Document fuzzer usage (#3478)

* 📝 document fuzzer usage

* 📝 address review comments
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Niels Lohmann 3 months ago committed by GitHub
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  1. 4
      .gitignore
  2. 81
      tests/fuzzing.md

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.gitignore vendored

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# build directories (vscode-cmake-tools, user-defined, ...)
/build*/
# fuzzers
/tests/parse_*_fuzzer
/tests/corpus_*
/docs/mkdocs/docs/examples/
/docs/mkdocs/docs/__pycache__/
/docs/mkdocs/site/

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# Fuzz testing
Each parser of the library (JSON, BJData, BSON, CBOR, MessagePack, and UBJSON) can be fuzz tested. Currently,
[libFuzzer](https://llvm.org/docs/LibFuzzer.html) and [afl++](https://github.com/AFLplusplus/AFLplusplus) are supported.
## Corpus creation
For most effective fuzzing, a [corpus](https://llvm.org/docs/LibFuzzer.html#corpus) should be provided. A corpus is a
directory with some simple input files that cover several features of the parser and is hence a good starting point
for mutations.
```shell
TEST_DATA_VERSION=3.1.0
wget https://github.com/nlohmann/json_test_data/archive/refs/tags/v$TEST_DATA_VERSION.zip
unzip v$TEST_DATA_VERSION.zip
rm v$TEST_DATA_VERSION.zip
for FORMAT in json bjdata bson cbor msgpack ubjson
do
rm -fr corpus_$FORMAT
mkdir corpus_$FORMAT
find json_test_data-$TEST_DATA_VERSION -size -5k -name "*.$FORMAT" -exec cp "{}" "corpus_$FORMAT" \;
done
rm -fr json_test_data-$TEST_DATA_VERSION
```
The generated corpus can be used with both libFuzzer and afl++. The remainder of this documentation assumes the corpus
directories have been created in the `tests` directory.
## libFuzzer
To use libFuzzer, you need to pass `-fsanitize=fuzzer` as `FUZZER_ENGINE`. In the `tests` directory, call
```shell
make fuzzers FUZZER_ENGINE="-fsanitize=fuzzer"
```
This creates a fuzz tester binary for each parser that supports these
[command line options](https://llvm.org/docs/LibFuzzer.html#options).
In case your default compiler is not a Clang compiler that includes libFuzzer (Clang 6.0 or later), you need to set the
`CXX` variable accordingly. Note the compiler provided by Xcode (AppleClang) does not contain libFuzzer. Please install
Clang via Homebrew calling `brew install llvm` and add `CXX=$(brew --prefix llvm)/bin/clang` to the `make` call:
```shell
make fuzzers FUZZER_ENGINE="-fsanitize=fuzzer" CXX=$(brew --prefix llvm)/bin/clang
```
Then pass the corpus directory as command-line argument (assuming it is located in `tests`):
```shell
./parse_cbor_fuzzer corpus_cbor
```
The fuzzer should be able to run indefinitely without crashing. In case of a crash, the tested input is dumped into
a file starting with `crash-`.
## afl++
To use afl++, you need to pass `-fsanitize=fuzzer` as `FUZZER_ENGINE`. It will be replaced by a `libAFLDriver.a` to
re-use the same code written for libFuzzer with afl++. Furthermore, set `afl-clang-fast++` as compiler.
```shell
CXX=afl-clang-fast++ make fuzzers FUZZER_ENGINE="-fsanitize=fuzzer"
```
Then the fuzzer is called like this in the `tests` directory:
```shell
afl-fuzz -i corpus_cbor -o out -- ./parse_cbor_fuzzer
```
The fuzzer should be able to run indefinitely without crashing. In case of a crash, the tested input is written to the
directory `out`.
## OSS-Fuzz
The library is further fuzz-tested 24/7 by Google's [OSS-Fuzz project](https://github.com/google/oss-fuzz). It uses
the same `fuzzers` target as above and also relies on the `FUZZER_ENGINE` variable. See the used
[build script](https://github.com/google/oss-fuzz/blob/master/projects/json/build.sh) for more information.
In case the build at OSS-Fuzz fails, an issue will be created automatically.
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