net: add build-time checks for msg->msg_name size

This is a follow-up patch to f3d3342602f8bc ("net: rework recvmsg
handler msg_name and msg_namelen logic").

DECLARE_SOCKADDR validates that the structure we use for writing the
name information to is not larger than the buffer which is reserved
for msg->msg_name (which is 128 bytes). Also use DECLARE_SOCKADDR
consistently in sendmsg code paths.

Signed-off-by: Steffen Hurrle <steffen@hurrle.net>
Suggested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 800ca61..29fc8be 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1450,7 +1450,7 @@
 	struct sock *sk = sock->sk;
 	struct net *net = sock_net(sk);
 	struct unix_sock *u = unix_sk(sk);
-	struct sockaddr_un *sunaddr = msg->msg_name;
+	DECLARE_SOCKADDR(struct sockaddr_un *, sunaddr, msg->msg_name);
 	struct sock *other = NULL;
 	int namelen = 0; /* fake GCC */
 	int err;
@@ -1912,7 +1912,7 @@
 	struct scm_cookie tmp_scm;
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
-	struct sockaddr_un *sunaddr = msg->msg_name;
+	DECLARE_SOCKADDR(struct sockaddr_un *, sunaddr, msg->msg_name);
 	int copied = 0;
 	int check_creds = 0;
 	int target;