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=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,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 0C849C432BE for ; Mon, 23 Aug 2021 21:54:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DB734611CB for ; Mon, 23 Aug 2021 21:54:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233001AbhHWVz0 convert rfc822-to-8bit (ORCPT ); Mon, 23 Aug 2021 17:55:26 -0400 Received: from foss.arm.com ([217.140.110.172]:57400 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232898AbhHWVzY (ORCPT ); Mon, 23 Aug 2021 17:55:24 -0400 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 CF9D41042; Mon, 23 Aug 2021 14:54:40 -0700 (PDT) Received: from [127.0.0.1] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5FB0B3F766; Mon, 23 Aug 2021 14:54:40 -0700 (PDT) Date: Mon, 23 Aug 2021 22:54:35 +0100 From: Steven Price To: dri-devel@lists.freedesktop.org, Alyssa Rosenzweig CC: Alyssa Rosenzweig , Rob Herring , Tomeu Vizoso , David Airlie , Daniel Vetter , linux-kernel@vger.kernel.org, Chris Morgan , stable@vger.kernel.org Subject: Re: [PATCH 1/3] drm/panfrost: Simplify lock_region calculation User-Agent: K-9 Mail for Android In-Reply-To: References: <20210820213117.13050-1-alyssa.rosenzweig@collabora.com> <20210820213117.13050-2-alyssa.rosenzweig@collabora.com> <192e5a1b-2caf-11a8-f090-ec5649ea16b5@arm.com> Message-ID: <7076BF0A-C40E-4E5A-9185-FDB3882EC539@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 23 August 2021 22:09:08 BST, Alyssa Rosenzweig wrote: >> > In lock_region, simplify the calculation of the region_width parameter. >> > This field is the size, but encoded as log2(ceil(size)) - 1. >> > log2(ceil(size)) may be computed directly as fls(size - 1). However, we >> > want to use the 64-bit versions as the amount to lock can exceed >> > 32-bits. >> > >> > This avoids undefined behaviour when locking all memory (size ~0), >> > caught by UBSAN. >> >> It might have been useful to mention what it is that UBSAN specifically >> picked up (it took me a while to spot) - but anyway I think there's a >> bigger issue with it being completely wrong when size == ~0 (see below). > >Indeed. I've updated the commit message in v2 to explain what goes >wrong (your analysis was spot on, but a mailing list message is more >ephermal than a commit message). I'll send out v2 tomorrow assuming >nobody objects to v1 in the mean time. > >Thanks for the review. > >> There is potentially a third bug which kbase only recently attempted to >> fix. The lock address is effectively rounded down by the hardware (the >> bottom bits are ignored). So if you have mask=(1<> (iova & mask) != ((iova + size) & mask) then you are potentially failing >> to lock the end of the intended region. kbase has added some code to >> handle this: >> >> > /* Round up if some memory pages spill into the next region. */ >> > region_frame_number_start = pfn >> (lockaddr_size_log2 - PAGE_SHIFT); >> > region_frame_number_end = >> > (pfn + num_pages - 1) >> (lockaddr_size_log2 - PAGE_SHIFT); >> > >> > if (region_frame_number_start < region_frame_number_end) >> > lockaddr_size_log2 += 1; >> >> I guess we should too? > >Oh, I missed this one. Guess we have 4 bugs with this code instead of >just 3, yikes. How could such a short function be so deeply and horribly >broken? ���� > >Should I add a fourth patch to the series to fix this? Yes please! Thanks, Steve