Fuzz-testing Bitcoin Core ========================== A special test harness `test_bitcoin_fuzzy` is provided to provide an easy entry point for fuzzers and the like. In this document we'll describe how to use it with AFL. Building AFL ------------- It is recommended to always use the latest version of afl: ``` wget http://lcamtuf.coredump.cx/afl/releases/afl-latest.tgz tar -zxvf afl-latest.tgz cd afl-<version> make export AFLPATH=$PWD ``` Instrumentation ---------------- To build Bitcoin Core using AFL instrumentation (this assumes that the `AFLPATH` was set as above): ``` ./configure --disable-ccache --disable-shared --enable-tests CC=${AFLPATH}/afl-gcc CXX=${AFLPATH}/afl-g++ export AFL_HARDEN=1 cd src/ make test/test_bitcoin_fuzzy ``` We disable ccache because we don't want to pollute the ccache with instrumented objects, and similarly don't want to use non-instrumented cached objects linked in. Preparing fuzzing ------------------ AFL needs an input directory with examples, and an output directory where it will place examples that it found. These can be anywhere in the file system, we'll define environment variables to make it easy to reference them. ``` mkdir inputs AFLIN=$PWD/inputs mkdir outputs AFLOUT=$PWD/outputs ``` Example inputs are available from: - https://download.visucore.com/bitcoin/bitcoin_fuzzy_in.tar.xz - http://strateman.ninja/fuzzing.tar.xz Extract these (or other starting inputs) into the `inputs` directory before starting fuzzing. Fuzzing -------- To start the actual fuzzing use: ``` $AFLPATH/afl-fuzz -i ${AFLIN} -o ${AFLOUT} -m52 -- test/test_bitcoin_fuzzy ``` You may have to change a few kernel parameters to test optimally - `afl-fuzz` will print an error and suggestion if so.