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wasmtime/fuzz
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Andrew Brown c3f8415ac7 fuzz: improve the spec interpreter (#4881) 
* fuzz: improve the API of the `wasm-spec-interpreter` crate

This change addresses key parts of #4852 by improving the bindings to
the OCaml spec interpreter. The new API allows users to `instantiate` a
module, `interpret` named functions on that instance, and `export`
globals and memories from that instance. This currently leaves the
existing implementation ("instantiate and interpret the first function in
a module") present under a new name: `interpret_legacy`.

* fuzz: adapt the differential spec engine to the new API

This removes the legacy uses in the differential spec engine, replacing
them with the new `instantiate`-`interpret`-`export` API from the
`wasm-spec-interpreter` crate.

* fix: make instance access thread-safe

This changes the OCaml-side definition of the instance so that each
instance carries round a reference to a "global store" that's specific
to that instantiation. Because everything is updated by reference there
should be no visible behavioural change on the Rust side, apart from
everything suddenly being thread-safe (modulo the fact that access to
the OCaml runtime still needs to be locked). This fix will need to be
generalised slightly in future if we want to allow multiple modules to
be instantiated in the same store.

Co-authored-by: conrad-watt <cnrdwtt@gmail.com>
Co-authored-by: Alex Crichton <alex@alexcrichton.com>
2022-09-12 14:23:03 -07:00
..
fuzz_targets
fuzz: improve the spec interpreter (#4881)
2022-09-12 14:23:03 -07:00
.gitignore
fuzzing: remove the in-repo corpus
2019-11-26 15:49:07 -08:00
build.rs
Don't let fuzz targets import arbitrary directly (#4806)
2022-08-29 23:06:41 +00:00
Cargo.toml
Don't let fuzz targets import arbitrary directly (#4806)
2022-08-29 23:06:41 +00:00
README.md
Port v8 fuzzer to the new framework (#4739)
2022-08-19 19:19:00 +00:00

README.md

cargo fuzz Targets for Wasmtime

This crate defines various libFuzzer fuzzing targets for Wasmtime, which can be run via cargo fuzz.

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:

cargo fuzz run $MY_FUZZ_TARGET

Available Fuzz Targets

At the time of writing, we have the following fuzz targets:

  • api_calls: stress the Wasmtime API by executing sequences of API calls; only the subset of the API is currently supported.
  • compile: Attempt to compile libFuzzer's raw input bytes with Wasmtime.
  • compile-maybe-invalid: Attempt to compile a wasm-smith-generated Wasm module with code sequences that may be invalid.
  • cranelift-fuzzgen: Generate a Cranelift function and check that it returns the same results when compiled to the host and when using the Cranelift interpreter; only a subset of Cranelift IR is currently supported.
  • cranelift-icache: Generate a Cranelift function A, applies a small mutation to its source, yielding a function A', and checks that A compiled + incremental compilation generates the same machine code as if A' was compiled from scratch.
  • differential: Generate a Wasm module, evaluate each exported function with random inputs, and check that Wasmtime returns the same results as a choice of another engine: the Wasm spec interpreter (see the wasm-spec-interpreter crate), the wasmi interpreter, V8 (through the v8 crate), or Wasmtime itself run with a different configuration.
  • instantiate: Generate a Wasm module and Wasmtime configuration and attempt to compile and instantiate with them.
  • instantiate-many: Generate many Wasm modules and attempt to compile and instantiate them concurrently.
  • spectests: Pick a random spec test and run it with a generated configuration.
  • table_ops: Generate a sequence of externref table operations and run them in a GC environment.

The canonical list of fuzz targets is the .rs files in the fuzz_targets directory:

ls wasmtime/fuzz/fuzz_targets/

Corpora

While you can start from scratch, libFuzzer will work better if it is given a 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.

You can use our corpora by cloning it and placing it at wasmtime/fuzz/corpus:

git clone \
    https://github.com/bytecodealliance/wasmtime-libfuzzer-corpus.git \
    wasmtime/fuzz/corpus

Reproducing a Fuzz Bug

When investigating a fuzz bug (especially one found by OSS-Fuzz), use the following steps to reproduce it locally:

  1. Download the test case (either the "Minimized Testcase" or "Unminimized Testcase" from OSS-Fuzz will do).
  2. Run the test case in the correct fuzz target: 
    cargo +nightly fuzz run <target> <test case>
    If all goes well, the bug should reproduce and libFuzzer will dump the failure stack trace to stdout
  3. For more debugging information, run the command above with RUST_LOG=debug to print the configuration and WebAssembly input used by the test case (see uses of log_wasm in the wasmtime-fuzzing crate).