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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT 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 C92C2C04EB9 for ; Wed, 5 Dec 2018 20:20:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8DD45214DB for ; Wed, 5 Dec 2018 20:20:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8DD45214DB Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.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 S1728316AbeLEUUh (ORCPT ); Wed, 5 Dec 2018 15:20:37 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43282 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727388AbeLEUUg (ORCPT ); Wed, 5 Dec 2018 15:20:36 -0500 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 02BD1C0740F1; Wed, 5 Dec 2018 20:20:36 +0000 (UTC) Received: from redhat.com (dhcp-17-208.bos.redhat.com [10.18.17.208]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 222441001F50; Wed, 5 Dec 2018 20:20:35 +0000 (UTC) Date: Wed, 5 Dec 2018 15:20:34 -0500 From: Joe Lawrence To: Petr Mladek Cc: Jiri Kosina , Josh Poimboeuf , Miroslav Benes , Jason Baron , Evgenii Shatokhin , live-patching@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v14 09/11] livepatch: Atomic replace and cumulative patches documentation Message-ID: <20181205202034.a66aulfbkl4abr3q@redhat.com> References: <20181129094431.7801-1-pmladek@suse.com> <20181129094431.7801-10-pmladek@suse.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181129094431.7801-10-pmladek@suse.com> User-Agent: Mutt/1.6.2-neo (2016-08-08) 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]); Wed, 05 Dec 2018 20:20:36 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 29, 2018 at 10:44:29AM +0100, Petr Mladek wrote: > User documentation for the atomic replace feature. It makes it easier > to maintain livepatches using so-called cumulative patches. > > Signed-off-by: Petr Mladek > --- Acked-by: Joe Lawrence > 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 ^^^^^^^^^ re-wording suggestion: "creation of" > +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 ^^^^^^^^ s/the life/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 ^^^^^^^^^^^^^^^ rewording suggestion: "Especially that they" > + 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 > -- Joe