Tint Fuzzers

Tint currently has two fuzzer target executables: tint_wgsl_fuzzer which takes WGSL source code as input and tint_ir_fuzzer which takes a protobuf binary file as input.

Both fuzzers are implemented using libFuzzer, and are automatically and continuously run by Chromium's ClusterFuzz infrastructure. Fuzzer targets are automatically found via gn refs. See tint.gni for the core fuzzer target rules.

Tint's fuzzers are implemented as functions registered with the macros:

TINT_WGSL_PROGRAM_FUZZER() registers a fuzzer function that is handed a tint::Program.
TINT_IR_MODULE_FUZZER() registers a fuzzer function that is handed a tint::core::ir::Module.

Building

The fuzzer targets can be build with either CMake or GN:

CMake: Define TINT_BUILD_FUZZERS=1 (pass -DTINT_BUILD_FUZZERS=1 to CMake)
GN: Define use_libfuzzer = true in args.gn.

Running fuzzers

Local fuzzing

The tint_wgsl_fuzzer and tint_ir_fuzzer executables accept the standard libFuzzer command line arguments with extended command line arguments described below.

There's also a helper tool to run the fuzzers locally:

To run the local fuzzers across the full number of CPU threads available on the system, seeded with the corpus in test/tint, and using the dictionary in src/tint/cmd/fuzz/wgsl/dictionary.txt run:
tools/run fuzz
To check that all the test files in the corpus directory, test/tint by default, pass the fuzzers without error and then exit, run:
tools/run fuzz --check
Note: This is run by Dawn‘s CQ presubmit to check that fuzzers aren’t accidentally broken.
To run the local fuzzers using the same corpus used by ClusterFuzz:
tools/run fuzz -corpus out/libfuzz/gen/fuzzers/wgsl_corpus
Note that this corpus directory is generated when building the GN target tint_generate_wgsl_corpus.

Generating the corpus

Generate the tint_wgsl_fuzzer corpus using the tint_generate_wgsl_corpus GN target, which produces a corpus in <build_dir>/gen/fuzzers/wgsl_corpus. Pass in the path to the corpus directory as an argument to the fuzzer executable to use it. It's also a good idea to pass in the dictionary with -dict=src/tint/cmd/fuzz/wgsl/dictionary.txt:

autoninja -C out/libfuzz tint_generate_wgsl_corpus
out/libfuzz/tint_wgsl_fuzzer.exe -dict=src/tint/cmd/fuzz/wgsl/dictionary.txt out/libfuzz/gen/fuzzers/wgsl_corpus

Similarly, the tint_ir_fuzzer corpus can be generated using the tint_generate_ir_corpus GN target, producing the corpus in <build_dir>/gen/fuzzers/ir_corpus. For the IR fuzzer, we don't pass in the dictionary, since we are using lpm for mutating (the proto file effectively defines the dictionary):

autoninja -C out/libfuzz tint_generate_ir_corpus
out/libfuzz/tint_ir_fuzzer.exe out/libfuzz/gen/fuzzers/ir_corpus

Writing fuzzers

Registering a new `tint::Program` fuzzer

Create a new source file with a _fuzz.cc suffix.
#include "src/tint/cmd/fuzz/wgsl/fuzz.h"
Define a function in a (possibly nested) anonymous namespace with one of the signatures:
- void MyFuzzer(const tint::Program& program /*, ...additional fuzzed parameters... */) {
- void MyFuzzer(const tint::Program& program, const tint::fuzz::wgsl::Context& context /*, ...additional fuzzed parameters... */) {
The optional context parameter holds information about the Program and the environment used to run the fuzzers.
Note: Any number of additional fuzzer-populated parameters can be appended to the function signature.
Implement your fuzzer function, using TINT_ICE() to catch invalid state. Return early if the fuzzer cannot handle the input.
At the bottom of the file, in the global namespace, register the fuzzer with: TINT_WGSL_PROGRAM_FUZZER(MyFuzzer);
Use tools/run gen build to generate the build files for this new fuzzer.

Example:

#include "src/tint/cmd/fuzz/wgsl/fuzz.h"

namespace tint::my_namespace {
namespace {

bool CanRun(const tint::Program& program) {
    if (program.AST().HasOverrides()) {
        return false;  // Overrides are not supported.
    }
    return true;
}

void MyWGSLFuzzer(const tint::Program& program, bool a_fuzzer_provided_value) {
    if (!CanRun(program)) {
        return;
    }

    // Check something with program.
}

}  // namespace
}  // namespace tint::my_namespace

TINT_WGSL_PROGRAM_FUZZER(tint::my_namespace::MyWGSLFuzzer);

Registering a new `tint::core::ir::Module` fuzzer

Create a new source file with a _fuzz.cc suffix.
#include "src/tint/cmd/fuzz/ir/fuzz.h"
Define a function in a (possibly nested) anonymous namespace with the signature:
- void MyFuzzer(core::ir::Module& module /*, ...additional fuzzed parameters... */) {
Note: Any number of additional fuzzer-populated parameters can be appended to the function signature.
Implement your fuzzer function, using TINT_ICE() to catch invalid state. Return early if the fuzzer cannot handle the input.
At the bottom of the file, in the global namespace, register the fuzzer with: TINT_IR_MODULE_FUZZER(MyFuzzer);
Use tools/run gen build to generate the build files for this new fuzzer.

Example:

#include "src/tint/cmd/fuzz/ir/fuzz.h"

namespace tint::my_namespace {
namespace {

void MyIRFuzzer(core::ir::Module& module) {
    // Do something interesting with module.
}

}  // namespace
}  // namespace tint::my_namespace

TINT_IR_MODULE_FUZZER(tint::my_namespace::MyIRFuzzer);

Additional fuzzer data

WGSL and IR fuzzer functions can also declare any number of additional parameters, which will be populated with fuzzer provided data. These additional parameters must come at the end of the signatures described above, and can be of the following types:

Any integer, float or bool type.
Any structure reflected with TINT_REFLECT. Note: It's recommended to use a const reference, for these to avoid pass-by-value overheads.
Any enum reflected with TINT_REFLECT_ENUM_RANGE.

Executable targets

Tint has two fuzzer executable targets:

`tint_wgsl_fuzzer`

tint_wgsl_fuzzer accepts WGSL textual input and parses line comments (//) as a base-64 binary encoded data stream for the additional fuzzer parameters.

The entry point for the fuzzer lives at src/tint/cmd/fuzz/wgsl/main_fuzz.cc.

Extended command line arguments

On top of the standard libFuzzer command line arguments, the fuzzers support the following extended command line arguments:

--help: lists the command line arguments.
--filter=<name>: only runs the fuzzer functions that contain the given string in its name.
--concurrent: each of the fuzzer functions will be run on a separate, concurrent thread. This potentially offers performance improvements, and also tests for concurrent execution.
--verbose : prints verbose information about what the fuzzer is doing.
--dump : prints shader source, including input WGSL, and genreated HLSL, MSL, and GLSL.

Behavior

The tint_wgsl_fuzzer will do the following:

Base-64 decode the line comments data from the WGSL source, used to populate the additional fuzzer parameters.
Parse and resolve the WGSL input, and will early-return if there are any parser errors.
Invoke each of the fuzzer functions registered with a call to TINT_WGSL_PROGRAM_FUZZER()
Automatically convert the Program to an IR module and run the function for each function registered with TINT_IR_MODULE_FUZZER(). Note: The Program is converted to an IR module for each registered IR fuzzer as the module is mutable.

`tint_ir_fuzzer`

TODO: Document when landed.

Debugging

To debug a specific registered fuzzer function, one strategy is to add a TINT_ICE call at the top of the function, and then run the fuzzer with -filter <name> to have it only run that specific fuzzer. When the function is called, the libfuzzer harness will emit a crash file that can be used as input on subsequent runs. Remove the TINT_ICE and run the fuzzer again using this crash file.

For example, if we wish to debug tint::msl::writer::IRFuzzer, we would first insert a TINT_ICE at the top:

Result<SuccessType> IRFuzzer(core::ir::Module& module,
                             const fuzz::ir::Context& context,
                             Options options) {
    TINT_ICE() << "Crash";
    // Comment out the rest of the body to avoid unreachable code warnings
}

Build and run the fuzzer, filtering in this function:

autoninja -C out/libfuzz tint_wgsl_fuzzer
out/libfuzz/tint_wgsl_fuzzer -filter=tint::msl::writer::IRFuzzer

It can take a little while before libfuzzer generates a valid input WGSL, but eventually it will call into the function and crash on the ICE:

...
#71607  NEW    cov: 3633 ft: 8316 corp: 1045/8248b lim: 25 exec/s: 2469 rss: 158Mb L: 8/25 MS: 2 ShuffleBytes-PersAutoDict- DE: "true"-
#71669  NEW    cov: 3633 ft: 8317 corp: 1046/8266b lim: 25 exec/s: 2471 rss: 158Mb L: 18/25 MS: 2 CMP-ChangeByte- DE: "if"-
#71697  REDUCE cov: 3633 ft: 8317 corp: 1046/8264b lim: 25 exec/s: 2472 rss: 158Mb L: 6/25 MS: 3 PersAutoDict-ChangeBit-EraseBytes- DE: "\001\002"-
ICE while running fuzzer: 'tint::msl::writer::IRFuzzer'
..\..\src\tint\lang\msl\writer\writer_fuzz.cc:63 internal compiler error: Crash
==25204== ERROR: libFuzzer: deadly signal
NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
MS: 1 PersAutoDict- DE: "or"-; base unit: b34d87c378ebbbfbfd475303dcc75d1d1b2a7c7a
0x2f,0x2f,0x33,0x33,0x33,0x33,0x6f,0x72,0x33,0x33,0x33,0x33,0x33,0x33,0x33,0x33,0x33,0x33,0x2a,0x2a,0x30,0x32,
//3333or3333333333**02
artifact_prefix='./'; Test unit written to ./crash-21563a85afd5322d9e17c1c43fd3d4029778d6e7
Base64: Ly8zMzMzb3IzMzMzMzMzMzMzKiowMg==

Note that the second to last line specifies that the input test was written to a file. Now we can remove the TINT_ICE and run the fuzzer with just this file as input:

out/libfuzz/tint_wgsl_fuzzer ./crash-21563a85afd5322d9e17c1c43fd3d4029778d6e7