Re: clone3: allow creation of time namespace with offset

[Adrian Reber <areber@redhat.com>]

On Wed, Mar 18, 2020 at 11:18:53AM +0100, Arnd Bergmann wrote:
> On Tue, Mar 17, 2020 at 9:32 AM 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;
> >  };
> 
> Wouldn't it be sufficient to have the two nanosecond values, rather
> than both seconds and nanoseconds? With 64-bit nanoseconds
> you can represent several hundred years, and these would
> always start at zero during boot.

I like this. Just using nanoseconds will make it easier and should
indeed be enough.

> Regardless of this, I think you need a signed offset, not unsigned.

Right, that was just a quick test at some point.

Christian and I have also been discussing this a bit and Christian
prefers a pointer to a struct. Maybe something like this:

        __aligned_u64 tls;
        __aligned_u64 set_tid;
        __aligned_u64 set_tid_size;
+       __aligned_u64 timens_offset;
 };

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;
 };

But setting the time namespace offset is probably something which does
not happen very often while using clone3(), so maybe the pointer to a
struct approach is better.

I will resend the patches using the pointer to a struct approach if
there are no other ideas how to do this.

		Adrian