Re: clone3: allow creation of time namespace with offset

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

[CC += linux-api; please CC on future versions]

On Tue, 17 Mar 2020 at 09:32, Adrian Reber <areber@redhat.com> 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;
>  };
>
> timens_offset is a pointer to an array just as previously done with
> set_tid and timens_offset_size is the size of the array.
>
> The timens_offset array is expected to contain a struct like this:
>
> struct set_timens_offset {
>        int clockid;
>        struct timespec val;
> };
>
> This way it is possible to pass the information of multiple clocks with
> seconds and nanonseconds to clone3().
>
> To me this seems the better approach, but I am not totally convinced
> that it is the right thing. If there are other ideas how to pass two
> clock offsets with seconds and nanonseconds to clone3() I would be happy
> to hear other ideas.
>
>                 Adrian
>
>


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/