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49 lines
1.6 KiB
Markdown
49 lines
1.6 KiB
Markdown
# FMT Fuzzer
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Fuzzing has revealed [several bugs](https://github.com/fmtlib/fmt/issues?&q=is%3Aissue+fuzz)
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in fmt. It is a part of the continous fuzzing at
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[oss-fuzz](https://github.com/google/oss-fuzz).
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The source code is modified to make the fuzzing possible without locking up on
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resource exhaustion:
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```cpp
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#ifdef FMT_FUZZ
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if(spec.precision>100000) {
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throw std::runtime_error("fuzz mode - avoiding large precision");
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}
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#endif
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```
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This macro `FMT_FUZZ` is enabled on OSS-Fuzz builds and makes fuzzing
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practically possible. It is used in fmt code to prevent resource exhaustion in
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fuzzing mode.
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The macro `FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION` is the
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defacto standard for making fuzzing practically possible to disable certain
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fuzzing-unfriendly features (for example, randomness), see [the libFuzzer
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documentation](https://llvm.org/docs/LibFuzzer.html#fuzzer-friendly-build-mode).
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## Running the fuzzers locally
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There is a [helper script](build.sh) to build the fuzzers, which has only been
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tested on Debian and Ubuntu linux so far. There should be no problems fuzzing on
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Windows (using clang>=8) or on Mac, but the script will probably not work out of
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the box.
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Something along
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```sh
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mkdir build
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cd build
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export CXX=clang++
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export CXXFLAGS="-fsanitize=fuzzer-no-link -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION= -g"
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cmake .. -DFMT_SAFE_DURATION_CAST=On -DFMT_FUZZ=On -DFMT_FUZZ_LINKMAIN=Off -DFMT_FUZZ_LDFLAGS="-fsanitize=fuzzer"
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cmake --build .
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```
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should work to build the fuzzers for all platforms which clang supports.
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Execute a fuzzer with for instance
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```sh
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cd build
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export UBSAN_OPTIONS=halt_on_error=1
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mkdir out_chrono
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bin/fuzzer_chrono_duration out_chrono
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```
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