1Fuzz-testing Litecoin Core 2========================== 3 4A special test harness in `src/test/fuzz/` is provided for each fuzz target to 5provide an easy entry point for fuzzers and the like. In this document we'll 6describe how to use it with AFL and libFuzzer. 7 8## Preparing fuzzing 9 10AFL needs an input directory with examples, and an output directory where it 11will place examples that it found. These can be anywhere in the file system, 12we'll define environment variables to make it easy to reference them. 13 14libFuzzer will use the input directory as output directory. 15 16Extract the example seeds (or other starting inputs) into the inputs 17directory before starting fuzzing. 18 19``` 20git clone https://github.com/bitcoin-core/qa-assets 21export DIR_FUZZ_IN=$PWD/qa-assets/fuzz_seed_corpus 22``` 23 24Only for AFL: 25 26``` 27mkdir outputs 28export AFLOUT=$PWD/outputs 29``` 30 31## AFL 32 33### Building AFL 34 35It is recommended to always use the latest version of afl: 36``` 37wget http://lcamtuf.coredump.cx/afl/releases/afl-latest.tgz 38tar -zxvf afl-latest.tgz 39cd afl-<version> 40make 41export AFLPATH=$PWD 42``` 43 44### Instrumentation 45 46To build Litecoin Core using AFL instrumentation (this assumes that the 47`AFLPATH` was set as above): 48``` 49./configure --disable-ccache --disable-shared --enable-tests --enable-fuzz --disable-wallet --disable-bench --with-utils=no --with-daemon=no --with-libs=no --with-gui=no CC=${AFLPATH}/afl-gcc CXX=${AFLPATH}/afl-g++ 50export AFL_HARDEN=1 51cd src/ 52make 53``` 54We disable ccache because we don't want to pollute the ccache with instrumented 55objects, and similarly don't want to use non-instrumented cached objects linked 56in. 57 58The fuzzing can be sped up significantly (~200x) by using `afl-clang-fast` and 59`afl-clang-fast++` in place of `afl-gcc` and `afl-g++` when compiling. When 60compiling using `afl-clang-fast`/`afl-clang-fast++` the resulting 61binary will be instrumented in such a way that the AFL 62features "persistent mode" and "deferred forkserver" can be used. See 63https://github.com/mcarpenter/afl/tree/master/llvm_mode for details. 64 65### Fuzzing 66 67To start the actual fuzzing use: 68 69``` 70export FUZZ_TARGET=fuzz_target_foo # Pick a fuzz_target 71mkdir ${AFLOUT}/${FUZZ_TARGET} 72$AFLPATH/afl-fuzz -i ${DIR_FUZZ_IN}/${FUZZ_TARGET} -o ${AFLOUT}/${FUZZ_TARGET} -m52 -- test/fuzz/${FUZZ_TARGET} 73``` 74 75You may have to change a few kernel parameters to test optimally - `afl-fuzz` 76will print an error and suggestion if so. 77 78## libFuzzer 79 80A recent version of `clang`, the address sanitizer and libFuzzer is needed (all 81found in the `compiler-rt` runtime libraries package). 82 83To build all fuzz targets with libFuzzer, run 84 85``` 86./configure --disable-ccache --disable-wallet --disable-bench --with-utils=no --with-daemon=no --with-libs=no --with-gui=no --enable-fuzz --with-sanitizers=fuzzer,address CC=clang CXX=clang++ 87make 88``` 89 90The fuzzer needs some inputs to work on, but the inputs or seeds can be used 91interchangeably between libFuzzer and AFL. 92 93See https://llvm.org/docs/LibFuzzer.html#running on how to run the libFuzzer 94instrumented executable. 95 96Alternatively run the script in `./test/fuzz/test_runner.py` and provide it 97with the `${DIR_FUZZ_IN}` created earlier. 98