net: tcp_probe: kprobes: adapt jtcp_rcv_established signature

This patches fixes a rather unproblematic function signature mismatch
as the const specifier was missing for the th variable; and next to
that it adds a build-time assertion so that future function signature
mismatches for kprobes will not end badly, similarly as commit 22222997
("net: sctp: add build check for sctp_sf_eat_sack_6_2/jsctp_sf_eat_sack")
did it for SCTP.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_probe.c

@@ -92,7 +92,7 @@ static inline int tcp_probe_avail(void)
  * Note: arguments must match tcp_rcv_established()!
  */
 static int jtcp_rcv_established(struct sock *sk, struct sk_buff *skb,
-			       struct tcphdr *th, unsigned int len)
+				const struct tcphdr *th, unsigned int len)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	const struct inet_sock *inet = inet_sk(sk);
@@ -225,6 +225,13 @@ static __init int tcpprobe_init(void)
 {
 	int ret = -ENOMEM;
 
+	/* Warning: if the function signature of tcp_rcv_established,
+	 * has been changed, you also have to change the signature of
+	 * jtcp_rcv_established, otherwise you end up right here!
+	 */
+	BUILD_BUG_ON(__same_type(tcp_rcv_established,
+				 jtcp_rcv_established) == 0);
+
 	init_waitqueue_head(&tcp_probe.wait);
 	spin_lock_init(&tcp_probe.lock);