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Jamey Sharp f242975c49 cranelift-frontend: Allow jump table reuse (#4429) 
* Allow using jump-tables multiple times (fixes #3347)

If there are multiple `br_table` instructions using the same jump table,
then `append_jump_argument` must not modify the jump table in-place.

When this function is called, we don't know if more `br_table`
instructions might be added later. So this patch conservatively assumes
that all jump tables might be reused. If Cranelift needs to add a block
argument to a block that's the target of some jump table, then the jump
table will be unconditionally cloned.

I'm not sure if having duplicated and unused jump tables will turn out
to be a compile-time performance issue. If it is, there's discussion in
issue #3347 about ways to determine that there can't be any more uses of
a jump table, so that it's safe to modify in-place.

* Re-enable cranelift-fuzzgen fuzz target

I've been running this fuzz target for an hour without finding new bugs.
Let's see if oss-fuzz finds anything now.
2022-07-11 15:09:51 -05:00
..
fuzz_targets
cranelift-fuzzgen-verify is redundant with cranelift-fuzzgen (#4411)
2022-07-07 22:05:23 +00:00
.gitignore
fuzzing: remove the in-repo corpus
2019-11-26 15:49:07 -08:00
Cargo.toml
cranelift-frontend: Allow jump table reuse (#4429)
2022-07-11 15:09:51 -05:00
README.md
cranelift-fuzzgen-verify is redundant with cranelift-fuzzgen (#4411)
2022-07-07 22:05:23 +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.
  • differential: Generate a Wasm module and check that Wasmtime returns the same results when run with two different configurations.
  • differential_spec: Generate a Wasm module and check that Wasmtime returns the same results as the Wasm spec interpreter (see the wasm-spec-interpreter crate).
  • differential_v8: Generate a Wasm module and check that Wasmtime returns the same results as V8.
  • differential_wasmi: Generate a Wasm module and check that Wasmtime returns the same results as the wasmi interpreter.
  • 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).