net: sctp: add build check for sctp_sf_eat_sack_6_2/jsctp_sf_eat_sack

In order to avoid any future surprises of kernel panics due to jprobes
function mismatches (as e.g. fixed in 4cb9d6ea: sctp: jsctp_sf_eat_sack:
fix jprobes function signature mismatch), we should check both function
types during build and scream loudly if they do not match. __same_type
resolves to __builtin_types_compatible_p, which is 1 in case both types
are the same and 0 otherwise, qualifiers are ignored. Tested by myself.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/probe.c

@@ -183,6 +183,13 @@ static __init int sctpprobe_init(void)
 {
 	int ret = -ENOMEM;
 
+	/* Warning: if the function signature of sctp_sf_eat_sack_6_2,
+	 * has been changed, you also have to change the signature of
+	 * jsctp_sf_eat_sack, otherwise you end up right here!
+	 */
+	BUILD_BUG_ON(__same_type(sctp_sf_eat_sack_6_2,
+				 jsctp_sf_eat_sack) == 0);
+
 	init_waitqueue_head(&sctpw.wait);
 	spin_lock_init(&sctpw.lock);
 	if (kfifo_alloc(&sctpw.fifo, bufsize, GFP_KERNEL))