Run our fuzz targets on our corpora in CI

.github/workflows/main.yml

@@ -62,6 +62,30 @@ jobs:
         name: doc-api
         path: target/doc
 
+  # Download our libFuzzer corpus and make sure that we can still handle all the
+  # inputs.
+  fuzz_corpora:
+    name: Fuzz Corpora
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/checkout@v1
+      with:
+        repository: bytecodealliance/wasmtime-libfuzzer-corpus
+        path: ./wasmtime/fuzz/corpus
+        ref: refs/heads/master
+    - uses: ./.github/actions/install-rust
+      with:
+        toolchain: nightly
+    - run: cargo install cargo-fuzz
+    - run: cargo fetch
+      working-directory: ./fuzz
+    # NB: the `-runs=0` means that libFuzzer won't generate new inputs, only run
+    # the seeds from the corpus.
+    - run: cargo fuzz run compile -- -runs=0
+    - run: cargo fuzz run instantiate -- -runs=0
+    - run: cargo fuzz run instantiate_translated -- -runs=0
+
   # Perform all tests (debug mode) for `wasmtime`. This runs stable/beta/nightly
   # channels of Rust as well as macOS/Linux/Windows.
   test:

fuzz/README.md

@@ -0,0 +1,52 @@
+# `cargo fuzz` Targets for Wasmtime
+
+This crate defines various [libFuzzer](https://www.llvm.org/docs/LibFuzzer.html)
+fuzzing targets for Wasmtime, which can be run via [`cargo
+fuzz`](https://rust-fuzz.github.io/book/cargo-fuzz.html).
+
+These fuzz targets just glue together pre-defined test case generators with
+oracles and pass libFuzzer-provided inputs to them. The test case generators and
+oracles themselves are independent from the fuzzing engine that is driving the
+fuzzing process and are defined in `wasmtime/crates/fuzzing`.
+
+## Example
+
+To start fuzzing run the following command, where `$MY_FUZZ_TARGET` is one of
+the [available fuzz targets](#available-fuzz-targets):
+
+```shell
+cargo fuzz run $MY_FUZZ_TARGET
+```
+
+## Available Fuzz Targets
+
+At the time of writing, we have the following fuzz targets:
+
+* `compile`: Attempt to compile libFuzzer's raw input bytes with Wasmtime.
+* `instantiate`: Attempt to compile and instantiate libFuzzer's raw input bytes
+  with Wasmtime.
+* `instantiate_translated`: Pass libFuzzer's input bytes to `wasm-opt -ttf` to
+  generate a random, valid Wasm module, and then attempt to instantiate it.
+
+The canonical list of fuzz targets is the `.rs` files in the `fuzz_targets`
+directory:
+
+```shell
+ls wasmtime/fuzz/fuzz_targets/
+```
+
+## Corpora
+
+While you *can* start from scratch, libFuzzer will work better if it is given a
+[corpus](https://www.llvm.org/docs/LibFuzzer.html#corpus) of seed inputs to kick
+start the fuzzing process. We maintain a corpus for each of these fuzz targets
+in [a dedicated repo on
+github](https://github.com/bytecodealliance/wasmtime-libfuzzer-corpus).
+
+You can use our corpora by cloning it and placing it at `wasmtime/fuzz/corpus`:
+
+```shell
+git clone \
+    https://github.com/bytecodealliance/wasmtime-libfuzzer-corpus.git \
+    wasmtime/fuzz/corpus
+```