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=-4.0 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 4D0ECC433E0 for ; Wed, 22 Jul 2020 13:14:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2A2012065F for ; Wed, 22 Jul 2020 13:14:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730018AbgGVNO0 convert rfc822-to-8bit (ORCPT ); Wed, 22 Jul 2020 09:14:26 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:53483 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726003AbgGVNO0 (ORCPT ); Wed, 22 Jul 2020 09:14:26 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-116-sW_rWl1MOCeWFYsGvVYncg-1; Wed, 22 Jul 2020 14:14:22 +0100 X-MC-Unique: sW_rWl1MOCeWFYsGvVYncg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Wed, 22 Jul 2020 14:14:22 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Wed, 22 Jul 2020 14:14:22 +0100 From: David Laight To: 'Catalin Marinas' , Linus Torvalds CC: Al Viro , linux-arch , Linux Kernel Mailing List Subject: RE: [RFC] raw_copy_from_user() semantics Thread-Topic: [RFC] raw_copy_from_user() semantics Thread-Index: AQHWYBxygAO6HUS840aFW/LsMa1rTakTksjw Date: Wed, 22 Jul 2020 13:14:21 +0000 Message-ID: <8fde1b9044a34ff59eb5ff3dafbf2b97@AcuMS.aculab.com> References: <20200719031733.GI2786714@ZenIV.linux.org.uk> <20200722113707.GC27540@gaia> In-Reply-To: <20200722113707.GC27540@gaia> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Catalin Marinas > Sent: 22 July 2020 12:37 > > On Sun, Jul 19, 2020 at 12:34:11PM -0700, Linus Torvalds wrote: > > On Sun, Jul 19, 2020 at 12:28 PM Linus Torvalds > > wrote: > > > I think we should try to get rid of the exact semantics. > > > > Side note: I think one of the historical reasons for the exact > > semantics was that we used to do things like the mount option copying > > with a "copy_from_user()" iirc. > > > > And that could take a fault at the end of the stack etc, because > > "copy_mount_options()" is nasty and doesn't get a size, and just > > copies "up to 4kB" of data. > > > > It's a mistake in the interface, but it is what it is. But we've > > always handled the inexact count there anyway by originally doing byte > > accesses, and at some point you optimized it to just look at where > > page boundaries might be.. > > And we may have to change this again since, with arm64 MTE, the page > boundary check is insufficient: > > https://lore.kernel.org/linux-fsdevel/20200715170844.30064-25-catalin.marinas@arm.com/ > > While currently the fault path is unlikely to trigger, with MTE in user > space it's a lot more likely since the buffer (e.g. a string) is > normally less than 4K and the adjacent addresses would have a different > colour. > > I looked (though briefly) into passing the copy_from_user() problem to > filesystems that would presumably know better how much to copy. In most > cases the options are string, so something like strncpy_from_user() > would work. For mount options as binary blobs (IIUC btrfs) maybe the fs > has a better way to figure out how much to copy. What about changing the mount code to loop calling get_user() to read aligned words until failure? Mount is fairly uncommon and the extra cost is probably small compared to the rest of doing a mount. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)