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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 6D6D0C49EA6 for ; Thu, 24 Jun 2021 16:38:46 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 0AF89613D8 for ; Thu, 24 Jun 2021 16:38:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0AF89613D8 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id CAA936B006C; Thu, 24 Jun 2021 12:38:44 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C592E6B0070; Thu, 24 Jun 2021 12:38:44 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AF9E06B0071; Thu, 24 Jun 2021 12:38:44 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0088.hostedemail.com [216.40.44.88]) by kanga.kvack.org (Postfix) with ESMTP id 80A036B006C for ; Thu, 24 Jun 2021 12:38:44 -0400 (EDT) Received: from smtpin12.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 9B767824C43A for ; Thu, 24 Jun 2021 16:38:44 +0000 (UTC) X-FDA: 78289176168.12.FF532C3 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf28.hostedemail.com (Postfix) with ESMTP id 64004200109A for ; Thu, 24 Jun 2021 16:38:43 +0000 (UTC) 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 5DF24ED1; Thu, 24 Jun 2021 09:38:42 -0700 (PDT) Received: from [10.57.9.136] (unknown [10.57.9.136]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6DEA03F719; Thu, 24 Jun 2021 09:38:40 -0700 (PDT) Subject: Re: [BUG] arm64: an infinite loop in generic_perform_write() To: Al Viro Cc: Matthew Wilcox , Christoph Hellwig , Chen Huang , Mark Rutland , Andrew Morton , Stephen Rothwell , Randy Dunlap , Catalin Marinas , Will Deacon , Linux ARM , linux-mm , open list References: <20210623132223.GA96264@C02TD0UTHF1T.local> <1c635945-fb25-8871-7b34-f475f75b2caf@huawei.com> <27fbb8c1-2a65-738f-6bec-13f450395ab7@arm.com> From: Robin Murphy Message-ID: <7896a3c7-2e14-d0f4-dbb9-286b6f7181b5@arm.com> Date: Thu, 24 Jun 2021 17:38:35 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Authentication-Results: imf28.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf28.hostedemail.com: domain of robin.murphy@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=robin.murphy@arm.com X-Rspamd-Server: rspam02 X-Stat-Signature: xhdidmit6571nezct4pjd6hmapjcg9gw X-Rspamd-Queue-Id: 64004200109A X-HE-Tag: 1624552723-381847 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2021-06-24 17:27, Al Viro wrote: > On Thu, Jun 24, 2021 at 02:22:27PM +0100, Robin Murphy wrote: > >> FWIW I think the only way to make the kernel behaviour any more robust here >> would be to make the whole uaccess API more expressive, such that rather >> than simply saying "I only got this far" it could actually differentiate >> between stopping due to a fault which may be recoverable and worth retrying, >> and one which definitely isn't. > > ... and propagate that "more expressive" information through what, 3 or 4 > levels in the call chain? > > From include/linux/uaccess.h: > > * If raw_copy_{to,from}_user(to, from, size) returns N, size - N bytes starting > * at to must become equal to the bytes fetched from the corresponding area > * starting at from. All data past to + size - N must be left unmodified. > * > * If copying succeeds, the return value must be 0. If some data cannot be > * fetched, it is permitted to copy less than had been fetched; the only > * hard requirement is that not storing anything at all (i.e. returning size) > * should happen only when nothing could be copied. In other words, you don't > * have to squeeze as much as possible - it is allowed, but not necessary. > > arm64 instances violate the aforementioned hard requirement. Please, fix > it there; it's not hard. All you need is an exception handler in .Ltiny15 > that would fall back to (short) byte-by-byte copy if the faulting address > happened to be unaligned. Or just do one-byte copy, not that it had been > considerably cheaper than a loop. Will be cheaper than propagating that extra > information up the call chain, let alone paying for extra ->write_begin() > and ->write_end() for single byte in generic_perform_write(). And what do we do if we then continue to fault with an external abort because whatever it is that warranted being mapped as Device-type memory in the first place doesn't support byte accesses? Robin.