linux-kernel - Re: [PATCH 2/2] random: Account for entropy loss due to overwrites

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Message-ID: <51794141.3070205@zytor.com>
Date:	Thu, 25 Apr 2013 07:44:17 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"Ted Ts'o" <tytso@....edu>, DJ Johnston <dj.johnston@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH 2/2] random: Account for entropy loss due to overwrites

On 04/25/2013 07:38 AM, Linus Torvalds wrote:
> This doesn't work AT ALL. You even hint at the reason why in your message.
> 
> On Tue, Apr 23, 2013 at 9:26 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> As such, this conservatism seems more than justified.  Note, however,
>> that attempting to add one bit of entropy will never succeed; nor will
>> two bits unless the pool is completely empty.  These roundoff
>> artifacts could be improved by using fixed-point arithmetic and adding
>> some number of fractional entropy bits.
> 
> Take a look at "add_interrupt_randomness()". Hmm..
> 

<facepalm>

Right.  Before July of last year add_interrupt_randomness() called
add_timer_randomness() which credits up to 11 bits, but not anymore.

-ESTALEBRAIN.

OK, so we need to track fractional bits.

	-hpa


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