Re: [RFC 00/20] ns: Introduce Time Namespace

Dmitry Safonov [off-list ref] writes:

Discussions around time virtualization are there for a long time.
The first attempt to implement time namespace was in 2006 by Jeff Dike.
From that time, the topic appears on and off in various discussions.

There are two main use cases for time namespaces:
1. change date and time inside a container;
2. adjust clocks for a container restored from a checkpoint.

“It seems like this might be one of the last major obstacles keeping
migration from being used in production systems, given that not all
containers and connections can be migrated as long as a time dependency
is capable of messing it up.” (by github.com/dav-ell)

The kernel provides access to several clocks: CLOCK_REALTIME,
CLOCK_MONOTONIC, CLOCK_BOOTTIME. Last two clocks are monotonous, but the
start points for them are not defined and are different for each running
system. When a container is migrated from one node to another, all
clocks have to be restored into consistent states; in other words, they
have to continue running from the same points where they have been
dumped.

The main idea behind this patch set is adding per-namespace offsets for
system clocks. When a process in a non-root time namespace requests
time of a clock, a namespace offset is added to the current value of
this clock on a host and the sum is returned.

All offsets are placed on a separate page, this allows up to map it as 
part of vvar into user processes and use offsets from vdso calls.

Now offsets are implemented for CLOCK_MONOTONIC and CLOCK_BOOTTIME
clocks.

Questions to discuss:

* Clone flags exhaustion. Currently there is only one unused clone flag
bit left, and it may be worth to use it to extend arguments of the clone
system call.

* Realtime clock implementation details:
  Is having a simple offset enough?
  What to do when date and time is changed on the host?
  Is there a need to adjust vfs modification and creation times? 
  Implementation for adjtime() syscall.

Overall I support this effort.  In my quick skim this code looked good.

My feeling is that we need to be able to support running ntpd and
support one namespace doing googles smoothing of leap seconds while
another namespace takes the leap second.

What I was imagining when I was last thinking about this was one
instance of struct timekeeper aka tk_core per time namespace.  That
structure already keeps offsets for all of the various clocks from
the kerne internal time sources.  What would be needed would be to
pass in an appropriate time namespace pointer.

I could be completely wrong as I have not take the time to completely
trace through the code.  Have you looked at pushing the time namespace
down as far as tk_core?

What I think would be the big advantage (besides ntp working) is that
the bulk of the code could be reused.  Allowing testing of the kernel's
time code by setting up a new time namespace.  So a person in production
could setup a time namespace with the time set ahead a little  bit and
be able to verify that the kernel handles the upcoming leap second
properly.



I don't know about the vfs.  I think the danger is being able to write
dates in the future or in the past.  It appears that utimes(2) and
utimesnat(2) already allow this except for status change.  So it is
possible we simply don't care.  I seem to remember that what nfs does
is take the time stamp from the host writing to the file.

I think the guide for filesystem timestamps should be to first ensure
we don't introduce security issues, and then do what distributed
filesystems do when dealing with hosts with different clocks.

Given those those two guidlines above I don't think there is a need to
change timestamsp the way the user namespace changes uid when displayed.



As for the hardware like the real time clock we definitely should not
let a root in a time namespace change it.  We might even be able to get
away with leaving the real time clock out of the time namespace.  If not
we need to be very careful how the real time clock is abstracted.  I
would start by leaving the real time clock hardware out of the time
namespace and see if there is any part of userspace that cares.

Eric

Cc: Dmitry Safonov <redacted>
Cc: Adrian Reber <redacted>
Cc: Andrei Vagin <redacted>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Brauner <redacted>
Cc: Cyrill Gorcunov <redacted>
Cc: "Eric W. Biederman" <redacted>
Cc: "H. Peter Anvin" <hpa@zytor.com> 
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jeff Dike <redacted>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pavel Emelyanov <redacted>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <redacted>
Cc: containers@lists.linux-foundation.org
Cc: criu@openvz.org
Cc: linux-api@vger.kernel.org
Cc: x86@kernel.org

Andrei Vagin (12):
  ns: Introduce Time Namespace
  timens: Add timens_offsets
  timens: Introduce CLOCK_MONOTONIC offsets
  timens: Introduce CLOCK_BOOTTIME offset
  timerfd/timens: Take into account ns clock offsets
  kernel: Take into account timens clock offsets in clock_nanosleep
  x86/vdso/timens: Add offsets page in vvar
  x86/vdso: Use set_normalized_timespec() to avoid 32 bit overflow
  posix-timers/timens: Take into account clock offsets
  selftest/timens: Add test for timerfd
  selftest/timens: Add test for clock_nanosleep
  timens/selftest: Add timer offsets test

Dmitry Safonov (8):
  timens: Shift /proc/uptime
  x86/vdso: Restrict splitting vvar vma
  x86/vdso: Purge timens page on setns()/unshare()/clone()
  x86/vdso: Look for vvar vma to purge timens page
  timens: Add align for timens_offsets
  timens: Optimize zero-offsets
  selftest: Add Time Namespace test for supported clocks
  timens/selftest: Add procfs selftest

 arch/Kconfig                                     |   5 +
 arch/x86/Kconfig                                 |   1 +
 arch/x86/entry/vdso/vclock_gettime.c             |  52 +++++
 arch/x86/entry/vdso/vdso-layout.lds.S            |   9 +-
 arch/x86/entry/vdso/vdso2c.c                     |   3 +
 arch/x86/entry/vdso/vma.c                        |  67 +++++++
 arch/x86/include/asm/vdso.h                      |   2 +
 fs/proc/namespaces.c                             |   3 +
 fs/proc/uptime.c                                 |   3 +
 fs/timerfd.c                                     |  16 +-
 include/linux/nsproxy.h                          |   1 +
 include/linux/proc_ns.h                          |   1 +
 include/linux/time_namespace.h                   |  72 +++++++
 include/linux/timens_offsets.h                   |  25 +++
 include/linux/user_namespace.h                   |   1 +
 include/uapi/linux/sched.h                       |   1 +
 init/Kconfig                                     |   8 +
 kernel/Makefile                                  |   1 +
 kernel/fork.c                                    |   3 +-
 kernel/nsproxy.c                                 |  19 +-
 kernel/time/hrtimer.c                            |   8 +
 kernel/time/posix-timers.c                       |  89 ++++++++-
 kernel/time/posix-timers.h                       |   2 +
 kernel/time_namespace.c                          | 230 +++++++++++++++++++++++
 tools/testing/selftests/timens/.gitignore        |   5 +
 tools/testing/selftests/timens/Makefile          |   6 +
 tools/testing/selftests/timens/clock_nanosleep.c |  98 ++++++++++
 tools/testing/selftests/timens/config            |   1 +
 tools/testing/selftests/timens/log.h             |  21 +++
 tools/testing/selftests/timens/procfs.c          | 145 ++++++++++++++
 tools/testing/selftests/timens/timens.c          | 196 +++++++++++++++++++
 tools/testing/selftests/timens/timer.c           |  95 ++++++++++
 tools/testing/selftests/timens/timerfd.c         |  96 ++++++++++
 33 files changed, 1272 insertions(+), 13 deletions(-)
 create mode 100644 include/linux/time_namespace.h
 create mode 100644 include/linux/timens_offsets.h
 create mode 100644 kernel/time_namespace.c
 create mode 100644 tools/testing/selftests/timens/.gitignore
 create mode 100644 tools/testing/selftests/timens/Makefile
 create mode 100644 tools/testing/selftests/timens/clock_nanosleep.c
 create mode 100644 tools/testing/selftests/timens/config
 create mode 100644 tools/testing/selftests/timens/log.h
 create mode 100644 tools/testing/selftests/timens/procfs.c
 create mode 100644 tools/testing/selftests/timens/timens.c
 create mode 100644 tools/testing/selftests/timens/timer.c
 create mode 100644 tools/testing/selftests/timens/timerfd.c