Re: clone3: allow creation of time namespace with offset

[Christian Brauner <christian.brauner@ubuntu.com>]

On Tue, Mar 17, 2020 at 09:41:55AM +0100, Christian Brauner wrote:
> 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.)

Oh, and I should point out right way that I consider this material for
the v5.8 merge window. It's too late in this cycle to land this with any
confidence in v5.7. Just so there's no disappointment. :) The good news
is that this leaves us with ample time to figure this out.

Christian