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

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Message-ID: <20121108114708.GB5859@thunk.org>
Date:	Thu, 8 Nov 2012 06:47:08 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Stephan Mueller <stephan.mueller@...ec.com>
Cc:	lkml <linux-kernel@...r.kernel.org>
Subject: Re: drivers/char/random.c: variable type mismatch

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.

							- Ted
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