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

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Message-ID: <20200317084154.m2u76jqj5f47mxqc@wittgenstein>
Date:   Tue, 17 Mar 2020 09:41:54 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     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>,
        Andrei Vagin <avagin@...il.com>, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: clone3: allow creation of time namespace with offset

On Tue, Mar 17, 2020 at 09:30:40AM +0100, Adrian Reber wrote:
> This is an attempt to add time namespace support to clone3(). I am not
> really sure which way clone3() should handle time namespaces. The time
> namespace through /proc cannot be used with clone3() because the offsets
> for the time namespace need to be written before a process has been
> created in that time namespace. This means it is necessary to somehow
> tell clone3() the offsets for the clocks.
> 
> The time namespace offers the possibility to set offsets for
> CLOCK_MONOTONIC and CLOCK_BOOTTIME. My first approach was to extend
> 'struct clone_args` with '__aligned_u64 monotonic_offset' and
> '__aligned_u64 boottime_offset'. The problem with this approach was that
> it was not possible to set nanoseconds for the clocks in the time
> namespace.
> 
> One of the motivations for clone3() with CLONE_NEWTIME was to enable
> CRIU to restore a process in a time namespace with the corresponding
> offsets. And although the nanosecond value can probably never be
> restored to the same value it had during checkpointing, because the
> clock keeps on running between CRIU pausing all processes and CRIU
> actually reading the value of the clocks, the nanosecond value is still
> necessary for CRIU to not restore a process where the clock jumps back
> due to CRIU restoring it with a nanonsecond value that is too small.
> 
> Requiring nanoseconds as well as seconds for two clocks during clone3()
> means that it would require 4 additional members to 'struct clone_args':
> 
>         __aligned_u64 tls;
>         __aligned_u64 set_tid;
>         __aligned_u64 set_tid_size;
> +       __aligned_u64 boottime_offset_seconds;
> +       __aligned_u64 boottime_offset_nanoseconds;
> +       __aligned_u64 monotonic_offset_seconds;
> +       __aligned_u64 monotonic_offset_nanoseconds;
>  };
> 
> To avoid four additional members to 'struct clone_args' this patchset
> uses another approach:
> 
>         __aligned_u64 tls;
>         __aligned_u64 set_tid;
>         __aligned_u64 set_tid_size;
> +       __aligned_u64 timens_offset;
> +       __aligned_u64 timens_offset_size;

Hm, so for set_tid we did set_tid and set_tid_size which makes sense
because set_tid wasn't actually a struct. But I'm not a fan of
establishing a pattern whereby we always have to grow two member, the
object and it's size; at least when we're adding a struct.
So at a first glance here are two possible ideas:
- Don't add a size argument and assume that struct timens_offset won't
  grow. I'm not sure how likely it is it will grow.
- Make the size the first member of struct timens_offset the size of the
  struct. (See examples for this pattern in the sched syscalls.)

Christian