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=-5.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 9CFEFC433E5 for ; Mon, 20 Jul 2020 03:35:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7882020684 for ; Mon, 20 Jul 2020 03:35:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="T17Vkc70" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726284AbgGTDf1 (ORCPT ); Sun, 19 Jul 2020 23:35:27 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:39796 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726161AbgGTDf0 (ORCPT ); Sun, 19 Jul 2020 23:35:26 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1595216125; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=D7FqMYqDvd7eDbuQSOuxX0aLldcdVgd76BMYePyueXY=; b=T17Vkc70mXKk4ZP0enbSDKVxJkZER+jSyDARcSRr4Ic2gtNEarl1m4nduWemeaFc9xAEy9 NaRcxA3KRy/S8iZJ6Gk4p4F86ozXF1nvq/p59Jdst3HT8A29/lKpu6MfPjeFQVrRr9b30g 79k1r/sY/S6zdqUvk+WrEemSNv63yG8= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-358-sByBR4xhPNSvCj3WC_n2Ig-1; Sun, 19 Jul 2020 23:35:23 -0400 X-MC-Unique: sByBR4xhPNSvCj3WC_n2Ig-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9CE89E91A; Mon, 20 Jul 2020 03:35:21 +0000 (UTC) Received: from [10.10.114.255] (ovpn-114-255.rdu2.redhat.com [10.10.114.255]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1F3FF7849A; Mon, 20 Jul 2020 03:35:21 +0000 (UTC) Subject: Re: [PATCH] Revert "kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled" To: Josh Poimboeuf , live-patching@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Randy Dunlap References: <696262e997359666afa053fe7d1a9fb2bb373964.1595010490.git.jpoimboe@redhat.com> From: Joe Lawrence Message-ID: Date: Sun, 19 Jul 2020 23:35:20 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <696262e997359666afa053fe7d1a9fb2bb373964.1595010490.git.jpoimboe@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Sender: live-patching-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On 7/17/20 2:29 PM, Josh Poimboeuf wrote: > Use of the new -flive-patching flag was introduced with the following > commit: > > 43bd3a95c98e ("kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled") > > This flag has several drawbacks: > > [ ... snip ... ] > > - While there *is* a distro which relies on this flag for their distro > livepatch module builds, there's not a publicly documented way to > create safe livepatch modules with it. Its use seems to be based on > tribal knowledge. It serves no benefit to those who don't know how to > use it. > > (In fact, I believe the current livepatch documentation and samples > are misleading and dangerous, and should be corrected. Or at least > amended with a disclaimer. But I don't feel qualified to make such > changes.) FWIW, I'm not exactly qualified to document source-based creation either, however I have written a few of the samples and obviously the kselftest modules. The samples should certainly include a disclaimer (ie, they are only for API demonstration purposes!) and eventually it would be great if the kselftest modules could guarantee their safety as well. I don't know quite yet how we can automate that, but perhaps some kind of post-build sanity check could verify that they are in fact patching what they intend to patch. As for a more general, long-form warning about optimizations, I grabbed Miroslav's LPC slides from a few years back and poked around at some IPA-optimized disassembly... Here are my notes that attempt to capture some common cases: http://file.bos.redhat.com/~jolawren/klp-compiler-notes/livepatch/compiler-considerations.html It's not complete and I lost steam about 80% of the way through today. :) But if it looks useful enough to add to Documentation/livepatch, we can work on it on-list and try to steer folks into using the automated kpatch-build, objtool (eventually) or a source-based safety checklist. The source-based steps have been posted on-list a few times, but I think it only needs to be formalized in a doc. -- Joe