xdp_ddos01_blacklist_cmdline: Fix wrong usage of bpf_map_get_next_key

If inserting key 0 into hash, this code will cause issues listing IPs.

First change the variable name to be prev_key and key like:
  bpf_map_get_next_key(fd, prev_key, &key)

The prev_key need to be a real pointer, as we need to start with
prev_key = NULL.  Before prev_key was (a value on stack) implicit
zero. Thus, we basically asked to start the listing with key 0.
Thus, it is why it *looked* like it was all working when the hash
doesn't contain key=0, as then it fails-to-start-at-0 and kernel
provides the first-key.

Signed-off-by: Jesper Dangaard Brouer <[email protected]>

Author: netoptimizer

kernel/samples/bpf/xdp_ddos01_blacklist_cmdline.c

@@ -234,14 +234,14 @@ static void blacklist_print_port(int key, __u32 val, int countfds[])
 
 static void blacklist_list_all_ipv4(int fd)
 {
-	__u32 key = 0, next_key;
+	__u32 key, *prev_key = NULL;
 	__u64 value;
 
-	while (bpf_map_get_next_key(fd, &key, &next_key) == 0) {
+	while (bpf_map_get_next_key(fd, prev_key, &key) == 0) {
 		printf("%s", key ? "," : "" );
-		key = next_key;
 		value = get_key32_value64_percpu(fd, key);
 		blacklist_print_ipv4(key, value);
+		prev_key = &key;
 	}
 	printf("%s", key ? "," : "");
 }