net,socket: introduce DECLARE_SOCKADDR helper to catch overflow at build time

proto_ops->getname implies copying protocol specific data
into storage unit (particulary to __kernel_sockaddr_storage).
So when we implement new protocol support we should keep such
a detail in mind (which is easy to forget about).

Lets introduce DECLARE_SOCKADDR helper which check if
storage unit is not overfowed at build time.

Eventually inet_getname is switched to use DECLARE_SOCKADDR
(to show example of usage).
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/net.h

@@ -199,6 +199,9 @@ struct proto_ops {
 				       struct pipe_inode_info *pipe, size_t len, unsigned int flags);
 };
 
+#define DECLARE_SOCKADDR(type, dst, src)	\
+	type dst = ({ __sockaddr_check_size(sizeof(*dst)); (type) src; })
+
 struct net_proto_family {
 	int		family;
 	int		(*create)(struct net *net, struct socket *sock, int protocol);

include/linux/socket.h

@@ -24,6 +24,9 @@ struct __kernel_sockaddr_storage {
 #include <linux/types.h>		/* pid_t			*/
 #include <linux/compiler.h>		/* __user			*/
 
+#define __sockaddr_check_size(size)	\
+	BUILD_BUG_ON(((size) > sizeof(struct __kernel_sockaddr_storage)))
+
 #ifdef __KERNEL__
 # ifdef CONFIG_PROC_FS
 struct seq_file;

net/ipv4/af_inet.c

@@ -685,7 +685,7 @@ int inet_getname(struct socket *sock, struct sockaddr *uaddr,
 {
 	struct sock *sk		= sock->sk;
 	struct inet_sock *inet	= inet_sk(sk);
-	struct sockaddr_in *sin	= (struct sockaddr_in *)uaddr;
+	DECLARE_SOCKADDR(struct sockaddr_in *, sin, uaddr);
 
 	sin->sin_family = AF_INET;
 	if (peer) {