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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 42787C433FF for ; Thu, 1 Aug 2019 02:57:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1E569206B8 for ; Thu, 1 Aug 2019 02:57:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728032AbfHAC5w (ORCPT ); Wed, 31 Jul 2019 22:57:52 -0400 Received: from smtprelay0034.hostedemail.com ([216.40.44.34]:59120 "EHLO smtprelay.hostedemail.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726626AbfHAC5w (ORCPT ); Wed, 31 Jul 2019 22:57:52 -0400 Received: from filter.hostedemail.com (clb03-v110.bra.tucows.net [216.40.38.60]) by smtprelay07.hostedemail.com (Postfix) with ESMTP id 0E0C3181D3377; Thu, 1 Aug 2019 02:57:51 +0000 (UTC) X-Session-Marker: 6A6F6540706572636865732E636F6D X-HE-Tag: hot98_2748f26d67433 X-Filterd-Recvd-Size: 4139 Received: from XPS-9350 (cpe-23-242-196-136.socal.res.rr.com [23.242.196.136]) (Authenticated sender: joe@perches.com) by omf13.hostedemail.com (Postfix) with ESMTPA; Thu, 1 Aug 2019 02:57:49 +0000 (UTC) Message-ID: Subject: Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs From: Joe Perches To: Masahiro Yamada , Rikard Falkeborn Cc: Andrew Morton , Johannes Berg , Linux Kernel Mailing List Date: Wed, 31 Jul 2019 19:57:48 -0700 In-Reply-To: References: <0306bec0ec270b01b09441da3200252396abed27.camel@perches.com> <20190731190309.19909-1-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 Thu, 2019-08-01 at 11:50 +0900, Masahiro Yamada wrote: > On Thu, Aug 1, 2019 at 4:04 AM 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_OR_ZERO() must be > > used instead of BUILD_BUG_ON(), and __is_constexpr() must be used instead > > of __builtin_constant_p(). > > > > 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 > > --- > > Joe Perches sent a series to fix the existing misuses of GENMASK() that > > needs to be merged before this to avoid build failures. Currently, 7 of > > the patches were not in Linus tree, and 2 were not in linux-next. > > > > Also, there's currently no asm users of bits.h, but since it was made > > asm-compatible just two weeks ago it would be a shame to break it right > > away... [] > > diff --git a/include/linux/bits.h b/include/linux/bits.h [] > > #define GENMASK(h, l) \ > > + (GENMASK_INPUT_CHECK(h, l) + \ > > (((~UL(0)) - (UL(1) << (l)) + 1) & \ > > - (~UL(0) >> (BITS_PER_LONG - 1 - (h)))) > > + (~UL(0) >> (BITS_PER_LONG - 1 - (h))))) > > > > #define GENMASK_ULL(h, l) \ > > + (GENMASK_INPUT_CHECK(h, l) + \ > > (((~ULL(0)) - (ULL(1) << (l)) + 1) & \ > > - (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h)))) > > + (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h))))) > > This is getting cluttered with so many parentheses. > > One way of clean-up is to rename the current macros as follows: > > GENMASK() -> __GENMASK() > GENMASK_UL() -> __GENMASK_ULL() > > Then, > > #define GENMASK(h, l) (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > #define GENMASK_ULL(h, l) (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l)) Much nicer. It may be better still to use avoid multiple dereferences of each argument. Also it'd be useful to rename h and l to something like high_bit and low_bit or high and low.