init: allow to specify a custom path for rc(8)

This is useful for testing alternative service managers
without modifying /etc/rc

MFC After:	1 weeks
Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D56828

sbin/init/init.8

@@ -28,7 +28,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd July 22, 2021
+.Dd May 5, 2026
 .Dt INIT 8
 .Os
 .Sh NAME
@@ -396,6 +396,14 @@ back to the default, so that the
 .Pa /etc/rc
 script is executed with the standard shell
 .Pa /bin/sh .
+.It Va init_rc
+If set to a valid file name in the root file system,
+overrides the default path to
+.Xr rc 8
+.Pq Pa /etc/rc .
+This is useful for testing alternative service managers
+without modifying the default
+.Pa /etc/rc .
 .It Va init_path
 Specifies the path used to search for the init binary.
 More than one binary can be listed in the string,

sbin/init/init.c

@@ -1025,11 +1025,23 @@ static state_func_t
 runcom(void)
 {
 	state_func_t next_transition;
+	char runcom_path[PATH_MAX];
+	const char *rc_script;
 
-	BOOTTRACE("/etc/rc starting...");
-	if ((next_transition = run_script(_PATH_RUNCOM)) != NULL)
+	/*
+	 * Allow overriding /etc/rc via the init_rc kenv variable.
+	 * This is useful for testing alternative service managers
+	 * without modifying /etc/rc.
+	 */
+	if (kenv(KENV_GET, "init_rc", runcom_path, sizeof(runcom_path)) > 0)
+		rc_script = runcom_path;
+	else
+		rc_script = _PATH_RUNCOM;
+
+	BOOTTRACE("%s starting...", rc_script);
+	if ((next_transition = run_script(rc_script)) != NULL)
 		return next_transition;
-	BOOTTRACE("/etc/rc finished");
+	BOOTTRACE("%s finished", rc_script);
 
 	runcom_mode = AUTOBOOT;		/* the default */
 	return (state_func_t) read_ttys;