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=-6.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 1FFADC4727E for ; Wed, 7 Oct 2020 19:39:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C47AC207EA for ; Wed, 7 Oct 2020 19:39:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728364AbgJGTi7 (ORCPT ); Wed, 7 Oct 2020 15:38:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51814 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728291AbgJGTi5 (ORCPT ); Wed, 7 Oct 2020 15:38:57 -0400 Received: from sipsolutions.net (s3.sipsolutions.net [IPv6:2a01:4f8:191:4433::2]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AFE6EC061755 for ; Wed, 7 Oct 2020 12:38:56 -0700 (PDT) Received: by sipsolutions.net with esmtpsa (TLS1.3:ECDHE_SECP256R1__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim 4.94) (envelope-from ) id 1kQFGy-0019jE-FY; Wed, 07 Oct 2020 21:38:48 +0200 Message-ID: Subject: Re: [PATCH v2 04/11] drivers/base/devcoredump: convert devcd_count to counter_atomic32 From: Johannes Berg To: Shuah Khan , Kees Cook , gregkh@linuxfoundation.org Cc: rafael@kernel.org, linux-kernel@vger.kernel.org Date: Wed, 07 Oct 2020 21:38:47 +0200 In-Reply-To: (sfid-20201007_213346_492518_9DEB611E) References: <462fd514dfe2afbb8faa1dea4cdb4b0e75d8e8da.1602011710.git.skhan@linuxfoundation.org> <202010071114.EE9A2A47C@keescook> (sfid-20201007_213346_492518_9DEB611E) Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-1.fc32) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2020-10-07 at 13:33 -0600, Shuah Khan wrote: > On 10/7/20 12:15 PM, Kees Cook wrote: > > On Tue, Oct 06, 2020 at 02:44:35PM -0600, Shuah Khan wrote: > > > counter_atomic* is introduced to be used when a variable is used as > > > a simple counter and doesn't guard object lifetimes. This clearly > > > differentiates atomic_t usages that guard object lifetimes. > > > > > > counter_atomic* variables will wrap around to 0 when it overflows and > > > should not be used to guard resource lifetimes, device usage and > > > open counts that control state changes, and pm states. > > > > > > devcd_count is used to track dev_coredumpm device count and used in > > > device name string. It doesn't guard object lifetimes, device usage > > > counts, device open counts, and pm states. There is very little chance > > > of this counter overflowing. Convert it to use counter_atomic32. > > > > > > This conversion doesn't change the overflow wrap around behavior. > > > > > > Reviewed-by: Greg Kroah-Hartman > > > Signed-off-by: Shuah Khan > > > > I actually wonder if this should use refcount_t just because it is > > designed to be an alway-unique value. It is hard to imagine ever causing > > this to overflow, but why not let it be protected? > > > > This is one of the cases where devcd_count doesn't guard lifetimes, > however if it ever overflows, refcount_t is a better choice. > > If we decide refcount_t is a better choice, I can drop this patch > and send refcount_t conversion patch instead. > > Greg! Any thoughts on refcount_t for this being a better choice? I'm not Greg, but ... there's a 5 minute timeout. So in order to cause a clash you'd have to manage to overflow the counter within a 5 minute interval, otherwise you can actually reuse the numbers starting again from 0 without any ill effect. And even if you *do* manage to overflow it quickly enough it'll just fail device_add() and error out, and nothing happens. So I think it's fairly much pointless to think about protecting against some kind of overflows. It's just trying to get a "temporarily unique ID" here, could be doing anything else instead, but most other things would require bigger data structures and/or (higher level) locking. OTOH, if you *do* somehow create that many core dumps (huge uptimes and extremely frequent crashes?) it seems like refcount_t would be a bad choice because it saturates, and then you can only do one more dump per 5 minutes? Or maybe that's a good thing in these ill cases ... I don't think it'll really happen either way :) johannes