From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752017AbeFDW4J (ORCPT ); Mon, 4 Jun 2018 18:56:09 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:36318 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751515AbeFDW4I (ORCPT ); Mon, 4 Jun 2018 18:56:08 -0400 Date: Mon, 4 Jun 2018 17:56:05 -0500 From: Josh Poimboeuf To: Jessica Yu Cc: linux-kernel@vger.kernel.org, live-patching@vger.kernel.org Subject: Re: [PATCH] module: exclude SHN_UNDEF symbols from kallsyms api Message-ID: <20180604225605.khmlg7pn7fnu5de6@treble> References: <20180602173209.merjb4o5a3wsdnbh@treble> <20180604080528.4z6rbhapbhn7xf5e@d217.suse.de> <20180604095416.rlthplerswzznyoc@d217.suse.de> <20180604130131.slyhtgyywffusf67@d217.suse.de> <20180604131648.eykjibc3iubckeiu@treble> <20180604140510.jovmjhamo24mzh3l@d217.suse.de> <20180604145412.ko2jduwi4qplbwyz@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20180604145412.ko2jduwi4qplbwyz@treble> User-Agent: NeoMutt/20180323 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 04, 2018 at 09:54:12AM -0500, Josh Poimboeuf wrote: > On Mon, Jun 04, 2018 at 04:05:11PM +0200, Jessica Yu wrote: > > +++ Josh Poimboeuf [04/06/18 08:16 -0500]: > > > On Mon, Jun 04, 2018 at 03:01:31PM +0200, Jessica Yu wrote: > > > > +++ Jessica Yu [04/06/18 11:54 +0200]: > > > > > +++ Jessica Yu [04/06/18 10:05 +0200]: > > > > > > +++ Josh Poimboeuf [02/06/18 12:32 -0500]: > > > > > > > Hi Jessica, > > > > > > > > > > > > > > I found a bug: > > > > > > > > > > > > > > [root@f25 ~]# modprobe livepatch-sample > > > > > > > [root@f25 ~]# grep ' u ' /proc/kallsyms > > > > > > > ffffffff81161080 u klp_enable_patch [livepatch_sample] > > > > > > > ffffffff81a01800 u __fentry__ [livepatch_sample] > > > > > > > ffffffff81161250 u klp_unregister_patch [livepatch_sample] > > > > > > > ffffffff81161870 u klp_register_patch [livepatch_sample] > > > > > > > ffffffff8131f0b0 u seq_printf [livepatch_sample] > > > > > > > > > > > > > > Notice that livepatch modules' undefined symbols are showing up in > > > > > > > /proc/kallsyms. This can confuse klp_find_object_symbol() which can > > > > > > > cause subtle bugs in livepatch. > > > > > > > > > > > > > > I stared at the module kallsyms code for a bit, but I don't see the bug. > > > > > > > Maybe it has something to do with how we save the symbol table in > > > > > > > copy_module_elf(). Any ideas? > > > > > > > > > > > > Hi Josh! > > > > > > > > > > > > This is because we preserve the entire symbol table for livepatch > > > > > > modules, including the SHN_UNDEF symbols. IIRC, this is so that we can > > > > > > still apply relocations properly with apply_relocate_add() after a > > > > > > to-be-patched object is loaded. Normally we don't save these SHN_UNDEF > > > > > > symbols for modules so they do not appear in /proc/kallsyms. > > > > > > > > > > Hm, if having the full symtab in kallsyms is causing trouble, one > > > > > possibility would be to just have the module kallsyms code simply > > > > > skip/ignore undef symbols. That's what we technically do for normal > > > > > modules anyway (we normally cut undef syms out of the symtab). Haven't > > > > > tested this idea but does that sound like it'd help? > > > > > > > > See if the following patch (untested) helps. It does not fix the > > > > /proc/kallsyms lookup, that requires a separate patch. But it should > > > > exclude the undef symbols from module_kallsyms_on_each_symbol() and > > > > thus also from klp_find_object_symbol(). > > > > > > That seems like it would work. But wouldn't it be more robust if we > > > don't store the SHN_UNDEF symbols to start with? Really it's only the > > > SHN_LIVEPATCH symbols that we need to keep, right? > > > > Hm, didn't we have a reason for preserving the original symbol tables? > > It's not that we're storing the undef symbols on purpose, it's just > > that in the case of a livepatch module we keep *all* the symbols. > > IIRC, the original reasoning was that we needed to keep the original > > symtab indices in place, otherwise apply_relocate() (which uses the > > original symbol index encoded in the relocation) won't work. > > Selectively copying only SHN_LIVEPATCH symbols when building the > > module's symbol table will unfortunately mess the symbol indices up :/ > > Under these restrictions, it just seemed easier to make kallsyms > > ignore undef symbols. > > Ah, you're right. I'll try out your patch. I can confirm that your patch fixes the issue. Thanks! -- Josh