From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755416AbbALUOz (ORCPT ); Mon, 12 Jan 2015 15:14:55 -0500 Received: from mail-la0-f52.google.com ([209.85.215.52]:34406 "EHLO mail-la0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754189AbbALUOy (ORCPT ); Mon, 12 Jan 2015 15:14:54 -0500 MIME-Version: 1.0 In-Reply-To: <20150112201145.GH3904@pd.tnic> References: <1421017655-25561-1-git-send-email-dvlasenk@redhat.com> <20150112192314.GF3904@pd.tnic> <20150112193704.GG3904@pd.tnic> <20150112201145.GH3904@pd.tnic> From: Andy Lutomirski Date: Mon, 12 Jan 2015 12:14:32 -0800 Message-ID: Subject: Re: [PATCH] x86: introduce push/pop macros which generate CFI_REL_OFFSET and CFI_RESTORE To: Borislav Petkov Cc: Denys Vlasenko , "linux-kernel@vger.kernel.org" , Linus Torvalds , Oleg Nesterov , "H. Peter Anvin" , Frederic Weisbecker , X86 ML , Alexei Starovoitov , Will Drewry , Kees Cook Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 12, 2015 at 12:11 PM, Borislav Petkov wrote: > On Mon, Jan 12, 2015 at 11:46:53AM -0800, Andy Lutomirski wrote: >> Dumb example: >> >> pushq_cfi $__KERNEL_DS /* ss */ >> >> This doesn't save anything that the unwinder would care about. > > And? The unwinder or whatever looks at that info simply ignores stuff it > is not interested in, no? But CFI_REL_OFFSET $__KERNEL_DS, 0 probably isn't even well-formed and won't build. > >> Better example: >> >> pushq_cfi \child_rip /* rip */ >> CFI_REL_OFFSET rip,0 >> >> Doing this with a macro would need a fancier macro. > > I'd ask first whether we really need this at all. > >> Then there's crap like: >> >> pushq_cfi %rdi >> SCHEDULE_USER >> popq_cfi %rdi > > I guess we can add a gas regname argument optional and if it is set, use > it and if not, use the reg itself... Or something like that in the best > effort type of approach. > >> I would need to look a lot more carefully to figure out whether this >> would need CFI_REL_OFFSET. >> >> If we actually had a DWARF unwinder in the kernel, maybe we could have >> real test cases :-/ > > I don't think that's ever going to happen. > > I'd say we do the CFI annotation on a best effort basis but not > sacrifice readability in the process. If it can't be annotated, well, > tough luck. > This stuff is at least useful (in theory) for debugging with gdb. And I wouldn't mind an optional DWARF unwinder to get higher quality backtraces. Obviously any such thing would need to be quite robust. I think SuSE has one. > -- > Regards/Gruss, > Boris. > > Sent from a fat crate under my desk. Formatting is fine. > -- -- Andy Lutomirski AMA Capital Management, LLC