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=-8.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_GIT autolearn=ham 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 8F367C04AA5 for ; Mon, 15 Oct 2018 12:37:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5DCB820892 for ; Mon, 15 Oct 2018 12:37:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5DCB820892 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726932AbeJOUWs (ORCPT ); Mon, 15 Oct 2018 16:22:48 -0400 Received: from mx2.suse.de ([195.135.220.15]:52510 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726453AbeJOUWr (ORCPT ); Mon, 15 Oct 2018 16:22:47 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 09581AE19; Mon, 15 Oct 2018 12:37:39 +0000 (UTC) From: Petr Mladek To: Jiri Kosina , Josh Poimboeuf , Miroslav Benes Cc: Jason Baron , Joe Lawrence , Evgenii Shatokhin , live-patching@vger.kernel.org, linux-kernel@vger.kernel.org, Petr Mladek Subject: [PATCH v13 10/12] livepatch: Atomic replace and cumulative patches documentation Date: Mon, 15 Oct 2018 14:37:11 +0200 Message-Id: <20181015123713.25868-11-pmladek@suse.com> X-Mailer: git-send-email 2.13.7 In-Reply-To: <20181015123713.25868-1-pmladek@suse.com> References: <20181015123713.25868-1-pmladek@suse.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org User documentation for the atomic replace feature. It makes it easier to maintain livepatches using so-called cumulative patches. Signed-off-by: Petr Mladek --- Documentation/livepatch/cumulative-patches.txt | 105 +++++++++++++++++++++++++ 1 file changed, 105 insertions(+) create mode 100644 Documentation/livepatch/cumulative-patches.txt diff --git a/Documentation/livepatch/cumulative-patches.txt b/Documentation/livepatch/cumulative-patches.txt new file mode 100644 index 000000000000..a8089f7fe306 --- /dev/null +++ b/Documentation/livepatch/cumulative-patches.txt @@ -0,0 +1,105 @@ +=================================== +Atomic Replace & Cumulative Patches +=================================== + +There might be dependencies between livepatches. If multiple patches need +to do different changes to the same function(s) then we need to define +an order in which the patches will be installed. And function implementations +from any newer livepatch must be done on top of the older ones. + +This might become a maintenance nightmare. Especially if anyone would want +to remove a patch that is in the middle of the stack. + +An elegant solution comes with the feature called "Atomic Replace". It allows +to create so called "Cumulative Patches". They include all wanted changes +from all older livepatches and completely replace them in one transition. + +Usage +----- + +The atomic replace can be enabled by setting "replace" flag in struct klp_patch, +for example: + + static struct klp_patch patch = { + .mod = THIS_MODULE, + .objs = objs, + .replace = true, + }; + +Such a patch is added on top of the livepatch stack when enabled. + +All processes are then migrated to use the code only from the new patch. +Once the transition is finished, all older patches are automatically +disabled and removed from the stack of patches. + +Ftrace handlers are transparently removed from functions that are no +longer modified by the new cumulative patch. + +As a result, the livepatch authors might maintain sources only for one +cumulative patch. It helps to keep the patch consistent while adding or +removing various fixes or features. + +Users could keep only the last patch installed on the system after +the transition to has finished. It helps to clearly see what code is +actually in use. Also the livepatch might then be seen as a "normal" +module that modifies the kernel behavior. The only difference is that +it can be updated at runtime without breaking its functionality. + + +Features +-------- + +The atomic replace allows: + + + Atomically revert some functions in a previous patch while + upgrading other functions. + + + Remove eventual performance impact caused by core redirection + for functions that are no longer patched. + + + Decrease user confusion about stacking order and what code + is actually in use. + + +Limitations: +------------ + + + Once the operation finishes, there is no straightforward way + to reverse it and restore the replaced patches atomically. + + A good practice is to set .replace flag in any released livepatch. + Then re-adding an older livepatch is equivalent to downgrading + to that patch. This is safe as long as the livepatches do _not_ do + extra modifications in (un)patching callbacks or in the module_init() + or module_exit() functions, see below. + + Also note that the replaced patch can be removed and loaded again + only when the transition was not forced. + + + + Only the (un)patching callbacks from the _new_ cumulative livepatch are + executed. Any callbacks from the replaced patches are ignored. + + In other words, the cumulative patch is responsible for doing any actions + that are necessary to properly replace any older patch. + + As a result, it might be dangerous to replace newer cumulative patches by + older ones. The old livepatches might not provide the necessary callbacks. + + This might be seen as a limitation in some scenarios. But it makes the life + easier in many others. Only the new cumulative livepatch knows what + fixes/features are added/removed and what special actions are necessary + for a smooth transition. + + In any case, it would be a nightmare to think about the order of + the various callbacks and their interactions if the callbacks from all + enabled patches were called. + + + + There is no special handling of shadow variables. Livepatch authors + must create their own rules how to pass them from one cumulative + patch to the other. Especially they should not blindly remove them + in module_exit() functions. + + A good practice might be to remove shadow variables in the post-unpatch + callback. It is called only when the livepatch is properly disabled. -- 2.13.7