time_namespaces.7: New page documenting time namespaces

Reviewed-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man7/time_namespaces.7 b/man7/time_namespaces.7
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+.\" Copyright (c) 2020 by Michael Kerrisk <mtk.manpages@gmail.com>
+.\"
+.\" %%%LICENSE_START(VERBATIM)
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+.\" professionally.
+.\"
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+.\" %%%LICENSE_END
+.\"
+.\"
+.TH TIME_NAMESPACES 7 2020-04-01 "Linux" "Linux Programmer's Manual"
+.SH NAME
+time_namespaces \- overview of Linux time namespaces
+.SH DESCRIPTION
+Time namespaces virtualize the values of two system clocks:
+.IP \(bu 2
+.BR CLOCK_MONOTONIC
+(and likewise
+.BR CLOCK_MONOTONIC_COARSE
+and
+.BR CLOCK_MONOTONIC_RAW ),
+a nonsettable clock that represents monotonic time  since\(emas
+described  by  POSIX\(em"some  unspecified  point in the past".
+.IP \(bu
+.BR CLOCK_BOOTTIME 
+(and likewise
+.BR CLOCK_BOOTTIME_ALARM ),
+a clock that is identical to
+.BR CLOCK_MONOTONIC ,
+except that it also includes any time that the system is suspended.
+.PP
+Thus, the processes in a time namespace share per-namespace values
+for these clocks.
+This affects various APIs that measure against these clocks, including:
+.BR clock_nanosleep (2),
+.BR nanosleep (2),
+.BR clock_gettime (2),
+and
+.IR /proc/uptime .
+.PP
+Currently, the only way to create a time namespace is by calling
+.BR unshare (2)
+with the
+.BR CLONE_NEWTIME
+flag.
+This call creates a new time namespace but does
+.I not
+place the calling process in the new namespace.
+Instead, the calling process's
+subsequently created children are placed in the new namespace.
+This allows clock offsets (see below) for the new namespace
+to be set before the first process is placed in the namespace.
+The
+.IR /proc/[pid]/ns/time_for_children
+symbolic link shows the time namespace in which
+the children of a process will be created.
+.\"
+.SS /proc/PID/timens_offsets
+Associated with each time namespace are offsets,
+expressed with respect to the initial time namespace,
+that define the values of the monotonic and boot clocks in that namespace.
+These offsets are exposed via the file
+.IR /proc/PID/timens_offsets .
+Within this file,
+the offsets are expressed as lines consisting of
+three space-delimited fields:
+.PP
+.in +4n
+.EX
+<clock-id> <offset-secs> <offset-nanosecs>
+.EE
+.in
+.PP
+The
+.I clock-id
+identifies the clock whose offsets are being shown.
+This field is either 1, for
+.BR CLOCK_MONOTONIC ,
+or 7, for
+.BR CLOCK_BOOTTIME .
+The remaining fields express the offset (seconds plus nanoseconds) for the
+clock in this time namespace.
+These offsets are expressed relative to the clock values in
+the initial time namespace.
+In the initial time namespace, the contents of this file are as follows:
+.PP
+.in +4n
+.EX
+$ \fBcat /proc/self/timens_offsets\fP
+1 0 0
+7 0 0
+.EE
+.in
+.PP
+In a new time namespace that has had no member processes,
+the clock offsets can be modified by writing newline-terminated
+records of the same form to the file.
+The file can be written to multiple times,
+but after the first process has been created in or has entered the namespace,
+.BR write (2)s
+on this file fail with the error
+.BR EACCES .
+In order to write to the
+.IR timens_offsets
+file, a process must have the
+.BR CAP_SYS_TIME
+capability in the user namespace that owns the time namespace.
+.PP
+In a new time namespace created by
+.BR unshare (2),
+the contents of the
+.I timens_offsets
+file are inherited from the time namespace of the creating process.
+.SH NOTES
+.PP
+Use of time namespaces requires a kernel that is configured with the
+.B CONFIG_TIME_NS
+option.
+.PP
+Note that time namespaces do not virtualize the
+.BR CLOCK_REALTIME
+clock.
+Virtualization of this clock was avoided for reasons of complexity
+and overhead within the kernel.
+.PP
+The motivation for adding time namespaces was to allow
+the monotonic and boot-time clocks to maintain consistent values
+during container migration and checkpoint/restore.
+.SH EXAMPLE
+.PP
+The following shell session demonstrates the operation of time namespaces.
+We begin by displaying the inode number of the time namespace
+of a shell in the initial time namespace:
+.PP
+.in +4n
+.EX
+$ \fBreadlink /proc/$$/ns/time\fP
+time:[4026531834]
+.EE
+.in
+.PP
+Continuing in the initial time namespace, we display the system uptime using
+.BR uptime (1)
+and use the
+.I clock_times
+example program shown in
+.BR clock_getres (2)
+to display the values of various clocks:
+.PP
+.in +4n
+.EX
+$ \fBuptime \-\-pretty\fP
+up 21 hours, 17 minutes
+$ \fB./clock_times\fP
+CLOCK_REALTIME : 1585989401.971 (18356 days +  8h 36m 41s)
+CLOCK_TAI      : 1585989438.972 (18356 days +  8h 37m 18s)
+CLOCK_MONOTONIC:      56338.247 (15h 38m 58s)
+CLOCK_BOOTTIME :      76633.544 (21h 17m 13s)
+.EE
+.in
+.PP
+We then use
+.BR unshare (1)
+to create a time namespace and execute a
+.BR bash (1)
+shell.
+From the new shell, we use the built-in
+.B echo
+command to write records to the
+.I timens_offsets
+file adjusting the offset for the
+.B CLOCK_MONOTONIC
+clock forward 2 days
+and the offset for the
+.B CLOCK_BOOTTIME
+clock forward 7 days:
+.PP
+.in +4n
+.EX
+$ \fBPS1="ns2# " sudo unshare \-T \-\- bash \-\-norc\fP
+ns2# \fBecho "1 $((2*24*60*60)) 0" > /proc/$$/timens_offsets\fP
+ns2# \fBecho "7 $((7*24*60*60)) 0" > /proc/$$/timens_offsets\fP
+.EE
+.in
+.PP
+Above, we started the
+.BR bash (1)
+shell with the
+.B \-\-norc
+options so that no start-up scripts were executed.
+This ensures that no child processes are created from the
+shell before we have a chance to update the
+.I timens_offsets
+file.
+.PP
+We then use
+.BR cat (1)
+to display the contents of the
+.I timens_offsets
+file.
+The execution of
+.BR cat (1)
+creates the first process in the new time namespace,
+after which further attempts to update the
+.I timens_offsets
+file produce an error.
+.PP
+.in +4n
+.EX
+ns2# \fBcat /proc/$$/timens_offsets\fP
+1 172800 0
+7 604800 0
+ns2# \fBecho "7 $((9*24*60*60)) 0" > /proc/$$/timens_offsets\fP
+bash: echo: write error: Permission denied
+.EE
+.in
+.PP
+Continuing in the new namespace, we execute
+.BR uptime (1)
+and the
+.I clock_times
+example program:
+.PP
+.in +4n
+.EX
+ns2# \fBuptime \-\-pretty\fP
+up 1 week, 21 hours, 18 minutes
+ns2# \fB./clock_times\fP
+CLOCK_REALTIME : 1585989457.056 (18356 days +  8h 37m 37s)
+CLOCK_TAI      : 1585989494.057 (18356 days +  8h 38m 14s)
+CLOCK_MONOTONIC:     229193.332 (2 days + 15h 39m 53s)
+CLOCK_BOOTTIME :     681488.629 (7 days + 21h 18m  8s)
+.EE
+.in
+.PP
+From the above output, we can see that the monotonic
+and boot-time clocks have new values in the new time namespace.
+.PP
+Examining the
+.I /proc/[pid]/ns/time
+and
+.I /proc/[pid]/ns/time_for_children
+symbolic links, we see that the shell is a member of the initial time
+namespace, but its children are created in a different namespace.
+
+.PP
+.in +4n
+.EX
+ns2# \fBreadlink /proc/$$/ns/time\fP
+time:[4026531834]
+ns2# \fBreadlink /proc/$$/ns/time_for_children\fP
+time:[4026532900]
+ns2# \fBreadlink /proc/self/ns/time\fP   # Creates a child process
+time:[4026532900]
+.EE
+.in
+.PP
+Returning to the shell in the initial time namespace,
+we see that the monotonic and boot-time clocks
+are unaffected by the
+.I timens_offsets
+changes that were made in the other time namespace:
+.PP
+.in +4n
+.EX
+$ \fBuptime \-\-pretty\fP
+up 21 hours, 19 minutes
+$ \fB./clock_times\fP
+CLOCK_REALTIME : 1585989401.971 (18356 days +  8h 38m 51s)
+CLOCK_TAI      : 1585989438.972 (18356 days +  8h 39m 28s)
+CLOCK_MONOTONIC:      56338.247 (15h 41m  8s)
+CLOCK_BOOTTIME :      76633.544 (21h 19m 23s)
+.EE
+.in
+.SH SEE ALSO
+.BR nsenter (1),
+.BR unshare (1),
+.BR clock_settime (2).
+.\" clone3() support for time namespaces is a work in progress
+.\" .BR clone3 (2),
+.BR setns (2),
+.BR unshare (2),
+.BR namespaces (7),
+.BR time (7)