linux-kernel - Re: [PATCH 01/10] random: make 'add_interrupt

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Message-ID: <1341524367.4020.1324.camel@calx>
Date:	Thu, 05 Jul 2012 16:39:27 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Theodore Ts'o <tytso@....edu>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	w@....eu, ewust@...ch.edu, zakir@...ch.edu, greg@...ah.com,
	nadiah@...ucsd.edu, jhalderm@...ch.edu, tglx@...utronix.de,
	davem@...emloft.net, stable@...nel.org
Subject: Re: [PATCH 01/10] random: make 'add_interrupt_randomness()' do
 something sane

On Thu, 2012-07-05 at 11:52 -0700, Linus Torvalds wrote:
> On Thu, Jul 5, 2012 at 11:47 AM, Matt Mackall <mpm@...enic.com> wrote:
> >
> > I think you should demand a minimum number of events > HZ to actually
> > credit any valid entropy.
> 
> There already is. It's 1.

> If we don't get a single non-timer interrupt, this code will never be called.

>From my read, this code path gets called on timer interrupts too. Thus
the number of events per HZ will be HZ at a minimum on systems that
haven't gone tickless. If such systems a) don't have a higher-res time
source and b) are halted or equivalent, such samples will be completely
deterministic. So the threshold for crediting entropy should be HZ + 1.

But perhaps I've missed something.

(As I expressed in my last message, this is strictly a correctness issue
and not a practical one. I've long held that the entropy counting model
is bogus and should be abandoned.)

-- 
Mathematics is the supreme nostalgia of our time.


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