From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753394AbeFDOFR (ORCPT ); Mon, 4 Jun 2018 10:05:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:38218 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753313AbeFDOFO (ORCPT ); Mon, 4 Jun 2018 10:05:14 -0400 Date: Mon, 4 Jun 2018 16:05:11 +0200 From: Jessica Yu To: Josh Poimboeuf Cc: linux-kernel@vger.kernel.org, live-patching@vger.kernel.org Subject: Re: [PATCH] module: exclude SHN_UNDEF symbols from kallsyms api Message-ID: <20180604140510.jovmjhamo24mzh3l@d217.suse.de> References: <20180602173209.merjb4o5a3wsdnbh@treble> <20180604080528.4z6rbhapbhn7xf5e@d217.suse.de> <20180604095416.rlthplerswzznyoc@d217.suse.de> <20180604130131.slyhtgyywffusf67@d217.suse.de> <20180604131648.eykjibc3iubckeiu@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20180604131648.eykjibc3iubckeiu@treble> X-OS: Linux d217 4.12.14-lp150.10-default x86_64 User-Agent: NeoMutt/20170912 (1.9.0) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org +++ 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. >What do you think about the following (untested)? > >diff --git a/kernel/module.c b/kernel/module.c >index c9bea7f2b43e..78ec9de856e3 100644 >--- a/kernel/module.c >+++ b/kernel/module.c >@@ -2586,6 +2586,9 @@ static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs, > { > const Elf_Shdr *sec; > >+ if (src->st_shndx == SHN_LIVEPATCH) >+ return true; >+ > if (src->st_shndx == SHN_UNDEF > || src->st_shndx >= shnum > || !src->st_name) >@@ -2632,9 +2635,9 @@ static void layout_symtab(struct module *mod, struct load_info *info) > > /* Compute total space required for the core symbols' strtab. */ > for (ndst = i = 0; i < nsrc; i++) { >- if (i == 0 || is_livepatch_module(mod) || >- is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum, >- info->index.pcpu)) { >+ if (i == 0 || is_core_symbol(src+i, info->sechdrs, >+ info->hdr->e_shnum, >+ info->index.pcpu)) { > strtab_size += strlen(&info->strtab[src[i].st_name])+1; > ndst++; > } >@@ -2691,9 +2694,9 @@ static void add_kallsyms(struct module *mod, const struct load_info *info) > mod->core_kallsyms.strtab = s = mod->core_layout.base + info->stroffs; > src = mod->kallsyms->symtab; > for (ndst = i = 0; i < mod->kallsyms->num_symtab; i++) { >- if (i == 0 || is_livepatch_module(mod) || >- is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum, >- info->index.pcpu)) { >+ if (i == 0 || is_core_symbol(src+i, info->sechdrs, >+ info->hdr->e_shnum, >+ info->index.pcpu)) { > dst[ndst] = src[i]; > dst[ndst++].st_name = s - mod->core_kallsyms.strtab; > s += strlcpy(s, &mod->kallsyms->strtab[src[i].st_name],