linux-kernel - Re: clone3: allow creation of time namespace with offset

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Message-ID: <20200324160945.orcm75avj2ol3eop@wittgenstein>
Date:   Tue, 24 Mar 2020 17:09:45 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Andrei Vagin <avagin@...il.com>, Arnd Bergmann <arnd@...db.de>,
        Adrian Reber <areber@...hat.com>
Cc:     Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Aleksa Sarai <cyphar@...har.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: clone3: allow creation of time namespace with offset

On Fri, Mar 20, 2020 at 11:33:55AM -0700, Andrei Vagin wrote:
> On Thu, Mar 19, 2020 at 11:29:55AM +0100, Christian Brauner wrote:
> > On Thu, Mar 19, 2020 at 09:16:43AM +0100, Arnd Bergmann wrote:
> > > On Thu, Mar 19, 2020 at 9:11 AM Adrian Reber <areber@...hat.com> wrote:
> > > 
> > > > With Arnd's idea of only using nanoseconds, timens_offset would then
> > > > contain something like this:
> > > >
> > > > struct timens_offset {
> > > >         __aligned_s64 monotonic_offset_ns;
> > > >         __aligned_s64 boottime_offset_ns;
> > > > };
> > > >
> > > > I kind of prefer adding boottime and monotonic directly to struct clone_args
> > > >
> > > >         __aligned_u64 tls;
> > > >         __aligned_u64 set_tid;
> > > >         __aligned_u64 set_tid_size;
> > > > +       __aligned_s64 monotonic_offset_ns;
> > > > +       __aligned_s64 boottime_offset_ns;
> > > >  };
> > > 
> > > I would also prefer the second approach using two 64-bit integers
> > > instead of a pointer, as it keeps the interface simpler to implement
> > > and simpler to interpret by other tools.
> > 
> > Why I don't like has two reasons. There's the scenario where we have
> > added new extensions after the new boottime member and then we introduce
> > another offset. Then you'd be looking at:
> > 
> > __aligned_u64 tls;
> > __aligned_u64 set_tid;
> > __aligned_u64 set_tid_size;
> > + __aligned_s64 monotonic_offset_ns;
> > + __aligned_s64 boottime_offset_ns;
> > __aligned_s64 something_1
> > __aligned_s64 anything_2
> > + __aligned_s64 sometime_offset_ns
> > 
> > which bothers me just by looking at it. That's in addition to adding two
> > new members to the struct when most people will never set CLONE_NEWTIME.
> > We'll also likely have more features in the future that will want to
> > pass down more info than we want to directly expose in struct
> > clone_args, e.g. for a long time I have been thinking about adding a
> > struct for CLONE_NEWUSER that allows you to specify the id mappings you
> > want the new user namespace to get. We surely don't want to force all
> > new info into the uppermost struct. So I'm not convinced we should here.
> 
> I think here we can start thinking about a netlink-like interface.

I think netlink is just not a great model for an API and I would not
want us to go down that route.

I kept thinking about this for a bit and I think that we will end up
growing more namespace-related functionality. So one thing that came to
my mind is the following layout:

struct {
	struct {
		__s64 monotonic;
		__s64 boot;
	} time;
} namespaces;

struct _clone_args {
	__aligned_u64 flags;
	__aligned_u64 pidfd;
	__aligned_u64 child_tid;
	__aligned_u64 parent_tid;
	__aligned_u64 exit_signal;
	__aligned_u64 stack;
	__aligned_u64 stack_size;
	__aligned_u64 tls;
	__aligned_u64 set_tid;
	__aligned_u64 set_tid_size;
	__aligned_u64 namespaces;
	__aligned_u64 namespaces_size;
};

Then when we end up adding id mapping support for CLONE_NEWUSER we can
extend this with:

struct {
	struct {
		__aligned_u64 monotonic;
		__aligned_u64 boot;
	} time;

	struct {
		/* id mapping members */
	} user;
} namespaces;

Thoughts? Other ideas?

Christian