A metadata in LibAFL is a self contained structure that holds associated data to the State or to a Testcase.

In terms of code, a metadata can be defined as a Rust struct registered in the SerdeAny register.

fn main() {
extern crate libafl;
extern crate serde;

use libafl::SerdeAny;
use serde::{Serialize, Deserialize};

#[derive(Debug, Serialize, Deserialize, SerdeAny)]
pub struct MyMetadata {

The struct must be static, so it cannot hold references to borrowed objects.

As an alternative to derive(SerdeAny) that is a proc-macro in libafl_derive the user can use libafl::impl_serdeany!(MyMetadata);.


Metadata objects are primarly intended to be used inside SerdeAnyMap and NamedSerdeAnyMap.

With these maps, the user can retrieve instances by type (and name). Internally, the instances are stored as SerdeAny trait objects.

Structs that want to have a set of metadata must implement the HasMetadata trait.

By default, Testcase and State implement it and hold a SerdeAnyMap testcase.


We are interested to store State's Metadata to not lose them in case of crash or stop of a fuzzer. To do that, they must be serialized and unserialized using Serde.

As Metadata is stored in a SerdeAnyMap as trait objects, they cannot be deserialized using Serde by default.

To cope with this problem, in LibAFL each SerdeAny struct must be registered in a global registry that keeps track of types and allows the (de)serialization of the registered types.

Normally, the impl_serdeany macro does that for the user creating a constructor function that fills the registry. However, when using LibAFL in no_std mode, this operation must be carried out manually before any other operation in the main function.

To do that, the developer needs to know each metadata type that is used inside the fuzzer and call RegistryBuilder::register::<MyMetadata>() for each of them at the beginning of main.