linux-kernel - Re: [BUG] perf: bogus correlation of kernel symbols

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Message-ID: <20110523193554.GA10975@elte.hu>
Date:	Mon, 23 May 2011 21:35:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Ray Lee <ray-lk@...rabbit.org>
Cc:	Dan Rosenberg <drosenberg@...curity.com>,
	Tony Luck <tony.luck@...il.com>, linux-kernel@...r.kernel.org,
	davej@...hat.com, kees.cook@...onical.com, davem@...emloft.net,
	eranian@...gle.com, torvalds@...ux-foundation.org,
	adobriyan@...il.com, penberg@...nel.org, hpa@...or.com,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [BUG] perf: bogus correlation of kernel symbols


* Ray Lee <ray-lk@...rabbit.org> wrote:

> On Mon, May 23, 2011 at 3:49 AM, Ingo Molnar <mingo@...e.hu> wrote:
> > Well, since entropy does not get reduced on addition of independent variables
> > the right sequence is (pseudocode):
> >
> >        rnd  = entropy_cycles();
> >        rnd += entropy_rdrand();
> >        rnd += entropy_RTC();
> >        rnd += entropy_system();
> 
> I think you mean concatenation rather than addition? Or perhaps XOR, or a 
> hash? [...]

Yeah.

In this special case probably concatenation works the best: the above 4 random 
variables have total randomness probably less than 32 bits, so we want to 
create 4 tight random numbers and concatenate them.

[ XOR would destroy some fair amount of entropy because most of these random 
  variables have their randomness in their low bits, and a hash would probably
  lose about 2 bits and would also be slower. A hash would probably be safer
  and more robust though, if we mis-identify any of the random variables. ]

Thanks,

	Ingo
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