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=-1.0 required=3.0 tests=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 9A33EC433E0 for ; Wed, 1 Jul 2020 18:41:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 851D42085B for ; Wed, 1 Jul 2020 18:41:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726364AbgGASlo (ORCPT ); Wed, 1 Jul 2020 14:41:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33630 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725440AbgGASln (ORCPT ); Wed, 1 Jul 2020 14:41:43 -0400 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [IPv6:2002:c35c:fd02::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7EFADC08C5C1 for ; Wed, 1 Jul 2020 11:41:43 -0700 (PDT) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jqhfn-003Vzq-Dz; Wed, 01 Jul 2020 18:41:31 +0000 Date: Wed, 1 Jul 2020 19:41:31 +0100 From: Al Viro To: Linus Torvalds Cc: Josh Poimboeuf , Peter Zijlstra , the arch/x86 maintainers , Linux Kernel Mailing List Subject: Re: objtool clac/stac handling change.. Message-ID: <20200701184131.GI2786714@ZenIV.linux.org.uk> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 01, 2020 at 11:22:01AM -0700, Linus Torvalds wrote: > Josh / PeterZ, > it turns out that clang seems to now have fixed the last known > nagging details with "asm goto" with outputs, so I'm looking at > actually trying to merge the support for that in the kernel. > > The main annoyance isn't actually using "asm goto" at all, the main > annoyance is just that it will all have to be conditional on whether > the compiler supports it or not. We have the config option for that > already, but it will just end up with two copies of the code depending > on that option. > > It's not a huge deal: the recent cleanups wrt the x86 uaccess code > have made the code _much_ more straightforward and legible, and I'm > not so worried about it all. > > Except that when I looked at this, I realized that I really had picked > the wrong model for how exceptions are handled wrt stac/clac. In > particular user access exceptions return with stac set, so we end up > having a code pattern where the error case will also have to do the > user_access_end() to finish the STAC region. > > Adding a user_access_end() to the user exception fault handler is trivial. > > But the thing I'm asking you for is how nasty it would be to change > objtool to have those rules? > > IOW, right now we have > > if (!user_acces_begin(...)) > goto efault; > unsafe_get/put_user(ptr, val, label); > user_access_end(); > return 0; > > label: > user_access_end(); > efaulr: > return -EFAULT; > > and I'd like to make the "label" case just go to "efault", with > objtool knowing that the exception handling already did the > user_access_end(). > > That would end up cleaning up the flow for a number of cases. > > Nasty? Trivial? Rather nasty for ppc; they have separate user_read_access_end() and user_write_access_end().