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=DKIM_SIGNED,DKIM_VALID, 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 6A91BC0650F for ; Sat, 3 Aug 2019 03:12:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 22A1B2086A for ; Sat, 3 Aug 2019 03:12:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nifty.com header.i=@nifty.com header.b="SUC1VGFb" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389259AbfHCDEK (ORCPT ); Fri, 2 Aug 2019 23:04:10 -0400 Received: from conssluserg-05.nifty.com ([210.131.2.90]:61780 "EHLO conssluserg-05.nifty.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388026AbfHCDEJ (ORCPT ); Fri, 2 Aug 2019 23:04:09 -0400 Received: from mail-vs1-f53.google.com (mail-vs1-f53.google.com [209.85.217.53]) (authenticated) by conssluserg-05.nifty.com with ESMTP id x7333p6K016272 for ; Sat, 3 Aug 2019 12:03:51 +0900 DKIM-Filter: OpenDKIM Filter v2.10.3 conssluserg-05.nifty.com x7333p6K016272 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nifty.com; s=dec2015msa; t=1564801432; bh=xefQyfTB9Thx4g2/D9xmFX+SV4M+SYdJ8qvtaBfCpAg=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=SUC1VGFbvjNg8TR0m4Z87KXocg7RZz45mbiRV7swbIZT0WRXi9ALyqgclZXRJEv4T yXqjNZp4g8F37AXTqUoreM5Hr2PGSJLNn9uysPtc4Xe77eIoxvfKK0prkFFTPFRWrA Q87gD+uj4EDPcPJg0a2UYvA6LXkSpThO0BQ/zgerrgWtthJcRj/5hwm/xZLeUBCvQu 1ctdBoHVof+BhMFHQJe4CG80jFdU6fowK3TEmlietpMBV6pBsoEWCkFVu2MTn7YPml vUwV8kREgQ1S/dUqys2svs8Pd5icHMpSkJ1d98JsXjrXrMAJ6D/fEIlUwq1dC3+ZwZ hoSdsMRO6ay3g== X-Nifty-SrcIP: [209.85.217.53] Received: by mail-vs1-f53.google.com with SMTP id h28so52556337vsl.12 for ; Fri, 02 Aug 2019 20:03:51 -0700 (PDT) X-Gm-Message-State: APjAAAVAL8PqoQMNGw7b5dthwDsGSygfwm+cV0KCLNueMtfMGIY0pNi4 PikYNEcDbxi4XLHJ7GOA6ooKvnsqTP9Q6w7TBcE= X-Google-Smtp-Source: APXvYqwGrIuOtQ1jgTbAYjL1Q5OITIAE7rGKKcJ5TAj3RMbTaYMdU2sPG3dVW1Sww+/uAQpMYJ+hlAb8wIyfbcEv2WY= X-Received: by 2002:a67:8e0a:: with SMTP id q10mr64840206vsd.215.1564801430468; Fri, 02 Aug 2019 20:03:50 -0700 (PDT) MIME-Version: 1.0 References: <0306bec0ec270b01b09441da3200252396abed27.camel@perches.com> <20190731190309.19909-1-rikard.falkeborn@gmail.com> <47d29791addc075431737aff4b64531a668d4c1b.camel@perches.com> <2b782cf609330f53b6ecc5b75a8a4b49898483eb.camel@perches.com> <20190802181853.GA809@rikard> In-Reply-To: <20190802181853.GA809@rikard> From: Masahiro Yamada Date: Sat, 3 Aug 2019 12:03:13 +0900 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs To: Rikard Falkeborn Cc: Joe Perches , Andrew Morton , Johannes Berg , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Aug 3, 2019 at 3:19 AM Rikard Falkeborn wrote: > > On Fri, Aug 02, 2019 at 12:25:06PM +0900, Masahiro Yamada wrote: > > On Fri, Aug 2, 2019 at 12:14 PM Joe Perches wrote: > > > > > > On Fri, 2019-08-02 at 10:40 +0900, Masahiro Yamada wrote: > > > > On Thu, Aug 1, 2019 at 4:27 AM Joe Perches wrote: > > > > > On Wed, 2019-07-31 at 21:03 +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_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 > > > > > [] > > > > > > @@ -18,12 +18,22 @@ > > > > > > * position @h. For example > > > > > > * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000. > > > > > > */ > > > > > > +#ifndef __ASSEMBLY__ > > > > > > +#include > > > > > > +#define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ > > > > > > + __is_constexpr(h) && __is_constexpr(l), (l) > (h), 0)) > > > > > > +#else > > > > > > +#define GENMASK_INPUT_CHECK(h, l) 0 > > > > > > > > > > A few things: > > > > > > > > > > o Reading the final code is a bit confusing. > > > > > Perhaps add a comment description saying it's not checked > > > > > in asm .h uses. > > > > > > > > > > o Maybe use: > > > > > #define GENMASK_INPUT_CHECK(h, l) UL(0) > > > > > > > > Why? > > > > > > Consistency with the uses in what's now called __GENMASK > > > > Inconsistent with __GENMASK_ULL. > > Would you prefer to add GENMASK_ULL_INPUT_CHECK? No. > Or replace UL(0) with > 0 and then probably move the cast of BUILD_BUG_OR_ZERO (to avoid > GENMASK be of type size_t) to GENMASK and GENMASK_ULL? No. Your original code is absolutely fine. C aligns the types to the wider one. (unsigned long) + (int) -> (unsigned long) (unsigned long long) + (int) -> (unsigned long long) Having GENMASK_INPUT_CHECK to return 'int' is OK. The resulted GENMASK(), GENMASK_ULL() still have unsigned long, unsigned long long, respectively. BTW, v2 is already inconsistent. If you wanted GENMASK_INPUT_CHECK() to return 'unsigned long',, you would have to cast (low) > (high) as well: (unsigned long)((low) > (high)), UL(0)))) This is totally redundant, and weird. -- Best Regards Masahiro Yamada