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Date: Mon, 30 Jan 2012 12:58:12 -0700
From: Jonathan Corbet <corbet@lwn.net>
To: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Pavel Emelyanov <xemul@parallels.com>,
        Serge Hallyn <serge.hallyn@canonical.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Kees Cook <keescook@chromium.org>, Tejun Heo <tj@kernel.org>,
        Andrew Vagin <avagin@openvz.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andi Kleen <andi@firstfloor.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>,
        Matt Helsley <matthltc@us.ibm.com>, Pekka Enberg <penberg@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Vasiliy Kulikov <segoon@openwall.com>, Valdis.Kletnieks@vt.edu
Subject: Re: [patch cr 2/4] [RFC] syscalls, x86: Add __NR_kcmp syscall v7
Message-ID: <20120130125812.0075dd04@dt>
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	<20120130141852.309402052@openvz.org>
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Just a couple of silly little things that came to mind while I was looking
at the code...

> +/*
> + * We don't expose real in-memory order of objects for security
> + * reasons, still the comparision results should be suitable for
> + * sorting. Thus, we obfuscate kernel pointers values (using random
> + * cookies obtaned at early boot stage) 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];
> +}

I don't understand the purpose of this at all.  Obfuscation will cause a
random shuffling in the ordering of the pointers - it's intended to - so
how is the result "suitable for sorting"?  More to the point, is there
ever a time when a user of this will care about some contrived ordering
value?  It seems like equality is all that really matters.

> +
> +/*
> + * 0 - equal
> + * 1 - less than
> + * 2 - greater than
> + * 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);
> +
> +	return (ret < 0) | ((ret > 0) << 1);
> +}

That's a cute trick, but do we know that every compiler that will ever see
this code will use 1 for a true integer comparison?  Simply spelling it
out with an if statement might be more robust, just as efficient, and, at
the same time, easier for others to understand.

Thanks,

jon