rds: fix an integer overflow test in rds_info_getsockopt()

"len" is a signed integer.  We check that len is not negative, so it
goes from zero to INT_MAX.  PAGE_SIZE is unsigned long so the comparison
is type promoted to unsigned long.  ULONG_MAX - 4095 is a higher than
INT_MAX so the condition can never be true.

I don't know if this is harmful but it seems safe to limit "len" to
INT_MAX - 4095.

Fixes: a8c879a7ee98 ('RDS: Info and stats')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/info.c

@@ -176,7 +176,7 @@ int rds_info_getsockopt(struct socket *sock, int optname, char __user *optval,
 
 	/* check for all kinds of wrapping and the like */
 	start = (unsigned long)optval;
-	if (len < 0 || len + PAGE_SIZE - 1 < len || start + len < start) {
+	if (len < 0 || len > INT_MAX - PAGE_SIZE + 1 || start + len < start) {
 		ret = -EINVAL;
 		goto out;
 	}