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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 0299CC433E0 for ; Wed, 20 Jan 2021 02:00:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B030B21D79 for ; Wed, 20 Jan 2021 02:00:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727688AbhATCAh (ORCPT ); Tue, 19 Jan 2021 21:00:37 -0500 Received: from out02.mta.xmission.com ([166.70.13.232]:38804 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731136AbhATB7a (ORCPT ); Tue, 19 Jan 2021 20:59:30 -0500 Received: from in02.mta.xmission.com ([166.70.13.52]) by out02.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1l22lj-000tTV-AH; Tue, 19 Jan 2021 18:58:47 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1l22li-00B0cw-Bf; Tue, 19 Jan 2021 18:58:46 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Alexey Gladkov Cc: Linus Torvalds , LKML , io-uring , Kernel Hardening , Linux Containers , Linux-MM , Andrew Morton , Christian Brauner , Jann Horn , Jens Axboe , Kees Cook , Oleg Nesterov References: <116c7669744404364651e3b380db2d82bb23f983.1610722473.git.gladkov.alexey@gmail.com> <20210118194551.h2hrwof7b3q5vgoi@example.org> <20210118205629.zro2qkd3ut42bpyq@example.org> Date: Tue, 19 Jan 2021 19:57:36 -0600 In-Reply-To: <20210118205629.zro2qkd3ut42bpyq@example.org> (Alexey Gladkov's message of "Mon, 18 Jan 2021 21:56:29 +0100") Message-ID: <87eeig74kv.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1l22li-00B0cw-Bf;;;mid=<87eeig74kv.fsf@x220.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/ggaLsmakqXtb9Zqya1vF2O+oxa+qmtw0= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Alexey Gladkov writes: > On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote: >> On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov >> wrote: >> > >> > Sorry about that. I thought that this code is not needed when switching >> > from int to refcount_t. I was wrong. >> >> Well, you _may_ be right. I personally didn't check how the return >> value is used. >> >> I only reacted to "it certainly _may_ be used, and there is absolutely >> no comment anywhere about why it wouldn't matter". > > I have not found examples where checked the overflow after calling > refcount_inc/refcount_add. > > For example in kernel/fork.c:2298 : > > current->signal->nr_threads++; > atomic_inc(¤t->signal->live); > refcount_inc(¤t->signal->sigcnt); > > $ semind search signal_struct.sigcnt > def include/linux/sched/signal.h:83 refcount_t sigcnt; > m-- kernel/fork.c:723 put_signal_struct if (refcount_dec_and_test(&sig->sigcnt)) > m-- kernel/fork.c:1571 copy_signal refcount_set(&sig->sigcnt, 1); > m-- kernel/fork.c:2298 copy_process refcount_inc(¤t->signal->sigcnt); > > It seems to me that the only way is to use __refcount_inc and then compare > the old value with REFCOUNT_MAX > > Since I have not seen examples of such checks, I thought that this is > acceptable. Sorry once again. I have not tried to hide these changes. The current ucount code does check for overflow and fails the increment in every case. So arguably it will be a regression and inferior error handling behavior if the code switches to the ``better'' refcount_t data structure. I originally didn't use refcount_t because silently saturating and not bothering to handle the error makes me uncomfortable. Not having to acquire the ucounts_lock every time seems nice. Perhaps the path forward would be to start with stupid/correct code that always takes the ucounts_lock for every increment of ucounts->count, that is later replaced with something more optimal. Not impacting performance in the non-namespace cases and having good performance in the other cases is a fundamental requirement of merging code like this. Eric