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Fuzzer: Pick an interesting function type for our functions #7936

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Oct 1, 2025
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Fuzzer: Pick an interesting function type for our functions #7936

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kripken Oct 1, 2025
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kripken Oct 1, 2025
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36 changes: 27 additions & 9 deletions src/tools/fuzzing/fuzzing.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1485,16 +1485,32 @@ Function* TranslateToFuzzReader::addFunction() {
auto allocation = std::make_unique<Function>();
auto* func = allocation.get();
func->name = Names::getValidFunctionName(wasm, "func");
Index numParams = upToSquared(fuzzParams->MAX_PARAMS);
std::vector<Type> params;
params.reserve(numParams);
for (Index i = 0; i < numParams; i++) {
auto type = getSingleConcreteType();
params.push_back(type);

// Pick params and results. There may be an interesting heap type we can use.
std::optional<HeapType> funcType;
auto& funcTypes = interestingHeapSubTypes[HeapTypes::func];
if (!funcTypes.empty() && oneIn(2)) {
auto type = pick(funcTypes);
if (type.getSignature().params.size() < (size_t)fuzzParams->MAX_PARAMS) {
// This is suitable for us.
funcType = type;
}
}
auto paramType = Type(params);
auto resultType = getControlFlowType();
func->type = Signature(paramType, resultType);
if (!funcType) {
// Generate a new type on the fly.
Index numParams = upToSquared(fuzzParams->MAX_PARAMS);
std::vector<Type> params;
params.reserve(numParams);
for (Index i = 0; i < numParams; i++) {
auto type = getSingleConcreteType();
params.push_back(type);
}
auto paramType = Type(params);
auto resultType = getControlFlowType();
funcType = Signature(paramType, resultType);
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Could also choose whether to make the new type open or not independently, but maybe it's not worth the effort.

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Yeah, I think it's ok to leave that part to the interesting heap types. Here we just create the simplest MVP-style type.

}
func->type = *funcType;

Index numVars = upToSquared(fuzzParams->MAX_VARS);
for (Index i = 0; i < numVars; i++) {
auto type = getConcreteType();
Expand Down Expand Up @@ -1524,6 +1540,8 @@ Function* TranslateToFuzzReader::addFunction() {
// at least one, though, to keep each testcase interesting. Avoid non-
// nullable params, as those cannot be constructed by the fuzzer on the
// outside.
auto paramType = func->getParams();
auto resultType = func->getResults();
bool validExportParams =
std::all_of(paramType.begin(), paramType.end(), [&](Type t) {
return t.isDefaultable() && isValidPublicType(t);
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