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 18B29C4332F for ; Thu, 17 Nov 2022 19:26:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240186AbiKQT0c (ORCPT ); Thu, 17 Nov 2022 14:26:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37334 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234540AbiKQT0a (ORCPT ); Thu, 17 Nov 2022 14:26:30 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F09DCDA4 for ; Thu, 17 Nov 2022 11:26:29 -0800 (PST) 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 A05C3B821C3 for ; Thu, 17 Nov 2022 19:26:28 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 68CD1C433D6; Thu, 17 Nov 2022 19:26:26 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="fKZGboZT" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1668713184; 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=4miayFE14wny2mfBEslDdups7GCejc3Lbk78v1jBxlM=; b=fKZGboZTtshrffcHojQQYR/YysoPKel7w/6GLqYGZ8JiWNU+ranJ6JYveLPMNJJ2DvXkev rxCBiFqrNY2svEhwcXYAN9GI80gZ5D1WeuPLewWnEELrHKr4SLCHEBZTFft8rA9RLuH09E pJdUbnAQ8btv25DZ1ugoP45nCRFu8S4= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 9093a911 (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Thu, 17 Nov 2022 19:26:24 +0000 (UTC) From: "Jason A. Donenfeld" To: linux-kernel@vger.kernel.org, tytso@mit.edu, kees@kernel.org, linux@armlinux.org.uk, ydroneaud@opteya.com, gregkh@linuxfoundation.org, rdunlap@infradead.org Cc: "Jason A. Donenfeld" Subject: [PATCH v3] random: add helpers for random numbers with given floor or range Date: Thu, 17 Nov 2022 20:26:20 +0100 Message-Id: <20221117192620.2304613-1-Jason@zx2c4.com> In-Reply-To: <2b3bc31e-c308-b04c-1759-26bcf8dba6d3@infradead.org> References: <2b3bc31e-c308-b04c-1759-26bcf8dba6d3@infradead.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Now that we have get_random_u32_below(), it's nearly trivial to make inline helpers to compute get_random_u32_above() and get_random_u32_inclusive(), which will help clean up open coded loops and manual computations throughout the tree. One snag is that in order to make get_random_u32_inclusive() operate on closed intervals, we have to do some (unlikely) special case handling if get_random_u32_interval(0, U32_MAX) is called. The least expensive way of doing this is actually to adjust the slowpath of get_random_u32_below() to have its undefined 0 result just return the output of get_random_u32(). We can make this basically free by calling get_random_u32() before the branch, so that the branch latency gets interleaved. Signed-off-by: Jason A. Donenfeld --- v2->v3: - Fix doc comment argument order. - Interleave !ceil check with get_random_u32() call to make branch ~free. drivers/char/random.c | 18 +++++++++++++++++- include/linux/random.h | 25 +++++++++++++++++++++++++ 2 files changed, 42 insertions(+), 1 deletion(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 6f323344d0b9..f5868dddbb61 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -161,6 +161,8 @@ EXPORT_SYMBOL(wait_for_random_bytes); * u16 get_random_u16() * u32 get_random_u32() * u32 get_random_u32_below(u32 ceil) + * u32 get_random_u32_above(u32 floor) + * u32 get_random_u32_inclusive(u32 floor, u32 ceil) * u64 get_random_u64() * unsigned long get_random_long() * @@ -522,7 +524,21 @@ u32 __get_random_u32_below(u32 ceil) * of `-ceil % ceil` is analogous to `2^32 % ceil`, but is computable * in 32-bits. */ - u64 mult = (u64)ceil * get_random_u32(); + u32 rand = get_random_u32(); + u64 mult; + + /* + * This function is technically undefined for ceil == 0, and in fact + * for the non-underscored constant version in the header, we build bug + * on that. But for the non-constant case, it's convenient to have that + * evaluate to being a straight call to get_random_u32(), so that + * get_random_u32_inclusive() can work over its whole range without + * undefined behavior. + */ + if (unlikely(!ceil)) + return rand; + + mult = (u64)ceil * rand; if (unlikely((u32)mult < ceil)) { u32 bound = -ceil % ceil; while (unlikely((u32)mult < bound)) diff --git a/include/linux/random.h b/include/linux/random.h index 3a82c0a8bc46..bd954ecbef90 100644 --- a/include/linux/random.h +++ b/include/linux/random.h @@ -91,6 +91,31 @@ static inline u32 get_random_u32_below(u32 ceil) } } +/* + * Returns a random integer in the interval (floor, U32_MAX], with uniform + * distribution, suitable for all uses. Fastest when floor is a constant, but + * still fast for variable floor as well. + */ +static inline u32 get_random_u32_above(u32 floor) +{ + BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && floor == U32_MAX, + "get_random_u32_above() must take floor < U32_MAX"); + return floor + 1 + get_random_u32_below(U32_MAX - floor); +} + +/* + * Returns a random integer in the interval [floor, ceil], with uniform + * distribution, suitable for all uses. Fastest when floor and ceil are + * constant, but still fast for variable floor and ceil as well. + */ +static inline u32 get_random_u32_inclusive(u32 floor, u32 ceil) +{ + BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && __builtin_constant_p(ceil) && + (floor > ceil || ceil - floor == U32_MAX), + "get_random_u32_inclusive() must take floor <= ceil"); + return floor + get_random_u32_below(ceil - floor + 1); +} + /* * On 64-bit architectures, protect against non-terminated C string overflows * by zeroing out the first byte of the canary; this leaves 56 bits of entropy. -- 2.38.1