net: Assert at build time the assumptions we make about the CMSG header.

It must always be the case that CMSG_ALIGN(sizeof(hdr)) == sizeof(hdr).

Otherwise there are missing adjustments in the various calculations
that parse and build these things.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/compat.c

@@ -130,6 +130,9 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg, struct sock *sk,
 	__kernel_size_t kcmlen, tmp;
 	int err = -EFAULT;
 
+	BUILD_BUG_ON(sizeof(struct compat_cmsghdr) !=
+		     CMSG_COMPAT_ALIGN(sizeof(struct compat_cmsghdr)));
+
 	kcmlen = 0;
 	kcmsg_base = kcmsg = (struct cmsghdr *)stackbuf;
 	ucmsg = CMSG_COMPAT_FIRSTHDR(kmsg);

net/socket.c

@@ -1948,6 +1948,8 @@ static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
 		ctl_buf = msg_sys->msg_control;
 		ctl_len = msg_sys->msg_controllen;
 	} else if (ctl_len) {
+		BUILD_BUG_ON(sizeof(struct cmsghdr) !=
+			     CMSG_ALIGN(sizeof(struct cmsghdr)));
 		if (ctl_len > sizeof(ctl)) {
 			ctl_buf = sock_kmalloc(sock->sk, ctl_len, GFP_KERNEL);
 			if (ctl_buf == NULL)