Move description of init_shell, init_script, and init_chroot kenv

tunables from loader(8) to init(8), since it's init that actually
uses them.  Add .Xrs at their old place.

MFC after:	2 weeks
Sponsored by:	DARPA, AFRL

1 files changed, 69 insertions, 1 deletions

diff --git a/sbin/init/init.8 b/sbin/init/init.8
index cefbf499d98a..89a94a38b448 100644
--- a/sbin/init/init.8
+++ b/sbin/init/init.8
@@ -31,7 +31,7 @@
 .\"     @(#)init.8	8.3 (Berkeley) 4/18/94
 .\" $FreeBSD$
 .\"
-.Dd October 3, 2016
+.Dd August 7, 2018
 .Dt INIT 8
 .Os
 .Sh NAME
@@ -294,6 +294,74 @@ as follows:
 .Xr ttys 5
 file
 .El
+.Sh KERNEL ENVIRONMENT VARIABLES
+The following
+.Xr kenv 2
+variables are available as
+.Xr loader 8
+tunables:
+.Bl -tag -width indent
+.It Va init_chroot
+If set to a valid directory in the root file system, it causes
+.Nm
+to perform a
+.Xr chroot 2
+operation on that directory, making it the new root directory.
+That happens before entering single-user mode or multi-user
+mode (but after executing the
+.Va init_script
+if enabled).
+This functionality has generally been eclipsed by rerooting.
+See
+.Xr reboot 8
+.Fl r
+for details.
+.It Va init_script
+If set to a valid file name in the root file system,
+instructs
+.Nm
+to run that script as the very first action,
+before doing anything else.
+Signal handling and exit code interpretation is similar to
+running the
+.Pa /etc/rc
+script.
+In particular, single-user operation is enforced
+if the script terminates with a non-zero exit code,
+or if a SIGTERM is delivered to the
+.Nm
+process (PID 1).
+This functionality has generally been eclipsed by rerooting.
+See
+.Xr reboot 8
+.Fl r
+for details.
+.It Va init_shell
+Defines the shell binary to be used for executing the various shell scripts.
+The default is
+.Dq Li /bin/sh .
+It is used for running the
+.Va init_script
+if set, as well as for the
+.Pa /etc/rc
+and
+.Pa /etc/rc.shutdown
+scripts.
+The value of the corresponding
+.Xr kenv 2
+variable is evaluated every time
+.Nm
+calls a shell script, so it can be changed later on using the
+.Xr kenv 1
+utility.
+In particular, if a non-default shell is used for running an
+.Va init_script ,
+it might be desirable to have that script reset the value of
+.Va init_shell
+back to the default, so that the
+.Pa /etc/rc
+script is executed with the standard shell
+.Pa /bin/sh .
 .Sh FILES
 .Bl -tag -width /var/log/init.log -compact
 .It Pa /dev/console