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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 519F7C43140 for ; Thu, 5 Sep 2019 14:28:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 79A3B206A5 for ; Thu, 5 Sep 2019 14:28:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389366AbfIEO2Q (ORCPT ); Thu, 5 Sep 2019 10:28:16 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48552 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727914AbfIEO2P (ORCPT ); Thu, 5 Sep 2019 10:28:15 -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 A34FB8980EA; Thu, 5 Sep 2019 14:28:15 +0000 (UTC) Received: from treble (ovpn-120-170.rdu2.redhat.com [10.10.120.170]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 23EEB100033E; Thu, 5 Sep 2019 14:28:12 +0000 (UTC) Date: Thu, 5 Sep 2019 09:28:09 -0500 From: Josh Poimboeuf To: Petr Mladek Cc: jikos@kernel.org, Joe Lawrence , Miroslav Benes , 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: <20190905142809.bpalx7wmihkvavxy@treble> References: <20190826145449.wyo7avwpqyriem46@treble> <5c649320-a9bf-ae7f-5102-483bc34d219f@redhat.com> <20190904084932.gndrtewubqiaxmzy@pathway.suse.cz> <20190905025055.36loaatxtkhdo4q5@treble> <20190905110955.wl4lwjbnpqybhkcn@pathway.suse.cz> <20190905130832.dznviqrrg6lfrxvx@treble> <20190905131502.mgiaplb3grlxsahp@treble> <20190905135259.7obdymb7c2wdgafw@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190905135259.7obdymb7c2wdgafw@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-4.6.2 (mx1.redhat.com [10.5.110.67]); Thu, 05 Sep 2019 14:28:15 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 05, 2019 at 03:52:59PM +0200, Petr Mladek wrote: > On Thu 2019-09-05 08:15:02, Josh Poimboeuf wrote: > > On Thu, Sep 05, 2019 at 08:08:32AM -0500, Josh Poimboeuf wrote: > > > On Thu, Sep 05, 2019 at 01:09:55PM +0200, Petr Mladek wrote: > > > > > I don't have a number, but it's very common to patch a function which > > > > > uses jump labels or alternatives. > > > > > > > > Really? My impression is that both alternatives and jump_labels > > > > are used in hot paths. I would expect them mostly in core code > > > > that is always loaded. > > > > > > > > Alternatives are often used in assembly that we are not able > > > > to livepatch anyway. > > > > > > > > Or are they spread widely via some macros or inlined functions? > > > > > > Jump labels are used everywhere. Looking at vmlinux.o in my kernel: > > > > > > Relocation section [19621] '.rela__jump_table' for section [19620] '__jump_table' at offset 0x197873c8 contains 11913 entries: > > > > > > Each jump label entry has 3 entries, so 11913/3 = 3971 jump labels. > > > > > > $ readelf -s vmlinux.o |grep FUNC |wc -l > > > 46902 > > > > > > 3971/46902 = ~8.5% > > > > > > ~8.5% of functions use jump labels. > > > > Obviously some functions may use more than one jump label so this isn't > > exactly bulletproof math. But it gives a rough idea of how widespread > > they are. > > It looks scary. I just wonder why we have never met this problem during > last few years. Who knows what can happen when you disable jump label patching. Sometimes it may be harmless. A panic is probably the worst case. There may be other fail modes which are harder to detect. > My only guess is that most of these functions are either in core > kernel or in code that we do not livepatch. This is definitely not the case. We recently introduced jump label checking in kpatch-build, and it complains a lot. The workaround is to replace such uses with static_key_enabled(). -- Josh