fmtlegacy/test/fuzzing

FMT Fuzzer

Fuzzing has revealed several bugs in fmt. It is a part of the continous fuzzing at oss-fuzz.

The source code is modified to make the fuzzing possible without locking up on resource exhaustion:

#ifdef FMT_FUZZ
if(spec.precision>100000) {
  throw std::runtime_error("fuzz mode - avoiding large precision");
}
#endif

This macro FMT_FUZZ is enabled on OSS-Fuzz builds and makes fuzzing practically possible. It is used in fmt code to prevent resource exhaustion in fuzzing mode.
The macro FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION is the defacto standard for making fuzzing practically possible to disable certain fuzzing-unfriendly features (for example, randomness), see the libFuzzer documentation.

Running the fuzzers locally

There is a helper script to build the fuzzers, which has only been tested on Debian and Ubuntu linux so far. There should be no problems fuzzing on Windows (using clang>=8) or on Mac, but the script will probably not work out of the box.

Something along

mkdir build
cd build
export CXX=clang++
export CXXFLAGS="-fsanitize=fuzzer-no-link -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION= -g"
cmake ..  -DFMT_SAFE_DURATION_CAST=On -DFMT_FUZZ=On -DFMT_FUZZ_LINKMAIN=Off -DFMT_FUZZ_LDFLAGS="-fsanitize=fuzzer"
cmake --build .

should work to build the fuzzers for all platforms which clang supports.

Execute a fuzzer with for instance

cd build
export UBSAN_OPTIONS=halt_on_error=1
mkdir out_chrono
bin/fuzzer_chrono_duration  out_chrono