Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754374AbbFJTvt (ORCPT ); Wed, 10 Jun 2015 15:51:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49228 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751813AbbFJTvk (ORCPT ); Wed, 10 Jun 2015 15:51:40 -0400 Date: Wed, 10 Jun 2015 14:51:37 -0500 From: Josh Poimboeuf To: Andy Lutomirski Cc: Andi Kleen , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Michal Marek , Peter Zijlstra , Andy Lutomirski , Borislav Petkov , Linus Torvalds , X86 ML , live-patching@vger.kernel.org, "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v5 00/10] x86/asm: Compile-time asm code validation Message-ID: <20150610195137.GD1125@treble.redhat.com> References: <20150610130814.GN19417@two.firstfloor.org> <20150610135203.GA19509@treble.redhat.com> <20150610141104.GQ19417@two.firstfloor.org> <20150610143209.GA28843@treble.redhat.com> <20150610150412.GR19417@two.firstfloor.org> <20150610153155.GB29724@treble.redhat.com> <87lhfr5rw9.fsf@tassilo.jf.intel.com> <20150610193618.GB1125@treble.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: 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: 3256 Lines: 81 On Wed, Jun 10, 2015 at 12:38:32PM -0700, Andy Lutomirski wrote: > On Wed, Jun 10, 2015 at 12:36 PM, Josh Poimboeuf wrote: > > On Wed, Jun 10, 2015 at 11:40:06AM -0700, Andi Kleen wrote: > >> Josh Poimboeuf writes: > >> > >> > On Wed, Jun 10, 2015 at 05:04:12PM +0200, Andi Kleen wrote: > >> >> > > > - duplicate the destination code inside the function > >> >> > > > - convert the jump to a call > >> >> > > > >> >> > > That all won't work for a lot of cases. > >> >> > > >> >> > Hm, could you give an example? > >> >> > >> >> Just a standard *_user exception handler. > >> > > >> > I'm afraid I don't follow. Exception handlers don't work via jump > >> > instructions, but rather via CPU exceptions. > >> > > >> > Or are you talking about something else? > >> > >> Let's take an example: > >> > >> 102: > >> .section .fixup,"ax" > >> 103: addl %ecx,%edx /* ecx is zerorest also */ > >> jmp copy_user_handle_tail > >> .previous > >> > >> _ASM_EXTABLE(100b,103b) > >> _ASM_EXTABLE(101b,103b) > >> > >> The exception handling code is part of the function, but it's out of line. > > > > The jump instruction is in the .fixup section, not in the callable > > function itself. So it doesn't violate the asmvalidate rules. > > It still won't unwind correctly unless .pushsection somehow magically > propagates CFI state. (Does it?) I don't think it does. We'll probably need some intelligence in the CFI generation tooling to deal properly with the extable stuff. > >> > Are you suggesting that we implement this gcc optimization in kernel asm > >> > code? > >> > >> It was how Linux traditionally implemented locking code for example. > >> Have the hot path handle the uncontended fast path, and the slow path > >> call. > >> > >> I don't know if there is much left of it (a lot of it was removed because > >> it was hard to describe in dwarf3, needs dwarf4). But it seems bad > >> to completely disallow it. > >> > >> But yes eventually gcc generated code should use it again, because it's > >> great for icache usage if you measure it correctly at run time > >> (not the broken "size" approach that is unfortunately far too common) > > > > This patch set has no relationship to gcc generated code whatsoever. So > > it doesn't disallow anything there. > > > > For kernel asm code, AFAIK, such a mechanism for hot/cold path > > separation in separate sections doesn't exist today. So it's not > > "disallowed" there either. It's just apparently not currently done. > > > > If somebody were to create such a mechanism, I think we could > > standardize it in such a way that it could be compatible with > > asmvalidate. > > Hopefully true. The entry code is full of tail calls, though. Well, I wasn't talking specifically about tail calls here. But either way, as long as they're not in a callable function (which is the case for most of the entry code), asmvalidate doesn't care. -- 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/