From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6896BC3A5A9 for ; Thu, 5 Sep 2019 02:51:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 47AB4206BB for ; Thu, 5 Sep 2019 02:51:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730804AbfIECvB (ORCPT ); Wed, 4 Sep 2019 22:51:01 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45350 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725965AbfIECvA (ORCPT ); Wed, 4 Sep 2019 22:51:00 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 56697C0578F8; Thu, 5 Sep 2019 02:51:00 +0000 (UTC) Received: from treble (ovpn-121-98.rdu2.redhat.com [10.10.121.98]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 051B11001958; Thu, 5 Sep 2019 02:50:56 +0000 (UTC) Date: Wed, 4 Sep 2019 21:50:55 -0500 From: Josh Poimboeuf To: Petr Mladek Cc: Miroslav Benes , Joe Lawrence , jikos@kernel.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org Subject: Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal Message-ID: <20190905025055.36loaatxtkhdo4q5@treble> References: <20190814151244.5xoaxib5iya2qjco@treble> <20190816094608.3p2z73oxcoqavnm4@pathway.suse.cz> <20190822223649.ptg6e7qyvosrljqx@treble> <20190823081306.kbkm7b4deqrare2v@pathway.suse.cz> <20190826145449.wyo7avwpqyriem46@treble> <5c649320-a9bf-ae7f-5102-483bc34d219f@redhat.com> <20190904084932.gndrtewubqiaxmzy@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190904084932.gndrtewubqiaxmzy@pathway.suse.cz> User-Agent: NeoMutt/20180716 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Thu, 05 Sep 2019 02:51:00 +0000 (UTC) Sender: live-patching-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On Wed, Sep 04, 2019 at 10:49:32AM +0200, Petr Mladek wrote: > On Tue 2019-09-03 15:02:34, Miroslav Benes wrote: > > On Mon, 2 Sep 2019, Joe Lawrence wrote: > > > > > On 9/2/19 12:13 PM, Miroslav Benes wrote: > > > >> I can easily foresee more problems like those in the future. Going > > > >> forward we have to always keep track of which special sections are > > > >> needed for which architectures. Those special sections can change over > > > >> time, or can simply be overlooked for a given architecture. It's > > > >> fragile. > > > > > > > > Indeed. It bothers me a lot. Even x86 "port" is not feature complete in > > > > this regard (jump labels, alternatives,...) and who knows what lurks in > > > > the corners of the other architectures we support. > > > > > > > > So it is in itself reason enough to do something about late module > > > > patching. > > > > > > > > > > Hi Miroslav, > > > > > > I was tinkering with the "blue-sky" ideas that I mentioned to Josh the other > > > day. > > > > > I dunno if you had a chance to look at what removing that code looks > > > like, but I can continue to flesh out that idea if it looks interesting: > > > > Unfortunately no and I don't think I'll come up with something useful > > before LPC, so anything is really welcome. > > > > > > > > https://github.com/joe-lawrence/linux/tree/blue-sky > > > > > > A full demo would require packaging up replacement .ko's with a livepatch, as > > > well as "blacklisting" those deprecated .kos, etc. But that's all I had time > > > to cook up last week before our holiday weekend here. > > > > Frankly, I'm not sure about this approach. I'm kind of torn. The current > > solution is far from ideal, but I'm not excited about the other options > > either. It seems like the choice is basically between "general but > > technically complicated fragile solution with nontrivial maintenance > > burden", or "something safer and maybe cleaner, but limiting for > > users/distros". Of course it depends on whether the limitation is even > > real and how big it is. Unfortunately we cannot quantify it much and that > > is probably why our opinions (in the email thread) differ. > > I wonder what is necessary for a productive discussion on Plumbers: > > + Josh would like to see what code can get removed when late > handling of modules gets removed. I think that it might be > partially visible from Joe's blue-sky patches. Yes, and I like what I see. Especially the removal of the .klp.arch nastiness! > + I would like to better understand the scope of the current > problems. It is about modifying code in the livepatch that > depends on position of the related code: > > + relocations are rather clear; we will need them anyway > to access non-public (static) API from the original code. > > + What are the other changes? I think the .klp.arch sections are the big ones: .klp.arch.altinstructions .klp.arch.parainstructions .klp.arch.jump_labels (doesn't exist yet) And that's just x86... And then of course there's the klp coming/going notifiers which have also been an additional source of complexity. > + Do we use them in livepatches? How often? I don't have a number, but it's very common to patch a function which uses jump labels or alternatives. > + How often new problematic features appear? I'm not exactly sure what you mean, but it seems that anytime we add a new feature, we have to try to wrap our heads around how it interacts with the weirdness of late module patching. > + Would be possible to detect potential problems, for example > by comparing the code in the binary and in memory when > the module is loaded the normal way? Perhaps, though I assume this would be some out-of-band testing thing. > + Would be possible to reset the livepatch code in memory > when the related module is unloaded and safe us half > of the troubles? Maybe, but I think that would solve a much lower percentage of our troubles than half :-/ > + It might be useful to prepare overview of the existing proposals > and agree on the positives and negatives. I am afraid that some > of them might depend on the customer base and > use cases. Sometimes we might not have enough information. > But it might be good to get on the same page where possible. I think we've already done that for the existing proposals. Maybe Miroslav can summarize them at the LPC session. > Anyway, it might rule out some variants so that we could better > concentrate on the acceptable ones. Or come with yet another > proposal that would avoid the real blockers. I'd like to hear more specific negatives about Joe's recent patches, which IMO, are the best option we've discussed so far. -- Josh