net: Dump tun device net id in img

This adds new tunfile_entry::ns_id field and populates
it in standard socket way. Restore uses this ns_id
to choose correct namespace. Note, we could completelly
skip set_netns() on restore in case of !has_ns_id, but
using top_net_ns invents some definite behaviour.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
ktkhai: comment written/code movings
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>

criu/tun.c

@@ -20,6 +20,7 @@
 #include "namespaces.h"
 #include "xmalloc.h"
 #include "kerndat.h"
+#include "sockets.h"
 
 #include "images/tun.pb-c.h"
 
@@ -304,6 +305,8 @@ static int dump_tunfile(int lfd, u32 id, const struct fd_parms *p)
 			pr_err("No net_ns for tun device\n");
 			return -1;
 		}
+		tfe.has_ns_id = true;
+		tfe.ns_id = ns->id;
 	}
 
 	if (dump_one_reg_file(lfd, id, p))
@@ -356,7 +359,7 @@ struct tunfile_info {
 
 static int tunfile_open(struct file_desc *d, int *new_fd)
 {
-	int fd;
+	int fd, ns_id;
 	struct tunfile_info *ti;
 	struct ifreq ifr;
 	struct tun_link *tl;
@@ -366,6 +369,10 @@ 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;
+	if (set_netns(ns_id))
+		return -1;
+
 	if (!ti->tfe->netdev)
 		/* just-opened tun file */
 		goto ok;;

images/tun.proto

@@ -6,6 +6,7 @@ message tunfile_entry {
 	required uint32		id		= 1;
 	optional string		netdev		= 2;
 	optional bool		detached	= 3;
+	optional uint32		ns_id		= 4;
 };
 
 message tun_link_entry {