Reproducing using fuzz

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:


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 = [ "-DSK_BUILD_FOR_LIBFUZZER", # enables fuzzer-constraints (see below)
                 "-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


Fuzzing Defines

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.

// Required for fuzzing with 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.