Implement switch_root functionality allowing Finit to serve as the init

in an initramfs, then transition to the real root filesystem.  Useful
for systems requiring early boot tasks like LUKS unlock, LVM activation,
or network boot before mounting the real root.

Adds INIT_CMD_SWITCH_ROOT API command, `initctl switch-root` subcommand,
and HOOK_SWITCH_ROOT plugin hook point.  The implementation gracefully
stops services, moves virtual filesystems (/dev, /proc, /sys, /run) to
the new root, deletes initramfs contents to free memory, then execs the
new init as PID 1.

See GitHub Discussion #292 for background.

src/initctl.c

@@ -646,6 +646,59 @@ int do_halt    (char *arg) { return do_cmd(INIT_CMD_HALT);     }
 int do_poweroff(char *arg) { return do_cmd(INIT_CMD_POWEROFF); }
 int do_suspend (char *arg) { return do_cmd(INIT_CMD_SUSPEND);  }
 
+/**
+ * do_switch_root - Switch to a new root filesystem (initramfs only)
+ * @argc: Number of arguments (1 or 2)
+ * @argv: [0] = newroot path, [1] = optional init path
+ *
+ * This command is only valid during runlevel S (bootstrap) or 1 when
+ * running from an initramfs.  It stops all services, moves virtual
+ * filesystems, and exec's the new init.
+ */
+int do_switch_root(int argc, char *argv[])
+{
+	struct init_request rq = {
+		.magic = INIT_MAGIC,
+		.cmd   = INIT_CMD_SWITCH_ROOT,
+	};
+	char *newroot, *newinit = NULL;
+	size_t off;
+
+	if (argc < 1)
+		ERRX(2, "Usage: initctl switch-root NEWROOT [INIT]");
+
+	newroot = argv[0];
+	if (argc > 1)
+		newinit = argv[1];
+
+	/* Verify newroot exists */
+	if (access(newroot, F_OK))
+		ERR(1, "Cannot access %s", newroot);
+
+	/*
+	 * Pack both strings into rq.data with null separators:
+	 * "newroot\0newinit\0"
+	 */
+	strlcpy(rq.data, newroot, sizeof(rq.data));
+	off = strlen(newroot) + 1;
+
+	if (newinit && off < sizeof(rq.data) - 1)
+		strlcpy(rq.data + off, newinit, sizeof(rq.data) - off);
+
+	printf("Switching root to %s", newroot);
+	if (newinit)
+		printf(", init %s", newinit);
+	printf(" ...\n");
+
+	/*
+	 * On success, finit exec's new init and we lose connection.
+	 * A "failure" to read reply is actually expected on success.
+	 */
+	client_send(&rq, sizeof(rq));
+
+	return 0;
+}
+
 int utmp_show(char *file)
 {
 	struct utmp *ut;
@@ -1474,7 +1527,8 @@ static int usage(int rc)
 		"  reboot                    Reboot system\n"
 		"  halt                      Halt system\n"
 		"  poweroff                  Halt and power off system\n"
-		"  suspend                   Suspend system\n");
+		"  suspend                   Suspend system\n"
+		"  switch-root ROOT [INIT]   Switch to new root filesystem (initramfs)\n");
 
 	if (utmp)
 		fprintf(stderr,
@@ -1654,6 +1708,7 @@ int main(int argc, char *argv[])
 		{ "halt",     NULL, do_halt,      NULL, NULL  },
 		{ "poweroff", NULL, do_poweroff,  NULL, NULL  },
 		{ "suspend",  NULL, do_suspend,   NULL, NULL  },
+		{ "switch-root", NULL, NULL,    NULL, do_switch_root },
 
 		{ "utmp",     NULL, do_utmp,     &utmp, NULL  },
 		{ NULL, NULL, NULL, NULL, NULL  }