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

From: Valdis.Kletnieks@vt.edu
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>,
	Oleg Nesterov <oleg@redhat.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Pavel Emelyanov <xemul@parallels.com>,
	Andrey Vagin <avagin@openvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
	Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Glauber Costa <glommer@parallels.com>,
	Andi Kleen <andi@firstfloor.org>, Tejun Heo <tj@kernel.org>,
	Matt Helsley <matthltc@us.ibm.com>,
	Pekka Enberg <penberg@kernel.org>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	Vasiliy Kulikov <segoon@openwall.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Michal Marek <mmarek@suse.cz>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree
Date: Tue, 10 Apr 2012 19:58:49 -0400	[thread overview]
Message-ID: <7939.1334102329@turing-police.cc.vt.edu> (raw)
In-Reply-To: Your message of "Mon, 09 Apr 2012 15:10:27 -0700." <20120409151027.7f3e0fa5.akpm@linux-foundation.org>

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On Mon, 09 Apr 2012 15:10:27 -0700, Andrew Morton said:
> Back on to kcmp.

I've totally forgotten what my original comment(s) on this patch were.. :)

> +/*
> + * 0 - equal, i.e. v1 = v2
> + * 1 - less than, i.e. v1 < v2
> + * 2 - greater than, i.e. v1 > v2
> + * 3 - not equal but ordering unavailable (reserved for future)
> + */
> +static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
> +{
> +	long ret;
> +
> +	ret = kptr_obfuscate((long)v1, type) - kptr_obfuscate((long)v2, type);

I'm not able to convince myself that "less than" and "greater than" mean
anything - do we have a good proof that for all v1 and v2, the obfuscated
pointers have the same ordering as the original pointers?

Hmm... consider the simplified example  v1 = 5 and v2= 16., and cookies[0] is
also 16. Then obfus(v1) == 21, and obfus(v2) == 0, and the ordering is
different.  So I'm thinking 0 and 3 are the only sane return values?

Or do I need more caffeine?

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