We assume that you can build Skia. Many fuzzes only reproduce when building with ASAN or MSAN; see those instructions for more details.
When building, you should add the following args to BUILD.gn to make reproducing less machine- and platform- dependent:
skia_use_fontconfig=false skia_use_freetype=true skia_use_system_freetype2=false skia_use_wuffs=true skia_enable_skottie=true skia_enable_fontmgr_custom_directory=false skia_enable_fontmgr_custom_embedded=false skia_enable_fontmgr_custom_empty=true
All that is needed to reproduce a fuzz downloaded from ClusterFuzz or oss-fuzz is to run something like:
out/ASAN/fuzz -b /path/to/downloaded/testcase
The fuzz binary will try its best to guess what the type/name should be based on the name of the testcase. Manually providing type and name is also supported, like:
out/ASAN/fuzz -t filter_fuzz -b /path/to/downloaded/testcase out/ASAN/fuzz -t api -n RasterN32Canvas -b /path/to/downloaded/testcase
To enumerate all supported types and names, run the following:
out/ASAN/fuzz --help # will list all types out/ASAN/fuzz -t api # will list all names
If the crash does not show up, try to add the flag --loops:
out/ASAN/fuzz -b /path/to/downloaded/testcase --loops <times-to-run>
Writing fuzzers with libfuzzer
libfuzzer is an easy way to write new fuzzers, and how we run them on oss-fuzz. Your fuzzer entry point should implement this API:
extern "C" int LLVMFuzzerTestOneInput(const uint8_t*, size_t);
First install Clang and libfuzzer, e.g.
sudo apt install clang-10 libc++-10-dev libfuzzer-10-dev
You should now be able to use
-fsanitize=fuzzer with Clang.
Set up GN args to use libfuzzer:
cc = "clang-10" cxx = "clang++-10" sanitize = "fuzzer" extra_cflags = [ "-O1" ] # Or whatever you want. ...
Build Skia and your fuzzer entry point:
ninja -C out/libfuzzer skia clang++-10 -I. -O1 -fsanitize=fuzzer fuzz/oss_fuzz/whatever.cpp out/libfuzzer/libskia.a
Run your new fuzzer binary
There are some defines that can help guide a fuzzer to be more productive (e.g. avoid OOMs, avoid unnecessarily slow code).
// Required for fuzzing with afl-fuzz to prevent OOMs from adding noise. SK_BUILD_FOR_AFL_FUZZ // Required for fuzzing with libfuzzer SK_BUILD_FOR_LIBFUZZER // This define adds in guards to abort when we think some code path will take a long time or // use a lot of RAM. It is set by default when either of the above defines are set. SK_BUILD_FOR_FUZZER