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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 42AC0C433EF for ; Tue, 28 Jun 2022 03:56:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243862AbiF1D4U (ORCPT ); Mon, 27 Jun 2022 23:56:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43646 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236892AbiF1D4T (ORCPT ); Mon, 27 Jun 2022 23:56:19 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 886F627B35; Mon, 27 Jun 2022 20:56:18 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 3DDE2B81C18; Tue, 28 Jun 2022 03:56:17 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9E9E4C3411D; Tue, 28 Jun 2022 03:56:13 +0000 (UTC) Date: Mon, 27 Jun 2022 23:56:12 -0400 From: Steven Rostedt To: Aaron Tomlin Cc: mcgrof@kernel.org, christophe.leroy@csgroup.eu, cl@linux.com, mbenes@suse.cz, akpm@linux-foundation.org, jeyu@kernel.org, linux-kernel@vger.kernel.org, linux-modules@vger.kernel.org, void@manifault.com, atomlin@atomlin.com, allen.lkml@gmail.com, joe@perches.com, msuchanek@suse.de, oleksandr@natalenko.name, jason.wessel@windriver.com, pmladek@suse.com, daniel.thompson@linaro.org, hch@infradead.org, Thomas Gleixner , linux-rt-users Subject: Re: [PATCH v11 10/14] module: kallsyms: Fix suspicious rcu usage Message-ID: <20220627235612.4796c837@gandalf.local.home> In-Reply-To: <20220310102413.3438665-11-atomlin@redhat.com> References: <20220310102413.3438665-1-atomlin@redhat.com> <20220310102413.3438665-11-atomlin@redhat.com> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: On Thu, 10 Mar 2022 10:24:09 +0000 Aaron Tomlin wrote: > No functional change. There most definitely is a functional change here. > > The purpose of this patch is to address the various Sparse warnings > due to the incorrect dereference/or access of an __rcu pointer. > > Signed-off-by: Aaron Tomlin > --- > kernel/module/kallsyms.c | 34 ++++++++++++++++++++++------------ > 1 file changed, 22 insertions(+), 12 deletions(-) > > diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c > index 1b0780e20aab..a3da0686a2a6 100644 > --- a/kernel/module/kallsyms.c > +++ b/kernel/module/kallsyms.c > @@ -171,14 +171,17 @@ void add_kallsyms(struct module *mod, const struct load_info *info) > Elf_Shdr *symsec = &info->sechdrs[info->index.sym]; > > /* Set up to point into init section. */ > - mod->kallsyms = mod->init_layout.base + info->mod_kallsyms_init_off; > + mod->kallsyms = (void __rcu *)mod->init_layout.base + > + info->mod_kallsyms_init_off; > > + preempt_disable(); ^^^^^^^^^^^^^^^^^^ > /* The following is safe since this pointer cannot change */ > - mod->kallsyms->symtab = (void *)symsec->sh_addr; > - mod->kallsyms->num_symtab = symsec->sh_size / sizeof(Elf_Sym); > + rcu_dereference_sched(mod->kallsyms)->symtab = (void *)symsec->sh_addr; > + rcu_dereference_sched(mod->kallsyms)->num_symtab = symsec->sh_size / sizeof(Elf_Sym); > /* Make sure we get permanent strtab: don't use info->strtab. */ > - mod->kallsyms->strtab = (void *)info->sechdrs[info->index.str].sh_addr; > - mod->kallsyms->typetab = mod->init_layout.base + info->init_typeoffs; > + rcu_dereference_sched(mod->kallsyms)->strtab = > + (void *)info->sechdrs[info->index.str].sh_addr; > + rcu_dereference_sched(mod->kallsyms)->typetab = mod->init_layout.base + info->init_typeoffs; > > /* > * Now populate the cut down core kallsyms for after init > @@ -187,20 +190,22 @@ void add_kallsyms(struct module *mod, const struct load_info *info) > mod->core_kallsyms.symtab = dst = mod->core_layout.base + info->symoffs; > mod->core_kallsyms.strtab = s = mod->core_layout.base + info->stroffs; > mod->core_kallsyms.typetab = mod->core_layout.base + info->core_typeoffs; > - src = mod->kallsyms->symtab; > - for (ndst = i = 0; i < mod->kallsyms->num_symtab; i++) { > - mod->kallsyms->typetab[i] = elf_type(src + i, info); > + src = rcu_dereference_sched(mod->kallsyms)->symtab; > + for (ndst = i = 0; i < rcu_dereference_sched(mod->kallsyms)->num_symtab; i++) { How big is this loop? > + rcu_dereference_sched(mod->kallsyms)->typetab[i] = elf_type(src + i, info); > if (i == 0 || is_livepatch_module(mod) || > is_core_symbol(src + i, info->sechdrs, info->hdr->e_shnum, > info->index.pcpu)) { > mod->core_kallsyms.typetab[ndst] = > - mod->kallsyms->typetab[i]; > + rcu_dereference_sched(mod->kallsyms)->typetab[i]; > dst[ndst] = src[i]; > dst[ndst++].st_name = s - mod->core_kallsyms.strtab; > - s += strscpy(s, &mod->kallsyms->strtab[src[i].st_name], > + s += strscpy(s, > + &rcu_dereference_sched(mod->kallsyms)->strtab[src[i].st_name], > KSYM_NAME_LEN) + 1; > } > } > + preempt_enable(); ^^^^^^^^^^^^^^^^^ This just introduced an unbounded latency and will fail PREEMPT_RT. -- Steve > mod->core_kallsyms.num_symtab = ndst; > } > > @@ -478,11 +483,16 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *, > > mutex_lock(&module_mutex); > list_for_each_entry(mod, &modules, list) { > - /* We hold module_mutex: no need for rcu_dereference_sched */ > - struct mod_kallsyms *kallsyms = mod->kallsyms; > + struct mod_kallsyms *kallsyms; > > if (mod->state == MODULE_STATE_UNFORMED) > continue; > + > + /* Use rcu_dereference_sched() to remain compliant with the sparse tool */ > + preempt_disable(); > + kallsyms = rcu_dereference_sched(mod->kallsyms); > + preempt_enable(); > + > for (i = 0; i < kallsyms->num_symtab; i++) { > const Elf_Sym *sym = &kallsyms->symtab[i]; >