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_2 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 7C656C2BA19 for ; Thu, 16 Apr 2020 01:25:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5D9922076D for ; Thu, 16 Apr 2020 01:25:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731629AbgDPBZd (ORCPT ); Wed, 15 Apr 2020 21:25:33 -0400 Received: from mail.kernel.org ([198.145.29.99]:33830 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730677AbgDPBY6 (ORCPT ); Wed, 15 Apr 2020 21:24:58 -0400 Received: from oasis.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id F1FB72076D; Thu, 16 Apr 2020 01:24:56 +0000 (UTC) Date: Wed, 15 Apr 2020 21:24:55 -0400 From: Steven Rostedt To: Bjorn Andersson Cc: John Stultz , "Paul E. McKenney" , Josh Triplett , lkml , Saravana Kannan , Todd Kjos , Stephen Boyd , Peter Zijlstra , Thomas Gleixner Subject: Re: On trace_*_rcuidle functions in modules Message-ID: <20200415212455.28f22041@oasis.local.home> In-Reply-To: <20200416010258.GM20625@builder.lan> References: <20200415161424.584d07d3@gandalf.local.home> <20200415164116.40564f2c@gandalf.local.home> <20200415174918.154a86d0@gandalf.local.home> <20200415220459.GE17661@paulmck-ThinkPad-P72> <20200415185121.381a4bc3@gandalf.local.home> <20200415204827.24f2c548@oasis.local.home> <20200416010258.GM20625@builder.lan> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 15 Apr 2020 18:02:58 -0700 Bjorn Andersson wrote: > Forgive me, but how is this problem related to the fact that the code is > dynamically loaded, i.e. encapsulated in a module? It's not. > > Per the example earlier in this thread, the thing we're worried about is > a use after free in the following scenario, right? > > cpu_pm_unregister_notifier(my_notifier); > kfree(my_data); > > But a driver implementing this snippet might do this regardless of being > builtin or module and afaict exiting probe() unsuccessfully or unbinding > the device would risk triggering this issue? I know my email was confusing. I was talking about a bug that does not exist. (There is no bug!) The reason is that rcu is enabled during the call to the notifiers. The above assumes that the my_data usage in the notifier callback is surrounded by rcu_read_lock() (otherwise it's broken regardless of this code or not). The above unregister will call synchronize_rcu() after it removes the notifier which will guarantee that the rcu_read_lock() critical sections would be completed. Then the kfree(my_data) can free my_data with no possible users. -- Steve