MFC r290548:

Userspace part of reroot support. This makes it possible to change
the root filesystem without full reboot, using "reboot -r". This can
be used to to eg. boot from a temporary md_image preloaded by loader(8),
setup an iSCSI session, and continue booting from rootfs mounted over
iSCSI.

Relnotes:	yes
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D3693

2 files changed, 35 insertions, 3 deletions

diff --git a/sbin/reboot/reboot.8 b/sbin/reboot/reboot.8
index 84dd887..420d4f8 100644
--- a/sbin/reboot/reboot.8
+++ b/sbin/reboot/reboot.8
@@ -28,7 +28,7 @@
 .\"	@(#)reboot.8	8.1 (Berkeley) 6/9/93
 .\" $FreeBSD$
 .\"
-.Dd October 11, 2010
+.Dd May 22, 2015
 .Dd Jan 06, 2016
 .Dt REBOOT 8
 .Os
@@ -43,7 +43,7 @@
 .Op Fl lNnpq
 .Op Fl k Ar kernel
 .Nm
-.Op Fl dlNnpq
+.Op Fl dlNnpqr
 .Op Fl k Ar kernel
 .Nm fasthalt
 .Op Fl lNnpq
@@ -122,6 +122,13 @@ the flushing of the file system cache is performed (if the
 .Fl n
 option is not specified).
 This option should probably not be used.
+.It Fl r
+The system kills all processes, unmounts all filesystems, mounts the new
+root filesystem, and begins the usual startup sequence.
+After changing vfs.root.mountfrom with
+.Xr kenv 8 ,
+.Nm Fl r
+can be used to change the root filesystem while preserving kernel state.
 .El
 .Pp
 The
@@ -139,6 +146,13 @@ Normally, the
 utility is used when the system needs to be halted or restarted, giving
 users advance warning of their impending doom and cleanly terminating
 specific programs.
+.Sh EXAMPLES
+Replace current root filesystem with UFS mounted from
+.Pa /dev/ada0s1a :
+.Bd -literal -offset indent
+kenv vfs.root.mountfrom=ufs:/dev/ada0s1a
+reboot -r
+.Ed
 .Sh SEE ALSO
 .Xr getutxent 3 ,
 .Xr boot 8 ,
diff --git a/sbin/reboot/reboot.c b/sbin/reboot/reboot.c
index 6d590ac..961ba41 100644
--- a/sbin/reboot/reboot.c
+++ b/sbin/reboot/reboot.c
@@ -77,7 +77,7 @@ main(int argc, char *argv[])
 	} else
 		howto = 0;
 	lflag = nflag = qflag = Nflag = 0;
-	while ((ch = getopt(argc, argv, "dk:lNnpq")) != -1)
+	while ((ch = getopt(argc, argv, "dk:lNnpqr")) != -1)
 		switch(ch) {
 		case 'd':
 			howto |= RB_DUMP;
@@ -102,6 +102,9 @@ main(int argc, char *argv[])
 		case 'q':
 			qflag = 1;
 			break;
+		case 'r':
+			howto |= RB_REROOT;
+			break;
 		case '?':
 		default:
 			usage();
@@ -113,6 +116,8 @@ main(int argc, char *argv[])
 		errx(1, "cannot dump (-d) when halting; must reboot instead");
 	if (Nflag && (howto & RB_NOSYNC) != 0)
 		errx(1, "-N cannot be used with -n");
+	if ((howto & RB_REROOT) != 0 && howto != RB_REROOT)
+		errx(1, "-r cannot be used with -d, -n, or -p");
 	if (geteuid()) {
 		errno = EPERM;
 		err(1, NULL);
@@ -143,6 +148,9 @@ main(int argc, char *argv[])
 		if (dohalt) {
 			openlog("halt", 0, LOG_AUTH | LOG_CONS);
 			syslog(LOG_CRIT, "halted by %s", user);
+		} else if (howto & RB_REROOT) {
+			openlog("reroot", 0, LOG_AUTH | LOG_CONS);
+			syslog(LOG_CRIT, "rerooted by %s", user);
 		} else {
 			openlog("reboot", 0, LOG_AUTH | LOG_CONS);
 			syslog(LOG_CRIT, "rebooted by %s", user);
@@ -176,6 +184,16 @@ main(int argc, char *argv[])
 	 */
 	(void)signal(SIGPIPE, SIG_IGN);
 
+	/*
+	 * Only init(8) can perform rerooting.
+	 */
+	if (howto & RB_REROOT) {
+		if (kill(1, SIGEMT) == -1)
+			err(1, "SIGEMT init");
+
+		return (0);
+	}
+
 	/* Just stop init -- if we fail, we'll restart it. */
 	if (kill(1, SIGTSTP) == -1)
 		err(1, "SIGTSTP init");