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, 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 DB170C433E0 for ; Wed, 20 Jan 2021 02:00:18 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id D299F21D79 for ; Wed, 20 Jan 2021 02:00:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D299F21D79 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=xmission.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-20670-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 26243 invoked by uid 550); 20 Jan 2021 02:00:10 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 26211 invoked from network); 20 Jan 2021 02:00:09 -0000 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> <87eeig74kv.fsf@x220.int.ebiederm.org> Date: Tue, 19 Jan 2021 19:58:44 -0600 In-Reply-To: <87eeig74kv.fsf@x220.int.ebiederm.org> (Eric W. Biederman's message of "Tue, 19 Jan 2021 19:57:36 -0600") Message-ID: <878s8o74iz.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=1l22mo-00B0pL-8x;;;mid=<878s8o74iz.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/T2B95TMqxtikO/3YyPpBrhK2ILrwugMw= 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) ebiederm@xmission.com (Eric W. Biederman) writes: > 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. So starting with something easy to comprehend and simple, may make it easier to figure out how to optimize the code. Eric