Spawning Instances

Multiple fuzzer instances can be spawned using different ways.

Manually, via a TCP port

The straightforward way to do Multi-Threading is to use the LlmpRestartingEventManager, specifically to use setup_restarting_mgr_std. It abstracts away all the pesky details about restarts on crash handling (for in-memory fuzzers) and multi-threading. With it, every instance you launch manually tries to connect to a TCP port on the local machine.

If the port is not yet bound, this instance becomes the broker, binding itself to the port to await new clients.

If the port is already bound, the EventManager will try to connect to it. The instance becomes a client and can now communicate with all other nodes.

Launching nodes manually has the benefit that you can have multiple nodes with different configurations, such as clients fuzzing with and without ASan.

While it's called "restarting" manager, it uses fork on Unix-like operating systems as optimization and only actually restarts from scratch on Windows.

Automated, with Launcher

The Launcher is the lazy way to do multiprocessing. You can use the Launcher builder to create a fuzzer that spawns multiple nodes with one click, all using restarting event managers and the same configuration.

To use launcher, first you need to write an anonymous function let mut run_client = |state: Option<_>, mut mgr, _core_id|{}, which uses three parameters to create an individual fuzzer. Then you can specify the shmem_provider,broker_port,monitor,cores and other stuff through Launcher::builder():

        .run_client(&mut run_client)

This first starts a broker, then spawns n clients, according to the value passed to cores. The value is a string indicating the cores to bind to, for example, 0,2,5 or 0-3. For each client, run_client will be called. If the launcher uses fork, it will hide child output, unless the settings indicate otherwise, or the LIBAFL_DEBUG_OUTPUT env variable is set. On Windows, the Launcher will restart each client, while on Unix-alikes, it will use fork.

Advanced use-cases:

  1. To connect multiple nodes together via TCP, you can use the remote_broker_addr. this requires the llmp_bind_public compile-time feature for LibAFL.
  2. To use multiple launchers for individual configurations, you can set spawn_broker to false on all instances but one.
  3. Launcher will not select the cores automatically, so you need to specify the cores that you want.
  4. On Unix, you can chose between a forking and non-forking version of Launcher by setting the fork feature in LibAFL. Some targets may not like forking, but it is faster than restarting processes from scratch. Windows will never fork.
  5. For simple debugging, first set the LIBAFL_DEBUG_OUTPUT env variable to see if a child process printed anything.
  6. For further debugging of fuzzer failures, it may make sense to replace Launcher temporarily with a SimpleEventManager and call your harness fn (run_client(None, mgr, 0);) directly, so that fuzzing runs in the same thread and is easier to debug, before moving back to Launcher after the bugfix.

For more examples, you can check out qemu_launcher and libfuzzer_libpng_launcher in ./fuzzers/.

Other ways

The LlmpEventManager family is the easiest way to spawn instances, but for obscure targets, you may need to come up with other solutions. LLMP is even, in theory, no_std compatible, and even completely different EventManagers can be used for message passing. If you are in this situation, please either read through the current implementations and/or reach out to us.