halt: add a -k kexec flag

--- a/man/halt.8
+++ b/man/halt.8
@@ -40,6 +40,7 @@ halt, reboot, poweroff \- stop the system.
 .RB [ \-d ]
 .RB [ \-f ]
 .RB [ \-i ]
+.RB [ \-k ]
 .br
 .B /sbin/poweroff
 .RB [ \-n ]
@@ -83,6 +84,8 @@ Put all hard drives on the system in stand-by mode just before halt or power-off
 .IP \fB\-p\fP
 When halting the system, switch off the power. This is the default when halt is
 called as \fBpoweroff\fP.
+.IP \fB\-k\fP
+Try to reboot using \fBkexec\fP, if kernel supports it.
 .\"}}}
 .\"{{{  Diagnostics
 .SH DIAGNOSTICS
--- a/src/halt.c
+++ b/src/halt.c
@@ -8,7 +8,7 @@
  *		execute an "shutdown -r". This is for compatibility with
  *		sysvinit 2.4.
  *
- * Usage:	halt [-n] [-w] [-d] [-f] [-h] [-i] [-p]
+ * Usage:	halt [-n] [-w] [-d] [-f] [-h] [-i] [-p] [-k]
  *		-n: don't sync before halting the system
  *		-w: only write a wtmp reboot record and exit.
  *		-d: don't write a wtmp record.
@@ -16,6 +16,7 @@
  *		-h: put harddisks in standby mode
  *		-i: shut down all network interfaces.
  *		-p: power down the system (if possible, otherwise halt).
+ *		-k: reboot the system using kexec.
  *
  *		Reboot and halt are both this program. Reboot
  *		is just a link to halt. Invoking the program
@@ -74,8 +75,10 @@ extern void write_wtmp(char *user, char *id, int pid, int type, char *line);
  */
 void usage(void)
 {
-	fprintf(stderr, "usage: %s [-n] [-w] [-d] [-f] [-h] [-i]%s\n",
-		progname, strcmp(progname, "halt") ? "" : " [-p]");
+	fprintf(stderr, "usage: %s [-n] [-w] [-d] [-f] [-h] [-i]%s%s\n",
+		progname,
+		strcmp(progname, "halt") ? "" : " [-p]",
+		strcmp(progname, "reboot") ? "" : " [-k]");
 	fprintf(stderr, "\t-n: don't sync before halting the system\n");
 	fprintf(stderr, "\t-w: only write a wtmp reboot record and exit.\n");
 	fprintf(stderr, "\t-d: don't write a wtmp record.\n");
@@ -84,6 +87,8 @@ void usage(void)
 	fprintf(stderr, "\t-i: shut down all network interfaces.\n");
 	if (!strcmp(progname, "halt"))
 		fprintf(stderr, "\t-p: power down the system (if possible, otherwise halt).\n");
+	if (!strcmp(progname, "reboot"))
+		fprintf(stderr, "\t-k: reboot the system using kexec.\n");
 	exit(1);
 }
 
@@ -182,6 +187,7 @@ int main(int argc, char **argv)
 	int do_ifdown = 0;
 	int do_hddown = 0;
 	int do_poweroff = 0;
+	int do_kexec = 0;
 	int c;
 	char *tm = NULL;
 
@@ -201,7 +207,7 @@ int main(int argc, char **argv)
 	/*
 	 *	Get flags
 	 */
-	while((c = getopt(argc, argv, ":ihdfnpwt:")) != EOF) {
+	while((c = getopt(argc, argv, ":ihdfnpwkt:")) != EOF) {
 		switch(c) {
 			case 'n':
 				do_sync = 0;
@@ -225,6 +231,9 @@ int main(int argc, char **argv)
 			case 'p':
 				do_poweroff = 1;
 				break;
+			case 'k':
+				do_kexec = 1;
+				break;
 			case 't':
 				tm = optarg;
 				break;
@@ -242,10 +251,24 @@ int main(int argc, char **argv)
 	(void)chdir("/");
 
 	if (!do_hard && !do_nothing) {
+		c = get_runlevel();
+
+		/*
+		 *	 We can't reboot using kexec through this path.
+		 */
+		if (c != '6' && do_reboot && do_kexec) {
+			fprintf(stderr, "ERROR: using -k at this"
+				" runlevel requires also -f\n"
+				"  (You probably want instead to reboot"
+				" normally and let your reboot\n"
+				"   script, usually /etc/init.d/reboot,"
+				" specify -k)\n");
+			exit(1);
+		}
+
 		/*
 		 *	See if we are in runlevel 0 or 6.
 		 */
-		c = get_runlevel();
 		if (c != '0' && c != '6')
 			do_shutdown(do_reboot ? "-r" : "-h", tm);
 	}
@@ -277,6 +300,15 @@ int main(int argc, char **argv)
 	if (do_nothing) exit(0);
 
 	if (do_reboot) {
+		/*
+		 *	kexec or reboot
+		 */
+		if (do_kexec)
+			init_reboot(BMAGIC_KEXEC);
+
+		/*
+		 *	Fall through if failed
+		 */
 		init_reboot(BMAGIC_REBOOT);
 	} else {
 		/*
--- a/src/reboot.h
+++ b/src/reboot.h
@@ -47,5 +47,8 @@
 #  define BMAGIC_POWEROFF	BMAGIC_HALT
 #endif
 
+/* for kexec support */
+#define BMAGIC_KEXEC  0x45584543
+
 #define init_reboot(magic)	reboot(magic)