Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755482AbbFLOKh (ORCPT ); Fri, 12 Jun 2015 10:10:37 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33713 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752713AbbFLOKc (ORCPT ); Fri, 12 Jun 2015 10:10:32 -0400 Date: Fri, 12 Jun 2015 09:10:30 -0500 From: Josh Poimboeuf To: Pedro Alves Cc: Ingo Molnar , Andy Lutomirski , Michal Marek , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "linux-kernel@vger.kernel.org" , Andi Kleen , live-patching@vger.kernel.org, X86 ML , "H. Peter Anvin" , Linus Torvalds , Peter Zijlstra Subject: Re: [PATCH v5 02/10] x86: Compile-time asm code validation Message-ID: <20150612141030.GA18807@treble.hsd1.ky.comcast.net> References: <20150610175320.GD29724@treble.redhat.com> <20150610185845.GA1125@treble.redhat.com> <20150611061050.GB30720@gmail.com> <20150611141000.GC11517@treble.redhat.com> <557ABFF8.5040304@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <557ABFF8.5040304@redhat.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1978 Lines: 40 On Fri, Jun 12, 2015 at 12:18:16PM +0100, Pedro Alves wrote: > On 06/11/2015 03:10 PM, Josh Poimboeuf wrote: > > > C would definitely make more sense when analyzing object code. In fact, > > asmvalidate is written in C. But then I guess we'd have to re-implement > > the .cfi stuff and populate the DWARF sections manually instead of > > letting the assembler do it. > > Was doing all this directly in the assembler considered? That is, > e.g., add some knob that makes it error/warn in the same conditions > you're making the validator catch. For tail calls, you'd e.g., add > some new ".nonlocal" directive that you'd use to whitelist the > following jump. And then if it's possible run a CFI generator > as a separate step over the source, it sounds like it should also > be possible to have the assembler do it instead too (again with > some new high level directive to trigger/help it). In general I think doing these types of things in the assembler would be a good idea. Missing or inaccurate debug data for asm code seems to be a common problem for other projects as well. As Andy pointed out, they're doing similar things in musl [1]. So it might be useful to add an option to the assembler which validates that the code conforms to certain structural rules, and then inserts frame pointer and/or .cfi directives. That said, the kernel has much more custom features than other projects. There are some sneaky macros, like _ASM_EXTABLE and ALTERNATIVE, which hide code in various sections. Unless we're able to somehow either stop using these macros or isolate them to a few places, I doubt that such a general purpose assembler option would work. [1] http://www.openwall.com/lists/musl/2015/05/31/5 -- Josh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/