From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CF423C433F5 for ; Fri, 8 Apr 2022 18:34:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238775AbiDHSgJ (ORCPT ); Fri, 8 Apr 2022 14:36:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54786 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238771AbiDHSgI (ORCPT ); Fri, 8 Apr 2022 14:36:08 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 76EFC131F7C for ; Fri, 8 Apr 2022 11:34:04 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3D5D41042; Fri, 8 Apr 2022 11:34:03 -0700 (PDT) Received: from lakrids (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 300E03F718; Fri, 8 Apr 2022 11:33:59 -0700 (PDT) Date: Fri, 8 Apr 2022 19:33:53 +0100 From: Mark Rutland To: "Jason A. Donenfeld" Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, arnd@arndb.de, Theodore Ts'o , Dominik Brodowski , Russell King , Catalin Marinas , Will Deacon , Geert Uytterhoeven , Thomas Bogendoerfer , Paul Walmsley , Palmer Dabbelt , Albert Ou , "David S . Miller" , Richard Weinberger , Anton Ivanov , Johannes Berg , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H . Peter Anvin" , Chris Zankel , Max Filippov , John Stultz , Stephen Boyd , linux-arm-kernel@lists.infradead.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org, sparclinux@vger.kernel.org, linux-um@lists.infradead.org, x86@kernel.org, linux-xtensa@linux-xtensa.org Subject: Re: [PATCH RFC v1 07/10] arm64: use sched_clock() for random_get_entropy() instead of zero Message-ID: References: <20220408182145.142506-1-Jason@zx2c4.com> <20220408182145.142506-8-Jason@zx2c4.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220408182145.142506-8-Jason@zx2c4.com> Precedence: bulk List-ID: X-Mailing-List: linux-m68k@vger.kernel.org On Fri, Apr 08, 2022 at 08:21:42PM +0200, Jason A. Donenfeld wrote: > In the event that random_get_entropy() can't access a cycle counter or > similar, falling back to returning 0 is really not the best we can do. > Instead, at least calling sched_clock() would be preferable, because > that always needs to return _something_, even falling back to jiffies > eventually. It's not as though sched_clock() is super high precision or > guaranteed to be entropic, but basically anything that's not zero all > the time is better than returning zero all the time. > > If CONFIG_ARM_ARCH_TIMER=n, then get_cycles() will return 0, so we only > need the fallback code for that case. In arch/arm64/Kconfig we unconditionally select CONFIG_ARM_ARCH_TIMER, so that configuration shouldn't be possible, and I don't think this patch is necessary. On arm64 we depend on the architected timer in a bunch of places, so anyone hacking that out has bigger problems. Thanks, Mark. > > Cc: Arnd Bergmann > Cc: Catalin Marinas > Cc: Will Deacon > Signed-off-by: Jason A. Donenfeld > --- > arch/arm64/include/asm/timex.h | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/arch/arm64/include/asm/timex.h b/arch/arm64/include/asm/timex.h > index cf59ce91b22d..bfebd2e1ce45 100644 > --- a/arch/arm64/include/asm/timex.h > +++ b/arch/arm64/include/asm/timex.h > @@ -13,6 +13,15 @@ > */ > #define get_cycles() arch_timer_read_counter() > > +#ifndef CONFIG_ARM_ARCH_TIMER > +/* > + * The default implementation of random_get_entropy() calls get_cycles(), > + * which will return 0 if CONFIG_ARM_ARCH_TIMER=n, so we fall back to > + * sched_clock() here. Not a great situation, but better than nothing. > + */ > +#define random_get_entropy() ((unsigned long)sched_clock()) > +#endif > + > #include > > #endif > -- > 2.35.1 > > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel