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, URIBL_BLOCKED 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 99FF2C10F14 for ; Tue, 8 Oct 2019 07:23:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6D17A205C9 for ; Tue, 8 Oct 2019 07:23:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730279AbfJHHXy (ORCPT ); Tue, 8 Oct 2019 03:23:54 -0400 Received: from mail-ot1-f68.google.com ([209.85.210.68]:37526 "EHLO mail-ot1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730112AbfJHHXy (ORCPT ); Tue, 8 Oct 2019 03:23:54 -0400 Received: by mail-ot1-f68.google.com with SMTP id k32so13208798otc.4 for ; Tue, 08 Oct 2019 00:23:53 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=mL6qxVhbFk2FmzE5VpHZF8kr6YW4l9t0yp9McD5cEJ4=; b=XCapfFQNlPJG8E4givd44mx0Z+lC2hoRde28L0KCYL8q6+9OYigTYjv4wH5OzOMKrP WeHR2rLdqURi09Oz81jw1opmE/lERxWSZE/d/pgeXbwvodpHTYQItGZd0jgNakwpQ2PO rT3u0It4ish6wo+Bv+pnFujo/i9Ama1ZwPDyGDSG98CJNRLVDv+fe6kLm1l2AdsRjFG7 OiTJgm9nDm0wW1Gi321UCUq+cWFR6NDm47RnsTLTlc3BzIC0Y70x4uwL6qzjmOtvMKra ukCzpuRU7I9MMTv0aAPBDj+sO5p8ysEz+algAD8zhFHkCzUsyX9H7q/5Ai93otQoXYUO 5jhg== X-Gm-Message-State: APjAAAUkjAe/5pg7XpN1QXB/aYO1O2YwFcSsuvfu+RwTbCVxigzRYni2 CuqcUTmVM62Kv43ca8gJ5x3HobO8IxPT1maRpj4= X-Google-Smtp-Source: APXvYqzBxBjA9SP9xgsbBX3nHfq6xX6r+cx08pQ0i3Mfs00X2jkl6wYy6mf5ODikVpXq7l8GMG+mtSk5/Q5opydrC/M= X-Received: by 2002:a9d:7311:: with SMTP id e17mr22833601otk.107.1570519432827; Tue, 08 Oct 2019 00:23:52 -0700 (PDT) MIME-Version: 1.0 References: <20190801230358.4193-1-rikard.falkeborn@gmail.com> <20190811184938.1796-1-rikard.falkeborn@gmail.com> <20190811184938.1796-4-rikard.falkeborn@gmail.com> In-Reply-To: <20190811184938.1796-4-rikard.falkeborn@gmail.com> From: Geert Uytterhoeven Date: Tue, 8 Oct 2019 09:23:41 +0200 Message-ID: Subject: Re: [PATCH v3 3/3] linux/bits.h: Add compile time sanity check of GENMASK inputs To: Rikard Falkeborn Cc: Andrew Morton , Joe Perches , Johannes Berg , Linux Kernel Mailing List , Masahiro Yamada , Haren Myneni 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 Hi Rikard, On Sun, Aug 11, 2019 at 8:52 PM 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 > 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(). > > __builtin_constant_p does not evaluate is argument, it only checks if it > is a constant or not at compile time, and __builtin_choose_expr does not > evaluate the expression that is not chosen. Therefore, GENMASK(x++, 0) > does only evaluate x++ once. > > Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends > available in assembly") made the macros in linux/bits.h available in > assembly. Since BUILD_BUG_OR_ZERO() is not asm compatible, disable the > checks if the file is included in an asm file. > > Signed-off-by: Rikard Falkeborn > --- > Changes in v3: > - Changed back to shorter macro argument names > - Remove casts and use 0 instead of UL(0) in GENMASK_INPUT_CHECK(), > since all results in GENMASK_INPUT_CHECK() are now ints. Update > commit message to reflect that. > > 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, 5 of > the patches are not in Linus tree, and 2 are not in linux-next. There is > also a patch pending by Nathan Chancellor that also needs to be merged > before this patch is merged to avoid build failures. > > include/linux/bits.h | 21 +++++++++++++++++++-- > 1 file changed, 19 insertions(+), 2 deletions(-) > > diff --git a/include/linux/bits.h b/include/linux/bits.h > index 669d69441a62..4ba0fb609239 100644 > --- a/include/linux/bits.h > +++ b/include/linux/bits.h > @@ -18,12 +18,29 @@ > * position @h. For example > * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000. > */ > -#define GENMASK(h, l) \ > +#ifndef __ASSEMBLY__ > +#include > +#define GENMASK_INPUT_CHECK(h, l) \ > + (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ > + __builtin_constant_p((l) > (h)), (l) > (h), 0))) > +#else > +/* > + * BUILD_BUG_ON_ZERO is not available in h files included from asm files, > + * disable the input check if that is the case. > + */ > +#define GENMASK_INPUT_CHECK(h, l) 0 > +#endif > + > +#define __GENMASK(h, l) \ > (((~UL(0)) - (UL(1) << (l)) + 1) & \ > (~UL(0) >> (BITS_PER_LONG - 1 - (h)))) > +#define GENMASK(h, l) \ > + (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > > -#define GENMASK_ULL(h, l) \ > +#define __GENMASK_ULL(h, l) \ > (((~ULL(0)) - (ULL(1) << (l)) + 1) & \ > (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h)))) > +#define GENMASK_ULL(h, l) \ > + (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l)) > > #endif /* __LINUX_BITS_H */ This is now commit 0fd35cd30a2fece1 ("linux/bits.h: add compile time sanity check of GENMASK inputs") in next-20191008. reported the following failure in sun3_defconfig, which I managed to reproduce with gcc-4.6.3: lib/842/842_compress.c: In function '__split_add_bits': lib/842/842_compress.c:164:25: error: first argument to '__builtin_choose_expr' not a constant lib/842/842_compress.c:164:25: error: bit-field '' width not an integer constant scripts/Makefile.build:265: recipe for target 'lib/842/842_compress.o' failed __split_add_bits() calls GENMASK_ULL() with a non-constant. However __split_add_bits() itself is called with constants only. Apparently gcc fails to inline __split_add_bits(). Adding inline or always_inline doesn't help. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds