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=-4.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, TVD_SUBJ_WIPE_DEBT,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 205B0C433E2 for ; Thu, 10 Sep 2020 19:22:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E23EC20882 for ; Thu, 10 Sep 2020 19:22:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727860AbgIJTWN (ORCPT ); Thu, 10 Sep 2020 15:22:13 -0400 Received: from gate.crashing.org ([63.228.1.57]:45384 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731446AbgIJPvQ (ORCPT ); Thu, 10 Sep 2020 11:51:16 -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 08AFKVl2000394; Thu, 10 Sep 2020 10:20:31 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 08AFKUG8000392; Thu, 10 Sep 2020 10:20:30 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 10 Sep 2020 10:20:30 -0500 From: Segher Boessenkool To: David Laight Cc: "'Christophe Leroy'" , "'Linus Torvalds'" , linux-arch , Kees Cook , the arch/x86 maintainers , Nick Desaulniers , Linux Kernel Mailing List , Alexey Dobriyan , Luis Chamberlain , Al Viro , linux-fsdevel , linuxppc-dev , Christoph Hellwig Subject: Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3 Message-ID: <20200910152030.GJ28786@gate.crashing.org> References: <20200903142242.925828-1-hch@lst.de> <20200903142803.GM1236603@ZenIV.linux.org.uk> <20200909184001.GB28786@gate.crashing.org> <3beb8b019e4a4f7b81fdb1bc68bd1e2d@AcuMS.aculab.com> <186a62fc-042c-d6ab-e7dc-e61b18945498@csgroup.eu> <59a64e9a210847b59f70f9bd2d02b5c3@AcuMS.aculab.com> <5050b43687c84515a49b345174a98822@AcuMS.aculab.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5050b43687c84515a49b345174a98822@AcuMS.aculab.com> User-Agent: Mutt/1.4.2.3i Sender: linux-arch-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Thu, Sep 10, 2020 at 12:26:53PM +0000, David Laight wrote: > Actually this is pretty sound: > __label__ label; > register int eax asm ("eax"); > // Ensure eax can't be reloaded from anywhere > // In particular it can't be reloaded after the asm goto line > asm volatile ("" : "=r" (eax)); This asm is fine. It says it writes the "eax" variable, which lives in the eax register *in that asm* (so *not* guaranteed after it!). > // Provided gcc doesn't save eax here... > asm volatile goto ("xxxxx" ::: "eax" : label); So this is incorrect. > // ... and reload the saved value here. > // The input value here will be that modified by the 'asm goto'. > // Since this modifies eax it can't be moved before the 'asm goto'. > asm volatile ("" : "+r" (eax)); > // So here eax must contain the value set by the "xxxxx" instructions. No, the register eax will contain the value of the eax variable. In the asm; it might well be there before or after the asm as well, but none of that is guaranteed. Segher