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 CC110C5517A for ; Sat, 24 Oct 2020 17:36:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A9DA522254 for ; Sat, 24 Oct 2020 17:36:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726758AbgJXRgp (ORCPT ); Sat, 24 Oct 2020 13:36:45 -0400 Received: from gate.crashing.org ([63.228.1.57]:45310 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762594AbgJXRgo (ORCPT ); Sat, 24 Oct 2020 13:36:44 -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 09OHTAUE029400; Sat, 24 Oct 2020 12:29:10 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 09OHT4g2029397; Sat, 24 Oct 2020 12:29:04 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Sat, 24 Oct 2020 12:29:03 -0500 From: Segher Boessenkool To: David Laight Cc: Al Viro , 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" , "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: <20201024172903.GK2672@gate.crashing.org> References: <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> <20201023182713.GG2672@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Fri, Oct 23, 2020 at 09:28:59PM +0000, David Laight wrote: > From: Segher Boessenkool > > Sent: 23 October 2020 19:27 > > 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: > > > 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. > > So, in essence, if the value is in a 64bit register the calling > code is independent of the actual type of the formal parameter. > Clearly a value might need explicit widening. No, this says that if you pass a 32-bit integer in a 64-bit register, then the top 32 bits of that register hold an undefined value. > I've found a copy of the 64 bit arm instruction set. > Unfortunately it is alpha sorted and repetitive so shows none > of the symmetry and makes things difficult to find. All of this is ABI, not ISA. Look at the AAPCS64 pointed to above. > But, contrary to what someone suggested most register writes > (eg from arithmetic) seem to zero/extend the high bits. Everything that writes a "w" does, yes. But that has nothing to do with the parameter passing rules, that is ABI. It just means that very often a 32-bit integer will be passed zero-extended in a 64-bit register, but that is just luck (or not, it makes finding bugs harder ;-) ) Segher