Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754289Ab3ILMgD (ORCPT ); Thu, 12 Sep 2013 08:36:03 -0400 Received: from mail.eperm.de ([89.247.134.16]:44897 "EHLO mail.eperm.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752949Ab3ILMgB (ORCPT ); Thu, 12 Sep 2013 08:36:01 -0400 From: Stephan Mueller To: Geert Uytterhoeven Cc: "Theodore Ts'o" , LKML , dave.taht@bufferbloat.net Subject: Re: [PATCH] /dev/random: Insufficient of entropy on many architectures Date: Thu, 12 Sep 2013 14:35:53 +0200 Message-ID: <2140053.ysf7239sM3@tauon> User-Agent: KMail/4.10.5 (Linux/3.10.10-200.fc19.x86_64; KDE/4.10.5; x86_64; ; ) In-Reply-To: References: <10005394.BRCyBMYWy3@tauon> <5896310.H6rEVunj63@tauon> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2011 Lines: 61 Am Donnerstag, 12. September 2013, 14:15:35 schrieb Geert Uytterhoeven: Hi Geert, >On Thu, Sep 12, 2013 at 2:08 PM, Stephan Mueller wrote: >>>BTW, I prefer a different name than "random_get_fast_cycles()", as >>>it's better to have something that returns different and >>>unpredictable numbers than an actual monotonic cycle counter. >>> >> A monotonic counter is fully ok. Note, for /dev/random, the >> occurrence >> of events delivers entropy. Thus, we have to be able to precisely >> measure that occurrence. The timer itself does not need to deliver >> any >> entropy as long as it is fast. > >Well, in my specific case (m68k/Amiga) I can use: > - a 24-bit counter running at only ca. 15 or 31 kHz (actual >frequency may vary), > - a 16-bit counter running at ca. 700 kHz. > >That is, if they have to be monotonic cycle counters. > >If not, I can mix the two (e.g. "a << 8 | (b & 0xff)") to get a 32-bit >value. That result would be fine for /dev/random, I guess, but it's >not really "get_cycles()". Note, get_cycles should return an u64. Not sure what a and b here is, but if a is the 24 bit value and b the faster 16 bit value, wouldn't there be a gap? I.e. wouldn't it be better to use the full 16 bit counter as low value and OR the 24 bit on bits 48 to 17? Yet, there is a break in that counter: the 16 low bits rotate several times (around 10 times) before bit 17 is changed once. > >Gr{oetje,eeting}s, > > Geert > >-- >Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- >geert@linux-m68k.org > >In personal conversations with technical people, I call myself a >hacker. But when I'm talking to journalists I just say "programmer" or >something like that. -- Linus Torvalds Ciao Stephan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/