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

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Message-ID: <507CE663.2060502@linux.intel.com>
Date:	Mon, 15 Oct 2012 21:45:23 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Theodore Ts'o <tytso@....edu>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	greg@...ah.com, w@....eu, ewust@...ch.edu, zakir@...ch.edu,
	mpm@...enic.com, nadiah@...ucsd.edu, jhalderm@...ch.edu,
	tglx@...utronix.de, davem@...emloft.net, mingo@...nel.org,
	DJ Johnston <dj.johnston@...el.com>, stable@...r.kernel.org
Subject: Re: [PATCH RFC] random: Account for entropy loss due to overwrites

On 10/15/2012 09:08 PM, Theodore Ts'o wrote:
> On Sat, Sep 29, 2012 at 12:47:04PM -0700, H. Peter Anvin wrote:
>>> -static struct poolinfo {
>>> +static const struct poolinfo {
>>> +	int poolshift;		/* log2(POOLBITS) */
>>>  	int poolwords;
>>>  	int tap1, tap2, tap3, tap4, tap5;
> 
> Poolshift is duplicated information; it's just log2(poolwords) + 5
> (since POOLBITS is poolwords*32).
> 
> Granted you don't want to recalculate it every single time you need to
> use it, but perhaps it would be better to add poolshift to struct
> entropy_store, and set it in init_std_data()?
> 

Or we could compute poolwords (and poolbits, and poolbytes) from it,
since shifts generally are cheap.  I don't strongly care, whatever your
preference is.

	-hpa



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