bpf: Handle BPF_EXIST and BPF_NOEXIST for LPM trie

Add the currently missing handling for the BPF_EXIST and BPF_NOEXIST
flags. These flags can be specified by users and are relevant since LPM
trie supports exact matches during update.

Fixes: b95a5c4 ("bpf: add a longest prefix match trie map implementation")
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20241206110622.1161752-4-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Hou Tao

kernel/bpf/lpm_trie.c

@@ -375,6 +375,10 @@ static long trie_update_elem(struct bpf_map *map,
 	 * simply assign the @new_node to that slot and be done.
 	 */
 	if (!node) {
+		if (flags == BPF_EXIST) {
+			ret = -ENOENT;
+			goto out;
+		}
 		rcu_assign_pointer(*slot, new_node);
 		goto out;
 	}
@@ -383,18 +387,31 @@ static long trie_update_elem(struct bpf_map *map,
 	 * which already has the correct data array set.
 	 */
 	if (node->prefixlen == matchlen) {
+		if (!(node->flags & LPM_TREE_NODE_FLAG_IM)) {
+			if (flags == BPF_NOEXIST) {
+				ret = -EEXIST;
+				goto out;
+			}
+			trie->n_entries--;
+		} else if (flags == BPF_EXIST) {
+			ret = -ENOENT;
+			goto out;
+		}
+
 		new_node->child[0] = node->child[0];
 		new_node->child[1] = node->child[1];
 
-		if (!(node->flags & LPM_TREE_NODE_FLAG_IM))
-			trie->n_entries--;
-
 		rcu_assign_pointer(*slot, new_node);
 		free_node = node;
 
 		goto out;
 	}
 
+	if (flags == BPF_EXIST) {
+		ret = -ENOENT;
+		goto out;
+	}
+
 	/* If the new node matches the prefix completely, it must be inserted
 	 * as an ancestor. Simply insert it between @node and *@slot.
 	 */