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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A6A8FC433FF for ; Thu, 1 Aug 2019 23:14:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7EA75206B8 for ; Thu, 1 Aug 2019 23:14:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732369AbfHAXOR (ORCPT ); Thu, 1 Aug 2019 19:14:17 -0400 Received: from smtprelay0235.hostedemail.com ([216.40.44.235]:40226 "EHLO smtprelay.hostedemail.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728834AbfHAXOQ (ORCPT ); Thu, 1 Aug 2019 19:14:16 -0400 Received: from filter.hostedemail.com (clb03-v110.bra.tucows.net [216.40.38.60]) by smtprelay04.hostedemail.com (Postfix) with ESMTP id 3B420180A814E; Thu, 1 Aug 2019 23:14:15 +0000 (UTC) X-Session-Marker: 6A6F6540706572636865732E636F6D X-HE-Tag: baby46_24fe664d8034b X-Filterd-Recvd-Size: 3608 Received: from XPS-9350.home (cpe-23-242-196-136.socal.res.rr.com [23.242.196.136]) (Authenticated sender: joe@perches.com) by omf09.hostedemail.com (Postfix) with ESMTPA; Thu, 1 Aug 2019 23:14:14 +0000 (UTC) Message-ID: Subject: Re: [PATCH v2 2/2] linux/bits.h: Add compile time sanity check of GENMASK inputs From: Joe Perches To: Rikard Falkeborn Cc: akpm@linux-foundation.org, johannes@sipsolutions.net, linux-kernel@vger.kernel.org, yamada.masahiro@socionext.com Date: Thu, 01 Aug 2019 16:14:12 -0700 In-Reply-To: <20190801230358.4193-2-rikard.falkeborn@gmail.com> References: <20190731190309.19909-1-rikard.falkeborn@gmail.com> <20190801230358.4193-1-rikard.falkeborn@gmail.com> <20190801230358.4193-2-rikard.falkeborn@gmail.com> Content-Type: text/plain; charset="ISO-8859-1" User-Agent: Evolution 3.30.5-0ubuntu0.18.10.1 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2019-08-02 at 01:03 +0200, Rikard Falkeborn wrote: > GENMASK() and GENMASK_ULL() are supposed to be called with the high bit > as the first argument and the low bit as the second argument. Mixing > them will return a mask with zero bits set. > > Recent commits show getting this wrong is not uncommon, see e.g. > commit aa4c0c9091b0 ("net: stmmac: Fix misuses of GENMASK macro") and > commit 9bdd7bb3a844 ("clocksource/drivers/npcm: Fix misuse of GENMASK > macro"). > > To prevent such mistakes from appearing again, add compile time sanity > checking to the arguments of GENMASK() and GENMASK_ULL(). If both the > arguments are known at compile time, and the low bit is higher than the > high bit, break the build to detect the mistake immediately. > > Since GENMASK() is used in declarations, BUILD_BUG_ON_ZERO() must be > used instead of BUILD_BUG_ON(), and __is_constexpr() must be used instead > of __builtin_constant_p(). > > If successful, BUILD_BUG_OR_ZERO() returns 0 of type size_t. To avoid > problems with implicit conversions, cast the result of BUILD_BUG_OR_ZERO > to unsigned long. > > Since both BUILD_BUG_ON_ZERO() and __is_constexpr() only uses sizeof() > on the arguments passed to them, neither of them evaluate the expression > unless it is a VLA. Therefore, GENMASK(1, x++) still behaves as > expected. > > Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends > available in assembly") made the macros in linux/bits.h available in > assembly. Since neither BUILD_BUG_OR_ZERO() or __is_constexpr() are asm > compatible, disable the checks if the file is included in an asm file. > > Signed-off-by: Rikard Falkeborn > --- > Changes in v2: > - Add comment about why inputs are not checked when used in asm file > - Use UL(0) instead of 0 > - Extract mask creation in a separate macro to improve readability > - Use high and low instead of h and l (part of this was extracted to a > separate patch) > - Updated commit message > > Joe Perches sent a series to fix the existing misuses of GENMASK() that > needs to be merged before this to avoid build failures. Currently, 6 of > the patches are not in Linus tree, and 2 are not in linux-next. Thanks Rikard. It wouldn't surprise me if this change finds more misuses as the compiler will perform substitutions on #define values where the search I did was just for decimal uses. For instance, this new macro should build error on: #define FOO 5 #define BAR 6 GENMASK(FOO, BAR)