From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751095AbdAMDLL (ORCPT ); Thu, 12 Jan 2017 22:11:11 -0500 Received: from mx1.redhat.com ([209.132.183.28]:40056 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751051AbdAMDLJ (ORCPT ); Thu, 12 Jan 2017 22:11:09 -0500 Date: Thu, 12 Jan 2017 21:11:07 -0600 From: Josh Poimboeuf To: Andy Lutomirski Cc: Linus Torvalds , Herbert Xu , Linux Kernel Mailing List , Linux Crypto Mailing List , Ingo Molnar , Thomas Gleixner , Andy Lutomirski , Ard Biesheuvel Subject: Re: x86-64: Maintain 16-byte stack alignment Message-ID: <20170113031107.mgitq54fmjnrvi6f@treble> References: <20170111043541.GA4944@gondor.apana.org.au> <20170112140215.rh247gwk55fjzmg7@treble> <20170112201511.yj5ekqmj76r2yv6t@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.6.0.1 (2016-04-01) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Fri, 13 Jan 2017 03:11:10 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 12, 2017 at 05:46:55PM -0800, Andy Lutomirski wrote: > On Thu, Jan 12, 2017 at 12:15 PM, Josh Poimboeuf wrote: > > On Thu, Jan 12, 2017 at 12:08:07PM -0800, Andy Lutomirski wrote: > >> On Thu, Jan 12, 2017 at 11:51 AM, Linus Torvalds > >> wrote: > >> > On Thu, Jan 12, 2017 at 6:02 AM, Josh Poimboeuf wrote: > >> >> > >> >> Just to clarify, I think you're asking if, for versions of gcc which > >> >> don't support -mpreferred-stack-boundary=3, objtool can analyze all C > >> >> functions to ensure their stacks are 16-byte aligned. > >> >> > >> >> It's certainly possible, but I don't see how that solves the problem. > >> >> The stack will still be misaligned by entry code. Or am I missing > >> >> something? > >> > > >> > I think the argument is that we *could* try to align things, if we > >> > just had some tool that actually then verified that we aren't missing > >> > anything. > >> > > >> > I'm not entirely happy with checking the generated code, though, > >> > because as Ingo says, you have a 50:50 chance of just getting it right > >> > by mistake. So I'd much rather have some static tool that checks > >> > things at a code level (ie coccinelle or sparse). > >> > >> What I meant was checking the entry code to see if it aligns stack > >> frames, and good luck getting sparse to do that. Hmm, getting 16-byte > >> alignment for real may actually be entirely a lost cause. After all, > >> I think we have some inline functions that do asm volatile ("call > >> ..."), and I don't see any credible way of forcing alignment short of > >> generating an entirely new stack frame and aligning that. > > > > Actually we already found all such cases and fixed them by forcing a new > > stack frame, thanks to objtool. For example, see 55a76b59b5fe. > > What I mean is: what guarantees that the stack is properly aligned for > the subroutine call? gcc promises to set up a stack frame, but does > it promise that rsp will be properly aligned to call a C function? Yes, I did an experiment and you're right. I had naively assumed that all stack frames would be aligned. -- Josh