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 43F70C4167B for ; Mon, 11 Dec 2023 15:40:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344159AbjLKPkA (ORCPT ); Mon, 11 Dec 2023 10:40:00 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35234 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344105AbjLKPj5 (ORCPT ); Mon, 11 Dec 2023 10:39:57 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 77138BD; Mon, 11 Dec 2023 07:40:03 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A6B83FEC; Mon, 11 Dec 2023 07:40:49 -0800 (PST) Received: from FVFF77S0Q05N.cambridge.arm.com (FVFF77S0Q05N.cambridge.arm.com [10.1.34.127]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B4D573F738; Mon, 11 Dec 2023 07:39:58 -0800 (PST) Date: Mon, 11 Dec 2023 15:39:56 +0000 From: Mark Rutland To: Will Deacon Cc: Robin Murphy , Joerg Roedel , Christoph Hellwig , Vineet Gupta , Russell King , Catalin Marinas , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , Paul Walmsley , Palmer Dabbelt , Albert Ou , Lorenzo Pieralisi , Hanjun Guo , Sudeep Holla , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Suravee Suthikulpanit , David Woodhouse , Lu Baolu , Niklas Schnelle , Matthew Rosato , Gerald Schaefer , Jean-Philippe Brucker , Rob Herring , Frank Rowand , Marek Szyprowski , Jason Gunthorpe , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-acpi@vger.kernel.org, iommu@lists.linux.dev, devicetree@vger.kernel.org Subject: Re: [PATCH 3/7] ACPI/IORT: Handle memory address size limits as limits Message-ID: References: <2ae6199a9cf035c1defd42e48675b827f41cdc95.1701268753.git.robin.murphy@arm.com> <20231211132757.GE25681@willie-the-truck> <91b22090-485f-49c9-a536-849fd7f92f8e@arm.com> <20231211153023.GA26048@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231211153023.GA26048@willie-the-truck> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 11, 2023 at 03:30:24PM +0000, Will Deacon wrote: > On Mon, Dec 11, 2023 at 03:01:27PM +0000, Robin Murphy wrote: > > On 2023-12-11 1:27 pm, Will Deacon wrote: > > > On Wed, Nov 29, 2023 at 05:43:00PM +0000, Robin Murphy wrote: > > > > Return the Root Complex/Named Component memory address size limit as an > > > > inclusive limit value, rather than an exclusive size. This saves us > > > > having to special-case 64-bit overflow, and simplifies our caller too. > > > > > > > > Signed-off-by: Robin Murphy > > > > --- > > > > drivers/acpi/arm64/dma.c | 9 +++------ > > > > drivers/acpi/arm64/iort.c | 18 ++++++++---------- > > > > include/linux/acpi_iort.h | 4 ++-- > > > > 3 files changed, 13 insertions(+), 18 deletions(-) > > > > > > [...] > > > > > > > diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c > > > > index 6496ff5a6ba2..eb64d8e17dd1 100644 > > > > --- a/drivers/acpi/arm64/iort.c > > > > +++ b/drivers/acpi/arm64/iort.c > > > > @@ -1367,7 +1367,7 @@ int iort_iommu_configure_id(struct device *dev, const u32 *input_id) > > > > { return -ENODEV; } > > > > #endif > > > > -static int nc_dma_get_range(struct device *dev, u64 *size) > > > > +static int nc_dma_get_range(struct device *dev, u64 *limit) > > > > { > > > > struct acpi_iort_node *node; > > > > struct acpi_iort_named_component *ncomp; > > > > @@ -1384,13 +1384,12 @@ static int nc_dma_get_range(struct device *dev, u64 *size) > > > > return -EINVAL; > > > > } > > > > - *size = ncomp->memory_address_limit >= 64 ? U64_MAX : > > > > - 1ULL<memory_address_limit; > > > > + *limit = (1ULL << ncomp->memory_address_limit) - 1; > > > > > > The old code handled 'ncomp->memory_address_limit >= 64' -- why is it safe > > > to drop that? You mention it in the cover letter, so clearly I'm missing > > > something! > > > > Because an unsigned shift by 64 or more generates 0 (modulo 2^64), thus > > subtracting 1 results in the correct all-bits-set value for an inclusive > > 64-bit limit. > > Oh, I'd have thought you'd have gotten one of those "left shift count >= > width of type" warnings if you did that. I think you'll get a UBSAN splat, but here the compiler doesn't know what 'ncomp->memory_address_limit' will be and so doesn't produce a compile-time warning. Regardless, it's undefined behaviour. Mark.