net: set_netns should set a root net ns if ns_id is zero

We need this to save backward compatibility with old images.
It is zero only if it is absent in an image file.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/include/net.h

@@ -47,6 +47,7 @@ extern int move_veth_to_bridge(void);
 
 extern int kerndat_link_nsid(void);
 extern int net_get_nsid(int rtsk, int fd, int *nsid);
+extern struct ns_id *net_get_root_ns();
 extern int kerndat_nsid(void);
 extern void check_has_netns_ioc(int fd, bool *kdat_val, const char *name);
 

criu/net.c

@@ -2664,7 +2664,7 @@ int collect_net_namespaces(bool for_dump)
 
 struct ns_desc net_ns_desc = NS_DESC_ENTRY(CLONE_NEWNET, "net");
 
-static struct ns_id *get_root_netns()
+struct ns_id *net_get_root_ns()
 {
 	static struct ns_id *root_netns = NULL;
 
@@ -2693,7 +2693,7 @@ struct ns_id *get_socket_ns(int lfd)
 	if (ns_fd < 0) {
 		/* backward compatiblity with old kernels */
 		if (netns_nr == 1)
-			return get_root_netns();
+			return net_get_root_ns();
 
 		pr_perror("Unable to get a socket net namespace");
 		return NULL;

criu/sockets.c

@@ -762,7 +762,14 @@ int set_netns(uint32_t ns_id)
 	if (ns_id == last_ns_id)
 		return 0;
 
-	ns = lookup_ns_by_id(ns_id, &net_ns_desc);
+	/*
+	 * The 0 ns_id means that it was not set. We need
+	 * this to be compatible with old images.
+	 */
+	if (ns_id == 0)
+		ns = net_get_root_ns();
+	else
+		ns = lookup_ns_by_id(ns_id, &net_ns_desc);
 	if (ns == NULL) {
 		pr_err("Unable to find a network namespace\n");
 		return -1;

criu/tun.c

@@ -369,7 +369,7 @@ static int tunfile_open(struct file_desc *d, int *new_fd)
 	if (fd < 0)
 		return -1;
 
-	ns_id = ti->tfe->has_ns_id ? ti->tfe->ns_id : top_net_ns->id;
+	ns_id = ti->tfe->ns_id;
 	if (set_netns(ns_id))
 		return -1;