linux-kernel - Re: drivers/char/random.c: variable type mismatch

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Date:	Wed, 14 Nov 2012 17:15:36 +0100
From:	Stephan Mueller <stephan.mueller@...ec.com>
To:	Theodore Ts'o <tytso@....edu>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: drivers/char/random.c: variable type mismatch

On 08.11.2012 12:47:08, +0100, Theodore Ts'o <tytso@....edu> wrote:

Hi Theodore,

> On Thu, Nov 08, 2012 at 01:32:38AM +0100, Stephan Mueller wrote:
>>
>> However, due to the fact that jiffies provides very few entropy, the
>> event value provides (almost) none, the majority of entropy comes from
>> the processor cycles. Assuming that the processor cycles increase once
>> per nanosecond, after 2**32 cycles (about 4 seconds), the counter wraps.
> 
> Sure, we can make this change, but it doesn't make as much difference
> as you think.  The high 32 bits gets incremented about ounce every 4
> seconds, while jiffies gets incremented once every 1/HZ seconds.  But
> the point is they are pretty well correlated (i.e., if you know the
> jiffies values A' and A'', and I know the high 32 bits of the cycles
> B', and you can determine the likely value of B'' to a very high
> degree of accuracy.  Values which are correlated don't actually
> entropy.
> 

I agree with the argument of correlation.

So, if the reduction of values with a known lack of entropy is of
interest, why not change the jiffies variable type too? There we know
that only the lower 32 bits are really relevant. Therefore, wouldn't be
a structure of

        struct {
                unsigned jiffies;
                unsigned cycles;
                unsigned num;
        } sample;

be more appropriate?

Thanks
Stephan

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