docs/development/testing/fuzzing/libfuzzer_fidl.md

# Fuzzing FIDL Servers with LibFuzzer on Fuchsia

## Quick-start guide

An example fuzzer is defined in `//garnet/examples/fidl/echo_server_cpp/fuzzer`. If you are not familiar with fuzzers, see [LibFuzzer on Fuchsia](libfuzzer.md). Fuzzing FIDL servers with `LibFuzzer` on fuchsia requires gn targets that will generate a fuzz target and writing some code to provide an instance of the server to be fuzzed.

1. Add `fuzzers = {protocol = "fully.qualified.fidl.ProtocolName"}` to your `fidl()` gn target.
1. Depending on the language you are using:
    * At the C++ level (easy mode):
      Use the `FIDL_FUZZER_DEFINITION()` macro in `//sdk/lib/fidl/cpp/fuzzing/server_provider.h` to define a server provider for your interface and server implementation class. This will automatically define the C symbols described below. See `//garnet/examples/fidl/echo_server_cpp/fuzzer` for a reference example.
    * At the C level (hard mode):
      Implement a library that defines the following symbols:
      * `zx_status_t fuzzer_init()`: Instantiate server implementation.
      * `zx_status_t fuzzer_connect(zx_handle_t, async_dispatcher_t*)`: Bind server implementation to the channel handle. Optionally, use the dispatcher if your server can be fuzzed on the same thread as fuzzer clients (see [note on threading][single-thread-dispatcher-preferred]).
      * `zx_status_t fuzzer_disconnect(zx_handle_t, async_dispatcher_t*)`: Unbind server implemementation from the channel handle.
      * `zx_status_t fuzzer_clean_up()`: Clean up server implementation.
      If any of these returns a status other than `ZX_OK`, then the fuzzer will cleanup and halt.
1. Define a `fidl_protocol_fuzzer()` gn target. Specify:
    * `fidl = //path/to:fidl_gn_target` (the `fidl()` target mentioned above).
    * `protocol = "fully.qualified.fidl.ProtocolName"`.
    * `deps = [... :your_library ...]` (the one mentioned above, defining `fuzzer_...` symbols).
    * Anything else needed for a `fuzzer()` gn target that fuzzes your server (see [LibFuzzer on Fuchsia](libfuzzer.md) for details).
1. Add `fidl_protocol_fuzzer()` target to `fuzzers = [ ... ]` in a new or existing `fuzzers_package()` gn target (see [LibFuzzer on Fuchsia](libfuzzer.md) for details).

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## Implementation details

The bulk of a FIDL server implementation fuzz target is C++ code generated by `fidlgen` that expects a handful of C symbols to provide an API to the FIDL server implementation. The generated code contains a global `async::Loop`, bound to its initial thread, that is reused for the client side of a FIDL connection on each run of the fuzz target. `LibFuzzer` repeatedly invokes the same fuzz target with different inputs. The generated fuzz target code will:

* Invoke `fuzzer_init()`, initializing the server to be fuzzed.
* Instantiates a pair of `zx::channel`s.
* Initializes an `fidl::InterfacePtr` of the appropriate type, bound to a channel end and the loop's dispatcher.
* Invoke `fuzzer_connect(raw_server_channel_handle, loop->dispatcher())`, establishing a connection with the server and allowing the server to opt in to using the same dispatcher as the client if its API is compatible with such a scheme (see [note on threading][single-thread-dispatcher-preferred]).
* Invoke a method through its `fidl::InterfacePtr`.
* Set its `async::Loop` to run-until-idle.
* Synchronize with the method's callback via a `zx::event`.
* Invoke `fuzzer_disconnect(raw_server_channel_handle, loop->dispatcher())`, allowing the server to clean up its end of the connection.
* Invoke `fuzzer_clean_up()` to tear down the server instance.

### Allocation of fuzz target input to FIDL messages

In broad strokes, the first two bytes are used to select a protocol and method pair from among those defined in the FIDL source file. In the case of a FIDL file that contains many protocols, but only one is enabled in the fuzzer, discovery of meaningful inputs relies on `LibFuzzer`'s coverage-guided engine to deduce that the first bytes of a certain form cause the fuzzer to exercise almost no code.

After a protocol and method pair are identified, the remaining bytes are carved up as follows:

* Each type has a trait that defines the minimum number of bytes it requires.
* If insufficient bytes are input, the fuzz target exits immediately.
* Otherwise the "slack" bytes that exceed the minimum required for the method parameters are divided evenly among parameters and an allocation trait for each type is invoked to construct an object of the appropriate type using at most `MinForParam + SlackForParam` bytes.

In terms of allocation trait details, collection and numeric types have relatively natural interpretations based on the collection of bytes to be transformed into an object. Handles are treated like numeric types, which can be expected to lead to errors when the server attempts to exercise them.

### A note about threading
{#a-note-about-threading}

It is **highly** desirable to keep the fuzz target single-threaded. That is, use `ServerProviderDispatcherMode::kFromCaller` in C++ or use the `async_dispatcher_t*` passed to `fuzzer_connect` in C. This is preferred because it increases the liklihood that bugs found by the fuzzer will be consistently reproducible.

## Future work

The following work is planned for improving fuzzing FIDL server implementations with `LibFuzzer` on Fuchsia:

1. Language bindings/libraries for C/C++, rust, and go.
1. Improved distribution of fuzz input data over method input signature. For example, some fuzz inputs should lead to using legitimate handles rather than treating handles as numeric types.
1. Fuzzing multiple method invocations in the same test run.
1. Fuzzing multiple connections in the same test run.

[single-thread-dispatcher-preferred]: #a-note-about-threading