restore: reserve the service descriptor for a control tty

I'm really lucky.

prepare_ctl_tty:
	e->fd           = get_service_fd(CTL_TTY_OFF); // -1 as fd is not set

send_fd_to_self:
if (dup2(fd, dfd) != dfd) { // tries to install sfd, but -1 == -1

tty_restore_ctl_terminal:
	if (!is_service_fd(fd, CTL_TTY_OFF))
		return fd == get_service_fd(type); // -1 = -1
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files.c

@@ -405,7 +405,7 @@ int prepare_ctl_tty(int pid, struct rst_info *rst_info, u32 ctl_tty_id)
 	fdinfo_entry__init(e);
 
 	e->id		= ctl_tty_id;
-	e->fd		= get_service_fd(CTL_TTY_OFF);
+	e->fd		= reserve_service_fd(CTL_TTY_OFF);
 	e->type		= FD_TYPES__TTY;
 
 	if (collect_fd(pid, e, rst_info)) {

include/crtools.h

@@ -132,6 +132,7 @@ enum sfd_type {
 extern int clone_service_fd(int id);
 extern int init_service_fd(void);
 extern int get_service_fd(enum sfd_type type);
+extern int reserve_service_fd(enum sfd_type type);
 extern int install_service_fd(enum sfd_type type, int fd);
 extern int close_service_fd(enum sfd_type type);
 extern bool is_service_fd(int fd, enum sfd_type type);

util.c

@@ -296,6 +296,16 @@ static int __get_service_fd(enum sfd_type type, int service_fd_id)
 
 static DECLARE_BITMAP(sfd_map, SERVICE_FD_MAX);
 
+int reserve_service_fd(enum sfd_type type)
+{
+	int sfd = __get_service_fd(type, service_fd_id);
+
+	BUG_ON((int)type <= SERVICE_FD_MIN || (int)type >= SERVICE_FD_MAX);
+
+	set_bit(type, sfd_map);
+	return sfd;
+}
+
 int install_service_fd(enum sfd_type type, int fd)
 {
 	int sfd = __get_service_fd(type, service_fd_id);