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