From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: x86-64: Maintain 16-byte stack alignment Date: Thu, 12 Jan 2017 17:46:55 -0800 Message-ID: References: <20170111031124.GA4515@gondor.apana.org.au> <20170111043541.GA4944@gondor.apana.org.au> <20170112140215.rh247gwk55fjzmg7@treble> <20170112201511.yj5ekqmj76r2yv6t@treble> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: Linus Torvalds , Herbert Xu , Linux Kernel Mailing List , Linux Crypto Mailing List , Ingo Molnar , Thomas Gleixner , Andy Lutomirski , Ard Biesheuvel To: Josh Poimboeuf Return-path: Received: from mail-ua0-f177.google.com ([209.85.217.177]:36848 "EHLO mail-ua0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751003AbdAMBrR (ORCPT ); Thu, 12 Jan 2017 20:47:17 -0500 Received: by mail-ua0-f177.google.com with SMTP id 96so27421466uaq.3 for ; Thu, 12 Jan 2017 17:47:17 -0800 (PST) In-Reply-To: <20170112201511.yj5ekqmj76r2yv6t@treble> Sender: linux-crypto-owner@vger.kernel.org List-ID: 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?