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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 DDD7EC55179 for ; Fri, 23 Oct 2020 18:34:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9E6F020882 for ; Fri, 23 Oct 2020 18:34:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754983AbgJWSes (ORCPT ); Fri, 23 Oct 2020 14:34:48 -0400 Received: from gate.crashing.org ([63.228.1.57]:46520 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754809AbgJWSep (ORCPT ); Fri, 23 Oct 2020 14:34:45 -0400 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 09NIREDK014153; Fri, 23 Oct 2020 13:27:15 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 09NIRD8Q014147; Fri, 23 Oct 2020 13:27:13 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Fri, 23 Oct 2020 13:27:13 -0500 From: Segher Boessenkool To: Al Viro Cc: David Hildenbrand , "linux-aio@kvack.org" , "linux-mips@vger.kernel.org" , David Howells , "linux-mm@kvack.org" , "keyrings@vger.kernel.org" , "sparclinux@vger.kernel.org" , Christoph Hellwig , "linux-arch@vger.kernel.org" , "linux-s390@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "kernel-team@android.com" , Arnd Bergmann , "linux-block@vger.kernel.org" , "io-uring@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , Jens Axboe , "linux-parisc@vger.kernel.org" , "'Greg KH'" , Nick Desaulniers , "linux-kernel@vger.kernel.org" , "linux-security-module@vger.kernel.org" , David Laight , "netdev@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , Andrew Morton , "linuxppc-dev@lists.ozlabs.org" Subject: Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c" Message-ID: <20201023182713.GG2672@gate.crashing.org> References: <5fd6003b-55a6-2c3c-9a28-8fd3a575ca78@redhat.com> <20201022104805.GA1503673@kroah.com> <20201022121849.GA1664412@kroah.com> <98d9df88-b7ef-fdfb-7d90-2fa7a9d7bab5@redhat.com> <20201022125759.GA1685526@kroah.com> <20201022135036.GA1787470@kroah.com> <134f162d711d466ebbd88906fae35b33@AcuMS.aculab.com> <935f7168-c2f5-dd14-7124-412b284693a2@redhat.com> <20201023175857.GA3576660@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201023175857.GA3576660@ZenIV.linux.org.uk> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Oct 23, 2020 at 06:58:57PM +0100, Al Viro wrote: > On Fri, Oct 23, 2020 at 03:09:30PM +0200, David Hildenbrand wrote: > > > Now, I am not a compiler expert, but as I already cited, at least on > > x86-64 clang expects that the high bits were cleared by the caller - in > > contrast to gcc. I suspect it's the same on arm64, but again, I am no > > compiler expert. > > > > If what I said and cites for x86-64 is correct, if the function expects > > an "unsigned int", it will happily use 64bit operations without further > > checks where valid when assuming high bits are zero. That's why even > > converting everything to "unsigned int" as proposed by me won't work on > > clang - it assumes high bits are zero (as indicated by Nick). > > > > As I am neither a compiler experts (did I mention that already? ;) ) nor > > an arm64 experts, I can't tell if this is a compiler BUG or not. > > On arm64 when callee expects a 32bit argument, the caller is *not* responsible > for clearing the upper half of 64bit register used to pass the value - it only > needs to store the actual value into the lower half. The callee must consider > the contents of the upper half of that register as undefined. See AAPCS64 (e.g. > https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst#parameter-passing-rules > ); AFAICS, the relevant bit is > "Unlike in the 32-bit AAPCS, named integral values must be narrowed by > the callee rather than the caller." Or the formal rule: C.9 If the argument is an Integral or Pointer Type, the size of the argument is less than or equal to 8 bytes and the NGRN is less than 8, the argument is copied to the least significant bits in x[NGRN]. The NGRN is incremented by one. The argument has now been allocated. Segher