net: make get_net_ns return error if NET_NS is disabled

There is a panic in socket ioctl cmd SIOCGSKNS when NET_NS is not enabled.
The reason is that nsfs tries to access ns->ops but the proc_ns_operations
is not implemented in this case.

[7.670023] Unable to handle kernel NULL pointer dereference at virtual address 00000010
[7.670268] pgd = 32b54000
[7.670544] [00000010] *pgd=00000000
[7.671861] Internal error: Oops: 5 [#1] SMP ARM
[7.672315] Modules linked in:
[7.672918] CPU: 0 PID: 1 Comm: systemd Not tainted 5.13.0-rc3-00375-g6799d4f2da49 #16
[7.673309] Hardware name: Generic DT based system
[7.673642] PC is at nsfs_evict+0x24/0x30
[7.674486] LR is at clear_inode+0x20/0x9c

The same to tun SIOCGSKNS command.

To fix this problem, we make get_net_ns() return -EINVAL when NET_NS is
disabled. Meanwhile move it to right place net/core/net_namespace.c.

Signed-off-by: Changbin Du <[e-mail]>
Fixes: c62cce2caee5 ("net: add an ioctl to get a socket network namespace")
Cc: Cong Wang <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: David Laight <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Suggested-by: Jakub Kicinski <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/socket.h

···
 			    int __user *usockvec);
 extern int __sys_shutdown_sock(struct socket *sock, int how);
 extern int __sys_shutdown(int fd, int how);
-
-extern struct ns_common *get_net_ns(struct ns_common *ns);
 #endif /* _LINUX_SOCKET_H */

include/net/net_namespace.h

···
 void net_ns_get_ownership(const struct net *net, kuid_t *uid, kgid_t *gid);
 
 void net_ns_barrier(void);
+
+struct ns_common *get_net_ns(struct ns_common *ns);
 #else /* CONFIG_NET_NS */
 #include <linux/sched.h>
 #include <linux/nsproxy.h>
···
 }
 
 static inline void net_ns_barrier(void) {}
+
+static inline struct ns_common *get_net_ns(struct ns_common *ns)
+{
+	return ERR_PTR(-EINVAL);
+}
 #endif /* CONFIG_NET_NS */
 
 

net/core/net_namespace.c

···
 }
 EXPORT_SYMBOL_GPL(__put_net);
 
+/**
+ * get_net_ns - increment the refcount of the network namespace
+ * @ns: common namespace (net)
+ *
+ * Returns the net's common namespace.
+ */
+struct ns_common *get_net_ns(struct ns_common *ns)
+{
+	return &get_net(container_of(ns, struct net, ns))->ns;
+}
+EXPORT_SYMBOL_GPL(get_net_ns);
+
 struct net *get_net_ns_by_fd(int fd)
 {
 	struct file *file;

net/socket.c

···
  *	what to do with it - that's up to the protocol still.
  */
 
-/**
- *	get_net_ns - increment the refcount of the network namespace
- *	@ns: common namespace (net)
- *
- *	Returns the net's common namespace.
- */
-
-struct ns_common *get_net_ns(struct ns_common *ns)
-{
-	return &get_net(container_of(ns, struct net, ns))->ns;
-}
-EXPORT_SYMBOL_GPL(get_net_ns);
-
 static long sock_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 {
 	struct socket *sock;