>> +		for (j = 0; j < 2; j++) {
>> +			get_random_bytes(&cookies[i][j], sizeof(cookies[i][j]));
>> +
>> +			if (cookies[i][j])
>> +				continue;
>> +
>> +			/*
>> +			 * This is impossible case but just to be sure.
>> +			 */
>> +			cookies_valid = false;
>> +			WARN_ONCE(1, "Can't get random bytes for k-pointers\n");
>> +		}
>> +	}
>> +
>> +	cookies_valid = true;
> 
> darn, this string of course should be on top,
> i'll update don't complain on this nit.
> 

This code is wrong.  You will have a zero cookie, legitimately, once in
2^32 or 2^64 attempts, depending on the bitness.

The other thing is that for the multiplicative cookie you should OR in
the value (~(~0UL >> 1) | 1) in order to make sure that the value is (a)
large and (b) odd.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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