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Merge pull request #3982 from embg/fuzzer_readme

Document the process for adding a new fuzzer
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@ -117,3 +117,45 @@ CC=clang CXX=clang++ ./fuzz.py build all --enable-msan
## Fuzzing a custom sequence producer plugin
Sequence producer plugin authors can use the zstd fuzzers to stress-test their code.
See the documentation in `fuzz_third_party_seq_prod.h` for details.
## Adding a new fuzzer
There are several steps involved in adding a new fuzzer harness.
### Build your harness
1. Create a new your fuzzer harness `tests/fuzz/your_harness.c`.
2. Add your harness to the Makefile
2.1 Follow [this example](https://github.com/facebook/zstd/blob/e124e39301381de8f323436a3e4c46539747ba24/tests/fuzz/Makefile#L216) if your fuzzer requires both compression and decompression symbols (prefix `rt_`). If your fuzzer only requires decompression symbols, follow [this example](https://github.com/facebook/zstd/blob/6a0052a409e2604bd40354b76b86272b712edd7d/tests/fuzz/Makefile#L194) (prefix `d_`).
2.2 Add your target to [`FUZZ_TARGETS`](https://github.com/facebook/zstd/blob/6a0052a409e2604bd40354b76b86272b712edd7d/tests/fuzz/Makefile#L108).
3. Add your harness to [`fuzz.py`](https://github.com/facebook/zstd/blob/6a0052a409e2604bd40354b76b86272b712edd7d/tests/fuzz/fuzz.py#L48).
### Generate seed data
Follow the instructions above to generate seed data:
```
make -C ../tests decodecorpus
./fuzz.py gen your_harness
```
### Run the harness
Follow the instructions above to run your harness and fix any crashes:
```
./fuzz.py build your_harness --enable-fuzzer --enable-asan --enable-ubsan --cc clang --cxx clang++
./fuzz.py libfuzzer your_harness
```
### Minimize and zip the corpus
After running the fuzzer for a while, you will have a large corpus at `tests/fuzz/corpora/your_harness*`.
This corpus must be minimized and zipped before uploading to GitHub for regression testing:
```
./fuzz.py minimize your_harness
./fuzz.py zip your_harness
```
### Upload the zip file to GitHub
The previous step should produce a `.zip` file containing the corpus for your new harness.
This corpus must be uploaded to GitHub here: https://github.com/facebook/zstd/releases/tag/fuzz-corpora