Fuzzing

Fuzz tests generate a ton of random parameter arguments to the program and then validate that none cause it to crash.

How does it work?

Typically, Travis CI will run travis-fuzz.sh on one of the environments the automated tests are configured for. This is the most time-consuming component of the continuous integration workflow, so it is recommended that you detect issues locally, and Travis merely acts as a sanity check. Fuzzing is further only effective with a lot of CPU time, indicating that if crash scenarios are discovered on Travis with its low runtime constraints, the crash is caused relatively easily.

How do I run fuzz tests locally?

You typically won't need to run the entire combination of different fuzzing tools. For local execution, honggfuzz should be more than sufficient.

Setup

To install honggfuzz, simply run

cargo update
cargo install --force honggfuzz

In some environments, you may want to pin the honggfuzz version to 0.5.52:

cargo update -p honggfuzz --precise "0.5.52"
cargo install --force honggfuzz --version "0.5.52"

Execution

To run the Hongg fuzzer, do

export CPU_COUNT=1 # replace as needed
export HFUZZ_BUILD_ARGS="--features honggfuzz_fuzz"
export HFUZZ_RUN_ARGS="-n $CPU_COUNT --exit_upon_crash"

export TARGET="msg_ping_target" # replace with the target to be fuzzed
cargo hfuzz run $TARGET

(Or, for a prettier output, replace the last line with cargo --color always hfuzz run $TARGET.)

To see a list of available fuzzing targets, run:

ls ./src/bin/

A fuzz test failed on Travis, what do I do?

You're trying to create a PR, but need to find the underlying cause of that pesky fuzz failure blocking the merge?

Worry not, for this is easily traced.

If your Travis output log looks like this:

Size:639 (i,b,hw,ed,ip,cmp): 0/0/0/0/0/1, Tot:0/0/0/2036/5/28604
Seen a crash. Terminating all fuzzing threads

… # a lot of lines in between

<0x0000555555565559> [func:UNKNOWN file: line:0 module:/home/travis/build/rust-bitcoin/rust-lightning/fuzz/hfuzz_target/x86_64-unknown-linux-gnu/release/full_stack_target]
<0x0000000000000000> [func:UNKNOWN file: line:0 module:UNKNOWN]
=====================================================================
2d3136383734090101010101010101010101010101010101010101010101
010101010100040101010101010101010101010103010101010100010101
0069d07c319a4961
The command "if [ "$(rustup show | grep default | grep stable)" != "" ]; then cd fuzz && cargo test --verbose && ./travis-fuzz.sh; fi" exited with 1.

Note that the penultimate stack trace line ends in release/full_stack_target]. That indicates that the failing target was full_stack. To reproduce the error locally, simply copy the hex, and run the following from the fuzz directory:

export TARGET="full_stack" # adjust for your output
export HEX="2d3136383734090101010101010101010101010101010101010101010101\
010101010100040101010101010101010101010103010101010100010101\
0069d07c319a4961" # adjust for your output

mkdir -p ./test_cases/$TARGET
echo $HEX | xxd -r -p > ./test_cases/$TARGET/any_filename_works

export RUST_BACKTRACE=1
export RUSTFLAGS="--cfg=fuzzing"
cargo test

Note that if the fuzz test failed locally, moving the offending run's trace to the test_cases folder should also do the trick; simply replace the echo $HEX | line above with (the trace file name is of course a bit longer than in the example):

mv hfuzz_workspace/fuzz_target/SIGABRT.PC.7ffff7e21ce1.STACK.[…].fuzz ./test_cases/$TARGET/

This will reproduce the failing fuzz input and yield a usable stack trace.

How do I add a new fuzz test?

1. The easiest approach is to take one of the files in fuzz/src/, such as process_network_graph.rs, and duplicate it, renaming the new file to something more suitable. For the sake of example, let's call the new fuzz target we're creating my_fuzzy_experiment.

2. In the newly created file fuzz/src/my_fuzzy_experiment.rs, run a string substitution of process_network_graph to my_fuzzy_experiment, such that the three methods in the file are do_test, my_fuzzy_experiment_test, and my_fuzzy_experiment_run.

3. Adjust the body (not the signature!) of do_test as necessary for the new fuzz test.

4. In fuzz/src/bin/gen_target.sh, add a line reading GEN_TEST my_fuzzy_experiment to the first group of GEN_TEST lines (starting in line 9).

5. If your test relies on a new local crate, add that crate as a dependency to fuzz/Cargo.toml.

6. In fuzz/src/lib.rs, add the line pub mod my_fuzzy_experiment. Additionally, if you added a new crate dependency, add the extern crate […] import line.

7. Run fuzz/src/bin/gen_target.sh.

8. There is no step eight: happy fuzzing!