Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933855AbbFJTjC (ORCPT ); Wed, 10 Jun 2015 15:39:02 -0400 Received: from mail-la0-f48.google.com ([209.85.215.48]:35344 "EHLO mail-la0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753174AbbFJTiy (ORCPT ); Wed, 10 Jun 2015 15:38:54 -0400 MIME-Version: 1.0 In-Reply-To: <20150610193618.GB1125@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> From: Andy Lutomirski Date: Wed, 10 Jun 2015 12:38:32 -0700 Message-ID: Subject: Re: [PATCH v5 00/10] x86/asm: Compile-time asm code validation To: Josh Poimboeuf 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" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2798 Lines: 83 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?) > >> > 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. --Andy > > -- > Josh -- Andy Lutomirski AMA Capital Management, LLC -- 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/