2020-04-04 09:23:02 +00:00
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.\" Copyright (c) 2020 by Michael Kerrisk <mtk.manpages@gmail.com>
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getent.1, ldd.1, locale.1, localedef.1, memusage.1, memusagestat.1, mtrace.1, _exit.2, _syscall.2, accept.2, access.2, acct.2, add_key.2, adjtimex.2, alloc_hugepages.2, arch_prctl.2, bdflush.2, bind.2, bpf.2, brk.2, cacheflush.2, capget.2, chdir.2, chmod.2, chown.2, chroot.2, clock_getres.2, clock_nanosleep.2, clone.2, close.2, close_range.2, connect.2, copy_file_range.2, create_module.2, delete_module.2, dup.2, epoll_create.2, epoll_ctl.2, epoll_wait.2, eventfd.2, execve.2, execveat.2, fanotify_init.2, fanotify_mark.2, fcntl.2, flock.2, fork.2, fsync.2, futex.2, get_kernel_syms.2, get_mempolicy.2, get_robust_list.2, getcpu.2, getdents.2, getdomainname.2, getgid.2, getgroups.2, gethostname.2, getitimer.2, getpagesize.2, getpeername.2, getpid.2, getpriority.2, getrandom.2, getresuid.2, getrlimit.2, getrusage.2, getsid.2, getsockname.2, getsockopt.2, gettid.2, gettimeofday.2, getuid.2, getunwind.2, getxattr.2, idle.2, init_module.2, inotify_add_watch.2, inotify_rm_watch.2, io_cancel.2, io_destroy.2, io_getevents.2, io_setup.2, io_submit.2, ioctl.2, ioctl_console.2, ioctl_fat.2, ioctl_ficlonerange.2, ioctl_fideduperange.2, ioctl_fslabel.2, ioctl_getfsmap.2, ioctl_ns.2, ioctl_tty.2, ioctl_userfaultfd.2, ioperm.2, iopl.2, ipc.2, kcmp.2, kexec_load.2, keyctl.2, kill.2, link.2, listen.2, listxattr.2, llseek.2, lookup_dcookie.2, lseek.2, madvise.2, mbind.2, membarrier.2, memfd_create.2, migrate_pages.2, mincore.2, mkdir.2, mknod.2, mlock.2, mmap.2, mmap2.2, modify_ldt.2, mount.2, move_pages.2, mprotect.2, mq_getsetattr.2, mremap.2, msgctl.2, msgget.2, msgop.2, msync.2, nanosleep.2, nfsservctl.2, nice.2, open.2, open_by_handle_at.2, openat2.2, pause.2, pciconfig_read.2, perf_event_open.2, perfmonctl.2, personality.2, pidfd_getfd.2, pidfd_open.2, pidfd_send_signal.2, pipe.2, pivot_root.2, pkey_alloc.2, poll.2, posix_fadvise.2, prctl.2, pread.2, process_vm_readv.2, ptrace.2, query_module.2, quotactl.2, read.2, readahead.2, readdir.2, readlink.2, readv.2, reboot.2, recv.2, remap_file_pages.2, removexattr.2, rename.2, request_key.2, restart_syscall.2, rmdir.2, rt_sigqueueinfo.2, s390_guarded_storage.2, s390_pci_mmio_write.2, s390_runtime_instr.2, s390_sthyi.2, sched_get_priority_max.2, sched_rr_get_interval.2, sched_setaffinity.2, sched_setattr.2, sched_setparam.2, sched_setscheduler.2, sched_yield.2, seccomp.2, select.2, select_tut.2, semctl.2, semget.2, semop.2, send.2, sendfile.2, set_thread_area.2, seteuid.2, setfsgid.2, setfsuid.2, setgid.2, setpgid.2, setresuid.2, setreuid.2, setsid.2, setuid.2, setup.2, setxattr.2, sgetmask.2, shmctl.2, shmget.2, shmop.2, shutdown.2, sigaction.2, sigaltstack.2, signal.2, signalfd.2, sigpending.2, sigprocmask.2, sigreturn.2, sigsuspend.2, sigwaitinfo.2, socket.2, socketcall.2, socketpair.2, splice.2, spu_create.2, spu_run.2, stat.2, statfs.2, statx.2, stime.2, subpage_prot.2, swapon.2, symlink.2, sync.2, sync_file_range.2, syscall.2, syscalls.2, sysctl.2, sysfs.2, sysinfo.2, syslog.2, time.2, timer_create.2, timer_delete.2, timer_getoverrun.2, timer_settime.2, timerfd_create.2, times.2, tkill.2, truncate.2, umask.2, umount.2, uname.2, unimplemented.2, unlink.2, unshare.2, uselib.2, userfaultfd.2, ustat.2, utime.2, utimensat.2, vfork.2, vhangup.2, vm86.2, vmsplice.2, wait.2, wait4.2, write.2, CPU_SET.3, __ppc_get_timebase.3, __ppc_set_ppr_med.3, __ppc_yield.3, __setfpucw.3, a64l.3, abort.3, abs.3, acos.3, acosh.3, addseverity.3, adjtime.3, aio_cancel.3, aio_error.3, aio_fsync.3, aio_read.3, aio_return.3, aio_suspend.3, aio_write.3, alloca.3, argz_add.3, asin.3, asinh.3, asprintf.3, assert.3, assert_perror.3, atan.3, atan2.3, atanh.3, atexit.3, atof.3, atoi.3, backtrace.3, basename.3, bcmp.3, bcopy.3, bindresvport.3, bsd_signal.3, bsearch.3, bstring.3, btowc.3, byteorder.3, bzero.3, cabs.3, cacos.3, cacosh.3, canonicalize_file_name.3, carg.3, casin.3, casinh.3, catan.3, catanh.3, catgets.3, catopen.3, cbrt.3, ccos.3, ccosh.3, ceil.3, cexp.3, cexp2.3, cfree.3, cimag.3, circleq.3, clearenv.3, clock.3, clock_getcpuclockid.3, clog.3, clog10.3, clog2.3, closedir.3, cmsg.3, confstr.3, conj.3, copysign.3, cos.3, cosh.3, cpow.3, cproj.3, creal.3, crypt.3, csin.3, csinh.3, csqrt.3, ctan.3, ctanh.3, ctermid.3, ctime.3, daemon.3, des_crypt.3, difftime.3, dirfd.3, div.3, dl_iterate_phdr.3, dladdr.3, dlerror.3, dlinfo.3, dlopen.3, dlsym.3, drand48.3, drand48_r.3, duplocale.3, dysize.3, ecvt.3, ecvt_r.3, encrypt.3, endian.3, envz_add.3, erf.3, erfc.3, err.3, errno.3, error.3, ether_aton.3, euidaccess.3, exec.3, exit.3, exp.3, exp10.3, exp2.3, expm1.3, fabs.3, fclose.3, fcloseall.3, fdim.3, fenv.3, ferror.3, fexecve.3, fflush.3, ffs.3, fgetc.3, fgetgrent.3, fgetpwent.3, fgetwc.3, fgetws.3, fileno.3, finite.3, flockfile.3, floor.3, fma.3, fmax.3, fmemopen.3, fmin.3, fmod.3, fmtmsg.3, fnmatch.3, fopen.3, fopencookie.3, fpathconf.3, fpclassify.3, fpurge.3, fputwc.3, fputws.3, fread.3, frexp.3, fseek.3, fseeko.3, ftime.3, ftok.3, fts.3, ftw.3, futimes.3, fwide.3, gamma.3, gcvt.3, get_nprocs_conf.3, get_phys_pages.3, getaddrinfo.3, getaddrinfo_a.3, getauxval.3, getcontext.3, getcwd.3, getdate.3, getdirentries.3, getdtablesize.3, getentropy.3, getenv.3, getfsent.3, getgrent.3, getgrent_r.3, getgrnam.3, getgrouplist.3, gethostbyname.3, gethostid.3, getifaddrs.3, getipnodebyname.3, getline.3, getloadavg.3, getlogin.3, getmntent.3, getnameinfo.3, getnetent.3, getnetent_r.3, getopt.3, getpass.3, getprotoent.3, getprotoent_r.3, getpt.3, getpw.3, getpwent.3, getpwent_r.3, getpwnam.3, getrpcent.3, getrpcent_r.3, getrpcport.3, gets.3, getservent.3, getservent_r.3, getspnam.3, getsubopt.3, getttyent.3, getumask.3, getusershell.3, getutent.3, getutmp.3, getw.3, getwchar.3, glob.3, gnu_get_libc_version.3, grantpt.3, group_member.3, gsignal.3, hsearch.3, hypot.3, iconv.3, iconv_close.3, iconv_open.3, if_nameindex.3, if_nametoindex.3, ilogb.3, index.3, inet.3, inet_net_pton.3, inet_ntop.3, inet_pton.3, initgroups.3, insque.3, isalpha.3, isatty.3, isfdtype.3, isgreater.3, iswalnum.3, iswalpha.3, iswblank.3, iswcntrl.3, iswctype.3, iswdigit.3, iswgraph.3, iswlower.3, iswprint.3, iswpunct.3, iswspace.3, iswupper.3, iswxdigit.3, j0.3, key_setsecret.3, killpg.3, ldexp.3, lgamma.3, lio_listio.3, list.3, localeconv.3, lockf.3, log.3, log10.3, log1p.3, log2.3, logb.3, login.3, lrint.3, lround.3, lsearch.3, lseek64.3, makecontext.3, makedev.3, mallinfo.3, malloc.3, malloc_get_state.3, malloc_hook.3, malloc_info.3, malloc_stats.3, malloc_trim.3, malloc_usable_size.3, mallopt.3, matherr.3, mblen.3, mbrlen.3, mbrtowc.3, mbsinit.3, mbsnrtowcs.3, mbsrtowcs.3, mbstowcs.3, mbtowc.3, mcheck.3, memccpy.3, memchr.3, memcmp.3, memcpy.3, memfrob.3, memmem.3, memmove.3, mempcpy.3, memset.3, mkdtemp.3, mkfifo.3, mkstemp.3, mktemp.3, modf.3, mpool.3, mq_close.3, mq_getattr.3, mq_notify.3, mq_open.3, mq_receive.3, mq_send.3, mq_unlink.3, mtrace.3, nan.3, newlocale.3, nextafter.3, nextup.3, nl_langinfo.3, ntp_gettime.3, on_exit.3, open_memstream.3, opendir.3, openpty.3, perror.3, popen.3, posix_fallocate.3, posix_madvise.3, posix_memalign.3, posix_openpt.3, posix_spawn.3, pow.3, pow10.3, printf.3, profil.3, psignal.3, pthread_attr_init.3, pthread_attr_setaffinity_np.3, pthread_attr_setdetachstate.3, pthread_attr_setguardsize.3, pthread_attr_setinheritsched.3, pthread_attr_setschedparam.3, pthread_attr_setschedpolicy.3, pthread_attr_setscope.3, pthread_attr_setsigmask_np.3, pthread_attr_setstack.3, pthread_attr_setstackaddr.3, pthread_attr_setstacksize.3, pthread_cancel.3, pthread_cleanup_push.3, pthread_cleanup_push_defer_np.3, pthread_create.3, pthread_detach.3, pthread_equal.3, pthread_exit.3, pthread_getattr_default_np.3, pthread_getattr_np.3, pthread_getcpuclockid.3, pthread_join.3, pthread_kill.3, pthread_kill_other_threads_np.3, pthread_mutex_consistent.3, pthread_mutexattr_getpshared.3, pthread_mutexattr_setrobust.3, pthread_rwlockattr_setkind_np.3, pthread_self.3, pthread_setaffinity_np.3, pthread_setcancelstate.3, pthread_setconcurrency.3, pthread_setname_np.3, pthread_setschedparam.3, pthread_setschedprio.3, pthread_sigmask.3, pthread_sigqueue.3, pthread_spin_init.3, pthread_spin_lock.3, pthread_testcancel.3, pthread_tryjoin_np.3, pthread_yield.3, ptsname.3, putenv.3, putgrent.3, putpwent.3, puts.3, putwchar.3, qecvt.3, qsort.3, raise.3, rand.3, random.3, random_r.3, rcmd.3, re_comp.3, readdir.3, readdir_r.3, realpath.3, regex.3, remainder.3, remove.3, remquo.3, resolver.3, rewinddir.3, rexec.3, rint.3, round.3, rpc.3, rpmatch.3, rtime.3, rtnetlink.3, scalb.3, scalbln.3, scandir.3, scanf.3, sched_getcpu.3, seekdir.3, sem_close.3, sem_destroy.3, sem_getvalue.3, sem_init.3, sem_open.3, sem_post.3, sem_unlink.3, sem_wait.3, setaliasent.3, setbuf.3, setenv.3, setjmp.3, setlocale.3, setlogmask.3, setnetgrent.3, shm_open.3, siginterrupt.3, signbit.3, significand.3, sigpause.3, sigqueue.3, sigset.3, sigsetops.3, sigvec.3, sigwait.3, sin.3, sincos.3, sinh.3, sleep.3, slist.3, sockatmark.3, sqrt.3, stailq.3, statvfs.3, stdarg.3, stdio.3, stdio_ext.3, stpcpy.3, stpncpy.3, strcasecmp.3, strcat.3, strchr.3, strcmp.3, strcoll.3, strcpy.3, strdup.3, strerror.3, strfmon.3, strfromd.3, strfry.3, strftime.3, string.3, strlen.3, strnlen.3, strpbrk.3, strptime.3, strsep.3, strsignal.3, strspn.3, strstr.3, strtod.3, strtoimax.3, strtok.3, strtol.3, strtoul.3, strverscmp.3, strxfrm.3, swab.3, sysconf.3, syslog.3, system.3, sysv_signal.3, tailq.3, tan.3, tanh.3, tcgetpgrp.3, tcgetsid.3, telldir.3, tempnam.3, termios.3, tgamma.3, timegm.3, timeradd.3, tmpfile.3, tmpnam.3, toascii.3, toupper.3, towctrans.3, towlower.3, towupper.3, trunc.3, tsearch.3, ttyname.3, ttyslot.3, tzset.3, ualarm.3, ulimit.3, undocumented.3, ungetwc.3, unlocked_stdio.3, unlockpt.3, updwtmp.3, uselocale.3, usleep.3, wcpcpy.3, wcpncpy.3, wcrtomb.3, wcscasecmp.3, wcscat.3, wcschr.3, wcscmp.3, wcscpy.3, wcscspn.3, wcsdup.3, wcslen.3, wcsncasecmp.3, wcsncat.3, wcsncmp.3, wcsncpy.3, wcsnlen.3, wcsnrtombs.3, wcspbrk.3, wcsrchr.3, wcsrtombs.3, wcsspn.3, wcsstr.3, wcstoimax.3, wcstok.3, wcstombs.3, wcswidth.3, wctob.3, wctomb.3, wctrans.3, wctype.3, wcwidth.3, wmemchr.3, wmemcmp.3, wmemcpy.3, wmemmove.3, wmemset.3, wordexp.3, wprintf.3, xcrypt.3, xdr.3, y0.3, cciss.4, console_codes.4, dsp56k.4, hpsa.4, initrd.4, loop.4, lp.4, msr.4, random.4, rtc.4, smartpqi.4, veth.4, wavelan.4, acct.5, core.5, elf.5, hosts.5, locale.5, proc.5, resolv.conf.5, rpc.5, slabinfo.5, sysfs.5, tmpfs.5, utmp.5, address_families.7, aio.7, attributes.7, bootparam.7, capabilities.7, cgroups.7, complex.7, ddp.7, environ.7, epoll.7, fanotify.7, feature_test_macros.7, hier.7, inode.7, inotify.7, ip.7, ipv6.7, keyrings.7, locale.7, man-pages.7, man.7, math_error.7, mount_namespaces.7, namespaces.7, netdevice.7, netlink.7, numa.7, packet.7, pkeys.7, pthreads.7, queue.7, raw.7, rtnetlink.7, sched.7, session-keyring.7, shm_overview.7, sigevent.7, signal-safety.7, signal.7, sock_diag.7, socket.7, spufs.7, symlink.7, system_data_types.7, tcp.7, time_namespaces.7, udp.7, udplite.7, unicode.7, unix.7, uri.7, user_namespaces.7, vdso.7, vsock.7, x25.7, iconvconfig.8, ld.so.8, ldconfig.8, sln.8, tzselect.8: tstamp
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2021-03-21 23:15:34 +00:00
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.TH TIME_NAMESPACES 7 2021-03-22 "Linux" "Linux Programmer's Manual"
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2020-04-04 09:23:02 +00:00
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.SH NAME
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time_namespaces \- overview of Linux time namespaces
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.SH DESCRIPTION
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Time namespaces virtualize the values of two system clocks:
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.IP \(bu 2
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.BR CLOCK_MONOTONIC
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(and likewise
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.BR CLOCK_MONOTONIC_COARSE
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and
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.BR CLOCK_MONOTONIC_RAW ),
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a nonsettable clock that represents monotonic time since\(emas
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described by POSIX\(em"some unspecified point in the past".
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.IP \(bu
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2020-04-11 20:32:33 +00:00
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.BR CLOCK_BOOTTIME
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2020-04-04 09:23:02 +00:00
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(and likewise
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.BR CLOCK_BOOTTIME_ALARM ),
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2020-04-06 08:26:10 +00:00
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a nonsettable clock that is identical to
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2020-04-04 09:23:02 +00:00
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.BR CLOCK_MONOTONIC ,
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except that it also includes any time that the system is suspended.
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.PP
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Thus, the processes in a time namespace share per-namespace values
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for these clocks.
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This affects various APIs that measure against these clocks, including:
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2020-04-07 12:45:00 +00:00
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.BR clock_gettime (2),
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.BR clock_nanosleep (2),
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.BR nanosleep (2),
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2020-04-07 12:45:00 +00:00
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.BR timer_settime (2),
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.BR timerfd_settime (2),
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2020-04-04 09:23:02 +00:00
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and
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.IR /proc/uptime .
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.PP
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Currently, the only way to create a time namespace is by calling
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.BR unshare (2)
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with the
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.BR CLONE_NEWTIME
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flag.
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This call creates a new time namespace but does
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.I not
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place the calling process in the new namespace.
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Instead, the calling process's
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subsequently created children are placed in the new namespace.
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This allows clock offsets (see below) for the new namespace
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to be set before the first process is placed in the namespace.
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The
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.IR /proc/[pid]/ns/time_for_children
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symbolic link shows the time namespace in which
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the children of a process will be created.
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2020-04-07 12:45:00 +00:00
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(A process can use a file descriptor opened on
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this symbolic link in a call to
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.BR setns (2)
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in order to move into the namespace.)
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2020-04-04 09:23:02 +00:00
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.\"
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.SS /proc/PID/timens_offsets
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Associated with each time namespace are offsets,
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expressed with respect to the initial time namespace,
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2020-04-06 08:26:10 +00:00
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that define the values of the monotonic and
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boot-time clocks in that namespace.
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2020-04-04 09:23:02 +00:00
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These offsets are exposed via the file
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.IR /proc/PID/timens_offsets .
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Within this file,
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the offsets are expressed as lines consisting of
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three space-delimited fields:
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.PP
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.in +4n
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.EX
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<clock-id> <offset-secs> <offset-nanosecs>
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.EE
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.in
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.PP
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The
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.I clock-id
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2020-04-11 11:32:29 +00:00
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is a string that identifies the clock whose offsets are being shown.
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This field is either
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.IR monotonic ,
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for
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2020-04-04 09:23:02 +00:00
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.BR CLOCK_MONOTONIC ,
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2020-04-11 11:32:29 +00:00
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or
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.IR boottime ,
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for
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2020-04-04 09:23:02 +00:00
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.BR CLOCK_BOOTTIME .
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The remaining fields express the offset (seconds plus nanoseconds) for the
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clock in this time namespace.
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These offsets are expressed relative to the clock values in
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the initial time namespace.
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2020-04-07 12:45:00 +00:00
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The
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.I offset-secs
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2020-04-07 13:07:51 +00:00
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value can be negative, subject to restrictions noted below;
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.I offset-nanosecs
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is an unsigned value.
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2020-04-07 12:45:00 +00:00
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.PP
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In the initial time namespace, the contents of the
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.I timens_offsets
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file are as follows:
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2020-04-04 09:23:02 +00:00
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.PP
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.in +4n
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.EX
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$ \fBcat /proc/self/timens_offsets\fP
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2020-04-11 11:32:29 +00:00
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monotonic 0 0
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boottime 0 0
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2020-04-04 09:23:02 +00:00
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.EE
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.in
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.PP
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In a new time namespace that has had no member processes,
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the clock offsets can be modified by writing newline-terminated
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2020-04-07 12:45:00 +00:00
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records of the same form to the
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.I timens_offsets
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file.
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2020-04-04 09:23:02 +00:00
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The file can be written to multiple times,
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but after the first process has been created in or has entered the namespace,
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.BR write (2)s
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on this file fail with the error
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.BR EACCES .
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In order to write to the
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.IR timens_offsets
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file, a process must have the
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.BR CAP_SYS_TIME
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capability in the user namespace that owns the time namespace.
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.PP
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2020-04-07 13:07:51 +00:00
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Writes to the
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.I timens_offsets
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file can fail with the following errors:
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.TP
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.B EINVAL
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An
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.I offset-nanosecs
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value is greater than 999,999,999.
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.TP
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.B EINVAL
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A
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.I clock-id
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value is not valid.
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.TP
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.B EPERM
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The caller does not have the
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.BR CAP_SYS_TIME
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capability.
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.TP
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.B ERANGE
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An
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.I offset-secs
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value is out of range.
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In particular;
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.RS
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.IP \(bu 2
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.I offset-secs
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can't be set to a value which would make the current
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time on the corresponding clock inside the namespace a negative value; and
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.IP \(bu
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.I offset-secs
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can't be set to a value such that the time on the corresponding clock
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inside the namespace would exceed half of the value of the kernel constant
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.BR KTIME_SEC_MAX
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(this limits the clock value to a maximum of approximately 146 years).
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.RE
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.PP
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2020-04-04 09:23:02 +00:00
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In a new time namespace created by
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.BR unshare (2),
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the contents of the
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.I timens_offsets
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file are inherited from the time namespace of the creating process.
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.SH NOTES
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Use of time namespaces requires a kernel that is configured with the
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.B CONFIG_TIME_NS
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option.
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.PP
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Note that time namespaces do not virtualize the
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.BR CLOCK_REALTIME
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clock.
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Virtualization of this clock was avoided for reasons of complexity
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and overhead within the kernel.
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.PP
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2020-04-11 11:32:29 +00:00
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For compatibility with the initial implementation, when writing a
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.I clock-id
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to the
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.IR /proc/[pid]/timens_offsets
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file, the numerical values of the IDs can be written
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instead of the symbolic names show above; i.e., 1 instead of
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.IR monotonic ,
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and 7 instead of
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.IR boottime .
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2021-02-20 14:21:58 +00:00
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For readability, the use of the symbolic names over the numbers is preferred.
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2020-04-11 11:32:29 +00:00
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.PP
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2020-04-04 09:23:02 +00:00
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The motivation for adding time namespaces was to allow
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the monotonic and boot-time clocks to maintain consistent values
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during container migration and checkpoint/restore.
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2020-05-21 08:00:37 +00:00
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.SH EXAMPLES
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2020-04-04 09:23:02 +00:00
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The following shell session demonstrates the operation of time namespaces.
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We begin by displaying the inode number of the time namespace
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of a shell in the initial time namespace:
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.PP
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.in +4n
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.EX
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$ \fBreadlink /proc/$$/ns/time\fP
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time:[4026531834]
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.EE
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.in
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.PP
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Continuing in the initial time namespace, we display the system uptime using
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.BR uptime (1)
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and use the
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.I clock_times
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example program shown in
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.BR clock_getres (2)
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to display the values of various clocks:
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.PP
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.in +4n
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.EX
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$ \fBuptime \-\-pretty\fP
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up 21 hours, 17 minutes
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$ \fB./clock_times\fP
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CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 36m 41s)
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CLOCK_TAI : 1585989438.972 (18356 days + 8h 37m 18s)
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CLOCK_MONOTONIC: 56338.247 (15h 38m 58s)
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CLOCK_BOOTTIME : 76633.544 (21h 17m 13s)
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.EE
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.in
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.PP
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We then use
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.BR unshare (1)
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to create a time namespace and execute a
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.BR bash (1)
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shell.
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From the new shell, we use the built-in
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.B echo
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command to write records to the
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.I timens_offsets
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file adjusting the offset for the
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.B CLOCK_MONOTONIC
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clock forward 2 days
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and the offset for the
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.B CLOCK_BOOTTIME
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clock forward 7 days:
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.PP
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.in +4n
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.EX
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$ \fBPS1="ns2# " sudo unshare \-T \-\- bash \-\-norc\fP
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2020-04-11 11:32:29 +00:00
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ns2# \fBecho "monotonic $((2*24*60*60)) 0" > /proc/$$/timens_offsets\fP
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ns2# \fBecho "boottime $((7*24*60*60)) 0" > /proc/$$/timens_offsets\fP
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2020-04-04 09:23:02 +00:00
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.EE
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.in
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.PP
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Above, we started the
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.BR bash (1)
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shell with the
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.B \-\-norc
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options so that no start-up scripts were executed.
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This ensures that no child processes are created from the
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shell before we have a chance to update the
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.I timens_offsets
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file.
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.PP
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We then use
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.BR cat (1)
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to display the contents of the
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.I timens_offsets
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file.
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The execution of
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.BR cat (1)
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creates the first process in the new time namespace,
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after which further attempts to update the
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.I timens_offsets
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file produce an error.
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.PP
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.in +4n
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.EX
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ns2# \fBcat /proc/$$/timens_offsets\fP
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2020-04-11 11:32:29 +00:00
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monotonic 172800 0
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boottime 604800 0
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ns2# \fBecho "boottime $((9*24*60*60)) 0" > /proc/$$/timens_offsets\fP
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2020-04-04 09:23:02 +00:00
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bash: echo: write error: Permission denied
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.EE
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.in
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.PP
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Continuing in the new namespace, we execute
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.BR uptime (1)
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and the
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.I clock_times
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example program:
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.PP
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.in +4n
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.EX
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ns2# \fBuptime \-\-pretty\fP
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up 1 week, 21 hours, 18 minutes
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ns2# \fB./clock_times\fP
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CLOCK_REALTIME : 1585989457.056 (18356 days + 8h 37m 37s)
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CLOCK_TAI : 1585989494.057 (18356 days + 8h 38m 14s)
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CLOCK_MONOTONIC: 229193.332 (2 days + 15h 39m 53s)
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CLOCK_BOOTTIME : 681488.629 (7 days + 21h 18m 8s)
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.EE
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.in
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.PP
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From the above output, we can see that the monotonic
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2020-04-06 08:26:10 +00:00
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and boot-time clocks have different values in the new time namespace.
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2020-04-04 09:23:02 +00:00
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.PP
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Examining the
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.I /proc/[pid]/ns/time
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and
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.I /proc/[pid]/ns/time_for_children
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symbolic links, we see that the shell is a member of the initial time
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2020-04-06 08:26:10 +00:00
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namespace, but its children are created in the new namespace.
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2020-04-04 09:23:02 +00:00
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.PP
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.in +4n
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.EX
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ns2# \fBreadlink /proc/$$/ns/time\fP
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time:[4026531834]
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ns2# \fBreadlink /proc/$$/ns/time_for_children\fP
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time:[4026532900]
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ns2# \fBreadlink /proc/self/ns/time\fP # Creates a child process
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time:[4026532900]
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.EE
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.in
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.PP
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Returning to the shell in the initial time namespace,
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we see that the monotonic and boot-time clocks
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are unaffected by the
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.I timens_offsets
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changes that were made in the other time namespace:
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.PP
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.in +4n
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.EX
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$ \fBuptime \-\-pretty\fP
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up 21 hours, 19 minutes
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$ \fB./clock_times\fP
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CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 38m 51s)
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CLOCK_TAI : 1585989438.972 (18356 days + 8h 39m 28s)
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CLOCK_MONOTONIC: 56338.247 (15h 41m 8s)
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CLOCK_BOOTTIME : 76633.544 (21h 19m 23s)
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.EE
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.in
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.SH SEE ALSO
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.BR nsenter (1),
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.BR unshare (1),
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2020-04-12 06:43:56 +00:00
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.BR clock_settime (2),
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2020-04-04 09:23:02 +00:00
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.\" clone3() support for time namespaces is a work in progress
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.\" .BR clone3 (2),
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.BR setns (2),
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.BR unshare (2),
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.BR namespaces (7),
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.BR time (7)
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