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=-0.8 required=3.0 tests=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 A98E0C34048 for ; Tue, 18 Feb 2020 17:20:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8B72820801 for ; Tue, 18 Feb 2020 17:20:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726754AbgBRRUZ (ORCPT ); Tue, 18 Feb 2020 12:20:25 -0500 Received: from rhlx01.hs-esslingen.de ([129.143.116.10]:44156 "EHLO rhlx01.hs-esslingen.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726415AbgBRRUY (ORCPT ); Tue, 18 Feb 2020 12:20:24 -0500 X-Greylist: delayed 535 seconds by postgrey-1.27 at vger.kernel.org; Tue, 18 Feb 2020 12:20:23 EST Received: by rhlx01.hs-esslingen.de (Postfix, from userid 1203) id F2D7B29C4924; Tue, 18 Feb 2020 18:11:26 +0100 (CET) Date: Tue, 18 Feb 2020 18:11:26 +0100 From: Adrian Reber To: Christian Brauner Cc: "Michael Kerrisk (man-pages)" , Dmitry Safonov , Linux Kernel , Dmitry Safonov <0x7f454c46@gmail.com>, Andy Lutomirski , Arnd Bergmann , Cyrill Gorcunov , "Eric W. Biederman" , "H. Peter Anvin" , Ingo Molnar , Jann Horn , Jeff Dike , Oleg Nesterov , Pavel Emelyanov , Shuah Khan , Thomas Gleixner , Vincenzo Frascino , containers , criu@openvz.org, Linux API , x86@kernel.org, Andrei Vagin Subject: Re: Time Namespaces: CLONE_NEWTIME and clone3()? Message-ID: <20200218171126.GF2322403@lisas.de> References: <20191112012724.250792-1-dima@arista.com> <20191112012724.250792-4-dima@arista.com> <20200217145908.7epzz5nescccwvzv@wittgenstein> <20200217230331.he6p5bs766zp6smx@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200217230331.he6p5bs766zp6smx@wittgenstein> X-Url: X-Operating-System: Linux (5.4.8-200.fc31.x86_64) X-Load-Average: 7.47 8.46 9.12 X-Unexpected: The Spanish Inquisition X-GnuPG-Key: gpg --recv-keys D3C4906A Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 18, 2020 at 12:03:31AM +0100, Christian Brauner wrote: > On Mon, Feb 17, 2020 at 10:47:53PM +0100, Michael Kerrisk (man-pages) wrote: > > Hello Christian, > > > > On Mon, 17 Feb 2020 at 16:15, Christian Brauner > > wrote: > > > > > > On Mon, Feb 17, 2020 at 03:20:55PM +0100, Michael Kerrisk wrote: > > > > Hello Dmitry, Andrei, > > > > > > > > Is the CLONE_NEWTIME flag intended to be usable with clone3()? The > > > > mail quoted below implies (in my reading) that this should be possible > > > > once clone3() is available, which it is by now. (See also [1].) > > > > > > > > If the answer is yes, CLONE_NEWTIME should be usable with clone3(), > > > > then I have a bug report and a question. > > > > > > > > I successfully used CLONE_NEWTIME with unshare(). But if I try to use > > > > CLONE_NEWSIGNAL with clone3(), it errors out with EINVAL, because of > > > > > > s/CLONE_NEWSIGNAL/CLONE_NEWTIME/ > > > > > > > the following check in clone3_args_valid(): > > > > > > > > /* > > > > * - make the CLONE_DETACHED bit reuseable for clone3 > > > > * - make the CSIGNAL bits reuseable for clone3 > > > > */ > > > > if (kargs->flags & (CLONE_DETACHED | CSIGNAL)) > > > > return false; > > > > > > > > The problem is that CLONE_NEWTIME matches one of the bits in the > > > > CSIGNAL mask. If the intention is to allow CLONE_NEWTIME with > > > > clone3(), then either the bit needs to be redefined, or the error > > > > checking in clone3_args_valid() needs to be reworked. > > > > > > If this is intended to be useable with clone3() the check should be > > > adapted to allow for CLONE_NEWTIME. (I asked about this a while ago I > > > think.) > > > But below rather sounds like it should simply be an unshare() flag. The > > > code seems to set frozen_offsets to true right after copy_namespaces() > > > in timens_on_fork(new_ns, tsk) and so the offsets can't be changed > > > anymore unless I'm reading this wrong. > > > Alternatives seem to either make timens_offsets writable once after fork > > > and before exec, I guess - though that's probably not going to work > > > with the vdso judging from timens_on_fork(). > > > > > > The other alternative is that Andrei and Dmitry send me a patch to > > > enable CLONE_NEWTIME with clone3() by exposing struct timens_offsets (or > > > a version of it) in the uapi and extend struct clone_args to include a > > > pointer to a struct timens_offset that is _only_ set when CLONE_NEWTIME > > > is set. > > > Though the unshare() way sounds way less invasive simpler. > > > > Actually, I think the alternative you propose just here is better. I > > imagine there are times when one will want to create multiple > > namespaces with a single call to clone3(), including a time namespace. > > I think this should be allowed by the API. And, otherwise, clone3() > > becomes something of a second-class citizen for creating namespaces. > > (I don't really get the "less invasive" argument. Implementing this is > > just a piece of kernel to code to make user-space's life a bit simpler > > and more consistent.) > > I don't particularly mind either way. If there's actual users that need > to set it at clone3() time then we can extend it. So I'd like to hear > what Adrian, Dmitry, and Thomas think since they are well-versed how > this will be used in the wild. I'm weary of exposing a whole new uapi > struct and extending clone3() without any real use-case but I'm happy to > if there is! Re-creating a time namespace during restore via clone3() would be CRIU's preferred way of doing this. If available CRIU is already using clone3(), especially because of set_tid, and for the time namespace we would also rely on clone3() to re-create it. I will provide a patch to extend clone3() to handle the creation of a new time namespace. Adrian