From patchwork Fri Apr 8 18:21:42 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Jason A. Donenfeld" X-Patchwork-Id: 558928 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 505FBC433F5 for ; Fri, 8 Apr 2022 18:23:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238655AbiDHSZe (ORCPT ); Fri, 8 Apr 2022 14:25:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43260 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238614AbiDHSZS (ORCPT ); Fri, 8 Apr 2022 14:25:18 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E5A99385F1B; Fri, 8 Apr 2022 11:23:13 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 9962DB82CE1; Fri, 8 Apr 2022 18:23:12 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 96180C385A3; Fri, 8 Apr 2022 18:23:07 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="bHvsH7he" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1649442186; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=78dSRR6cg4Cb6xE6wzqZl0y3T/C5RqbL9OTQfhaSus0=; b=bHvsH7heTJ22wd9kOVkjQRjB8qxqDYb0Neo0BG7ETnElqZmWD+KvzZiWfTFZwr1zQmUbC5 XXdPLoX/hfqwadSYoHEgXfmb2MBW+rOO7AO7VyqUuq9QbfjjauYGlTUsreemS7BXRmsC9S aoLAL9JjFgYgrOUBP32exCl+fVMfezY= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 2a1ba7e8 (TLSv1.3:AEAD-AES256-GCM-SHA384:256:NO); Fri, 8 Apr 2022 18:23:06 +0000 (UTC) From: "Jason A. Donenfeld" To: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, arnd@arndb.de Cc: "Jason A. Donenfeld" , 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: [PATCH RFC v1 07/10] arm64: use sched_clock() for random_get_entropy() instead of zero Date: Fri, 8 Apr 2022 20:21:42 +0200 Message-Id: <20220408182145.142506-8-Jason@zx2c4.com> In-Reply-To: <20220408182145.142506-1-Jason@zx2c4.com> References: <20220408182145.142506-1-Jason@zx2c4.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org 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. 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