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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 764D0C83001 for ; Tue, 28 Apr 2020 14:06:33 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 422F720775 for ; Tue, 28 Apr 2020 14:06:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 422F720775 Authentication-Results: mail.kernel.org; dmarc=none (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 C95378E0005; Tue, 28 Apr 2020 10:06:32 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C1E128E0001; Tue, 28 Apr 2020 10:06:32 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AE5948E0005; Tue, 28 Apr 2020 10:06:32 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0101.hostedemail.com [216.40.44.101]) by kanga.kvack.org (Postfix) with ESMTP id 930048E0001 for ; Tue, 28 Apr 2020 10:06:32 -0400 (EDT) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 1B9D88149 for ; Tue, 28 Apr 2020 14:06:32 +0000 (UTC) X-FDA: 76757439024.04.wren74_7330946a4b957 X-HE-Tag: wren74_7330946a4b957 X-Filterd-Recvd-Size: 3934 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf01.hostedemail.com (Postfix) with ESMTP for ; Tue, 28 Apr 2020 14:06:31 +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 D9E2731B; Tue, 28 Apr 2020 07:06:30 -0700 (PDT) Received: from gaia (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 30FA73F68F; Tue, 28 Apr 2020 07:06:29 -0700 (PDT) Date: Tue, 28 Apr 2020 15:06:27 +0100 From: Catalin Marinas To: Dave Martin Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, Richard Earnshaw , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , Peter Collingbourne , linux-mm@kvack.org, Alexander Viro , Vincenzo Frascino , Will Deacon Subject: Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass Message-ID: <20200428140626.GJ3868@gaia> References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-21-catalin.marinas@arm.com> <20200427165641.GC15808@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200427165641.GC15808@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Mon, Apr 27, 2020 at 05:56:42PM +0100, Dave P Martin wrote: > On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote: > > The copy_mount_options() function takes a user pointer argument but not > > a size. It tries to read up to a PAGE_SIZE. However, copy_from_user() is > > not guaranteed to return all the accessible bytes if, for example, the > > access crosses a page boundary and gets a fault on the second page. To > > work around this, the current copy_mount_options() implementations > > performs to copy_from_user() passes, first to the end of the current > > page and the second to what's left in the subsequent page. > > > > Some architectures like arm64 can guarantee an exact copy_from_user() > > depending on the size (since the arch function performs some alignment > > on the source register). Introduce an arch_has_exact_copy_from_user() > > function and allow copy_mount_options() to perform the user access in a > > single pass. > > > > While this function is not on a critical path, the single-pass behaviour > > is required for arm64 MTE (memory tagging) support where a uaccess can > > trigger intra-page faults (tag not matching). With the current > > implementation, if this happens during the first page, the function will > > return -EFAULT. > > Do you know how much extra overhead we'd incur if we read at must one > tag granule at a time, instead of PAGE_SIZE? Our copy routines already read 16 bytes at a time, so that's the tag granule. With current copy_mount_options() we have the issue that it assumes a fault in the first page is fatal. Even if we change it to a loop of smaller uaccess, we still have the issue of unaligned accesses which can fail without reading all that's possible (i.e. the access goes across a tag granule boundary). The previous copy_mount_options() implementation (from couple of months ago I think) had a fallback to byte-by-byte, didn't have this issue. > I'm guessing that in practice strcpy_from_user() type operations copy > much less than a page most of the time, so what we lose in uaccess > overheads we _might_ regain in less redundant copying. strncpy_from_user() has a fallback to byte by byte, so we don't have an issue here. The above is only for synchronous accesses. For async, in v3 I disabled such checks for the uaccess routines. -- Catalin