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    SubjectRe: clone3: allow creation of time namespace with offset
    Hi Adrian,

    If there was a revision to this patch, I missed it. Is there still a
    plan to bring CLONE_NEWTIME to clone3()?

    Thanks,

    Michael

    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/

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