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=-5.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 5E555C49EA5 for ; Thu, 24 Jun 2021 16:40:17 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 2F277613CC for ; Thu, 24 Jun 2021 16:40:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2F277613CC Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:Cc:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=ryDrs5Agpyh4GGhl6E/mBUU4TkOCYuhujB+jkzHK35k=; b=C35Kw1uuAor9eDw0nEy5/a/KjB ZDqbwc8uplqvhCYW1/HV9/NK4Mgg/pQ5v7hTsj06xj/C7SIXDmewMVXlwbT3bLcHlWWNm/80d4gEf jFTqbTbZ1XBJy8V5GM9+i3rC2zTE6f+glAdM6f5TP1RLYPIP40CPdjN2RKGurzPnMLp4XCKFYkq0v k7u4pjG8AKuQuFXRH7HKoOdYHc0rwbDxWLpJmVaQTwAn2gbz9vjdN/ZJ7xcqx08vHa6FeU3s7Wd+l raLk1e38CRS3V7XoBL97VTtRiUD4TqiDdarJ8TFpIWCZ1rByhdUeAkkBDduHQ1/ii2RhBdJx3V1vV QX1MqQvg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1lwSNO-00FVou-Uu; Thu, 24 Jun 2021 16:38:51 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1lwSNJ-00FVnQ-FL for linux-arm-kernel@lists.infradead.org; Thu, 24 Jun 2021 16:38:48 +0000 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-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210624_093845_612006_E727B230 X-CRM114-Status: GOOD ( 22.41 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel