libFuzzer Integration Reference

Additional Sanitizer Configuration


You need to download prebuilt instrumented libraries to use MSan (crbug/653712):

GYP_DEFINES='use_goma=1 msan=1 use_prebuilt_instrumented_libraries=1' gclient runhooks


By default, UBSan does not crash when undefined behavior is detected. To make it crash, the following option needs to be set in environment:

UBSAN_OPTIONS=halt_on_error=1 ./fuzzer <corpus_directory_or_single_testcase_path>

Other useful options are (also used by ClusterFuzz):

UBSAN_OPTIONS=symbolize=1:halt_on_error=1:print_stacktrace=1 ./fuzzer <corpus_directory_or_single_testcase_path>

Supported Platforms and Configurations

Builder configurations

The exact GN arguments that are used on our builders can be generated by running:

Linux ASantools/mb/ gen -m -b 'Libfuzzer Upload Linux ASan' out/Directory
Linux ASan Debugtools/mb/ gen -m -b 'Libfuzzer Upload Linux ASan Debug' out/Directory
Linux MSan [*]tools/mb/ gen -m -b 'Libfuzzer Upload Linux MSan' out/Directory
Linux UBSan [*]tools/mb/ gen -m -b 'Libfuzzer Upload Linux UBSan' out/Directory
Mac ASantools/mb/ gen -m -b 'Libfuzzer Upload Mac ASan' out/Directory
Windows ASantools/mb/ gen -m -b 'Libfuzzer Upload Windows ASan' out/Directory


Linux is fully supported by libFuzzer and ClusterFuzz with following sanitizer configurations:

GN ArgumentDescription
is_asan=trueenables Address Sanitizer to catch problems like buffer overruns.
is_msan=trueenables Memory Sanitizer to catch problems like uninitialized reads. [*]
is_ubsan_security=trueenables Undefined Behavior Sanitizer to catch undefined behavior like integer overflow. [*]

Configuration example:

# With address sanitizer
gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true' --check


Mac is supported by libFuzzer with is_asan configuration.

Configuration example:

gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true mac_deployment_target="10.7"' --check


Windows is supported by libFuzzer with is_asan configuration.

Configuration example:

gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true is_debug=false is_component_build=false' --check

On Windows you must use is_component_build=true as libFuzzer does not support component builds on Windows. If you are using is_asan=true then you must use is_debug=false as ASan does not support debug builds on Windows. You may also want to consider using symbol_level=1 which will reduce build size by reducing symbol level to the level necessary for libFuzzer (useful if building many fuzz targets).

fuzzer_test GN Template

Use fuzzer_test to define libFuzzer targets:

fuzzer_test("my_fuzzer") {

Following arguments are supported:

sourcesrequired list of fuzzer test source files
depsfuzzer dependencies
additional_configsadditional GN configurations to be used for compilation
dicta dictionary file for the fuzzer
libfuzzer_optionsruntime options file for the fuzzer. See Fuzzer Runtime Options
seed_corpussingle directory containing test inputs, parsed recursively
seed_corpusesmultiple directories with the same purpose as seed_corpus

Fuzzer Runtime Options

There are many different runtime options supported by libFuzzer. Options are passed as command line arguments:

./fuzzer [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ]

Most common flags are:

max_lenMaximum length of test input.
timeoutTimeout of seconds. Units slower than this value will be reported as bugs.

Full list of options can be found at libFuzzer options page and by running the binary with -help=1.

To specify these options for ClusterFuzz, list all parameters in libfuzzer_options target attribute:

fuzzer_test("my_fuzzer") {
  libfuzzer_options = [