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Date: Mon, 17 Feb 2020 22:47:53 +0100 From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> To: Christian Brauner <christian.brauner@...ntu.com> Cc: Dmitry Safonov <dima@...sta.com>, Andrei Vagin <avagin@...nvz.org>, Linux Kernel <linux-kernel@...r.kernel.org>, Dmitry Safonov <0x7f454c46@...il.com>, Adrian Reber <adrian@...as.de>, Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>, Cyrill Gorcunov <gorcunov@...nvz.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>, Oleg Nesterov <oleg@...hat.com>, Pavel Emelyanov <xemul@...tuozzo.com>, Shuah Khan <shuah@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Vincenzo Frascino <vincenzo.frascino@....com>, containers <containers@...ts.linux-foundation.org>, criu@...nvz.org, Linux API <linux-api@...r.kernel.org>, x86@...nel.org, Andrei Vagin <avagin@...il.com> Subject: Re: Time Namespaces: CLONE_NEWTIME and clone3()? Hello Christian, On Mon, 17 Feb 2020 at 16:15, Christian Brauner <christian.brauner@...ntu.com> 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.) Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
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