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//===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// Fix bitcasted functions.
///
/// WebAssembly requires caller and callee signatures to match, however in LLVM,
/// some amount of slop is vaguely permitted. Detect mismatch by looking for
/// bitcasts of functions and rewrite them to use wrapper functions instead.
///
/// This doesn't catch all cases, such as when a function's address is taken in
/// one place and casted in another, but it works for many common cases.
///
/// Note that LLVM already optimizes away function bitcasts in common cases by
/// dropping arguments as needed, so this pass only ends up getting used in less
/// common cases.
///
//===----------------------------------------------------------------------===//
#include "WebAssembly.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "wasm-fix-function-bitcasts"
namespace {
class FixFunctionBitcasts final : public ModulePass {
StringRef getPassName() const override {
return "WebAssembly Fix Function Bitcasts";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
ModulePass::getAnalysisUsage(AU);
}
bool runOnModule(Module &M) override;
public:
static char ID;
FixFunctionBitcasts() : ModulePass(ID) {}
};
} // End anonymous namespace
char FixFunctionBitcasts::ID = 0;
INITIALIZE_PASS(FixFunctionBitcasts, DEBUG_TYPE,
"Fix mismatching bitcasts for WebAssembly", false, false)
ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() {
return new FixFunctionBitcasts();
}
// Recursively descend the def-use lists from V to find non-bitcast users of
// bitcasts of V.
static void findUses(Value *V, Function &F,
SmallVectorImpl<std::pair<Use *, Function *>> &Uses,
SmallPtrSetImpl<Constant *> &ConstantBCs) {
for (Use &U : V->uses()) {
if (auto *BC = dyn_cast<BitCastOperator>(U.getUser()))
findUses(BC, F, Uses, ConstantBCs);
else if (auto *A = dyn_cast<GlobalAlias>(U.getUser()))
findUses(A, F, Uses, ConstantBCs);
else if (U.get()->getType() != F.getType()) {
CallSite CS(U.getUser());
if (!CS)
// Skip uses that aren't immediately called
continue;
Value *Callee = CS.getCalledValue();
if (Callee != V)
// Skip calls where the function isn't the callee
continue;
if (isa<Constant>(U.get())) {
// Only add constant bitcasts to the list once; they get RAUW'd
auto C = ConstantBCs.insert(cast<Constant>(U.get()));
if (!C.second)
continue;
}
Uses.push_back(std::make_pair(&U, &F));
}
}
}
// Create a wrapper function with type Ty that calls F (which may have a
// different type). Attempt to support common bitcasted function idioms:
// - Call with more arguments than needed: arguments are dropped
// - Call with fewer arguments than needed: arguments are filled in with undef
// - Return value is not needed: drop it
// - Return value needed but not present: supply an undef
//
// If the all the argument types of trivially castable to one another (i.e.
// I32 vs pointer type) then we don't create a wrapper at all (return nullptr
// instead).
//
// If there is a type mismatch that we know would result in an invalid wasm
// module then generate wrapper that contains unreachable (i.e. abort at
// runtime). Such programs are deep into undefined behaviour territory,
// but we choose to fail at runtime rather than generate and invalid module
// or fail at compiler time. The reason we delay the error is that we want
// to support the CMake which expects to be able to compile and link programs
// that refer to functions with entirely incorrect signatures (this is how
// CMake detects the existence of a function in a toolchain).
//
// For bitcasts that involve struct types we don't know at this stage if they
// would be equivalent at the wasm level and so we can't know if we need to
// generate a wrapper.
static Function *createWrapper(Function *F, FunctionType *Ty) {
Module *M = F->getParent();
Function *Wrapper = Function::Create(Ty, Function::PrivateLinkage,
F->getName() + "_bitcast", M);
BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
const DataLayout &DL = BB->getModule()->getDataLayout();
// Determine what arguments to pass.
SmallVector<Value *, 4> Args;
Function::arg_iterator AI = Wrapper->arg_begin();
Function::arg_iterator AE = Wrapper->arg_end();
FunctionType::param_iterator PI = F->getFunctionType()->param_begin();
FunctionType::param_iterator PE = F->getFunctionType()->param_end();
bool TypeMismatch = false;
bool WrapperNeeded = false;
Type *ExpectedRtnType = F->getFunctionType()->getReturnType();
Type *RtnType = Ty->getReturnType();
if ((F->getFunctionType()->getNumParams() != Ty->getNumParams()) ||
(F->getFunctionType()->isVarArg() != Ty->isVarArg()) ||
(ExpectedRtnType != RtnType))
WrapperNeeded = true;
for (; AI != AE && PI != PE; ++AI, ++PI) {
Type *ArgType = AI->getType();
Type *ParamType = *PI;
if (ArgType == ParamType) {
Args.push_back(&*AI);
} else {
if (CastInst::isBitOrNoopPointerCastable(ArgType, ParamType, DL)) {
Instruction *PtrCast =
CastInst::CreateBitOrPointerCast(AI, ParamType, "cast");
BB->getInstList().push_back(PtrCast);
Args.push_back(PtrCast);
} else if (ArgType->isStructTy() || ParamType->isStructTy()) {
LLVM_DEBUG(dbgs() << "createWrapper: struct param type in bitcast: "
<< F->getName() << "\n");
WrapperNeeded = false;
} else {
LLVM_DEBUG(dbgs() << "createWrapper: arg type mismatch calling: "
<< F->getName() << "\n");
LLVM_DEBUG(dbgs() << "Arg[" << Args.size() << "] Expected: "
<< *ParamType << " Got: " << *ArgType << "\n");
TypeMismatch = true;
break;
}
}
}
if (WrapperNeeded && !TypeMismatch) {
for (; PI != PE; ++PI)
Args.push_back(UndefValue::get(*PI));
if (F->isVarArg())
for (; AI != AE; ++AI)
Args.push_back(&*AI);
CallInst *Call = CallInst::Create(F, Args, "", BB);
Type *ExpectedRtnType = F->getFunctionType()->getReturnType();
Type *RtnType = Ty->getReturnType();
// Determine what value to return.
if (RtnType->isVoidTy()) {
ReturnInst::Create(M->getContext(), BB);
} else if (ExpectedRtnType->isVoidTy()) {
LLVM_DEBUG(dbgs() << "Creating dummy return: " << *RtnType << "\n");
ReturnInst::Create(M->getContext(), UndefValue::get(RtnType), BB);
} else if (RtnType == ExpectedRtnType) {
ReturnInst::Create(M->getContext(), Call, BB);
} else if (CastInst::isBitOrNoopPointerCastable(ExpectedRtnType, RtnType,
DL)) {
Instruction *Cast =
CastInst::CreateBitOrPointerCast(Call, RtnType, "cast");
BB->getInstList().push_back(Cast);
ReturnInst::Create(M->getContext(), Cast, BB);
} else if (RtnType->isStructTy() || ExpectedRtnType->isStructTy()) {
LLVM_DEBUG(dbgs() << "createWrapper: struct return type in bitcast: "
<< F->getName() << "\n");
WrapperNeeded = false;
} else {
LLVM_DEBUG(dbgs() << "createWrapper: return type mismatch calling: "
<< F->getName() << "\n");
LLVM_DEBUG(dbgs() << "Expected: " << *ExpectedRtnType
<< " Got: " << *RtnType << "\n");
TypeMismatch = true;
}
}
if (TypeMismatch) {
// Create a new wrapper that simply contains `unreachable`.
Wrapper->eraseFromParent();
Wrapper = Function::Create(Ty, Function::PrivateLinkage,
F->getName() + "_bitcast_invalid", M);
BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
new UnreachableInst(M->getContext(), BB);
Wrapper->setName(F->getName() + "_bitcast_invalid");
} else if (!WrapperNeeded) {
LLVM_DEBUG(dbgs() << "createWrapper: no wrapper needed: " << F->getName()
<< "\n");
Wrapper->eraseFromParent();
return nullptr;
}
LLVM_DEBUG(dbgs() << "createWrapper: " << F->getName() << "\n");
return Wrapper;
}
// Test whether a main function with type FuncTy should be rewritten to have
// type MainTy.
static bool shouldFixMainFunction(FunctionType *FuncTy, FunctionType *MainTy) {
// Only fix the main function if it's the standard zero-arg form. That way,
// the standard cases will work as expected, and users will see signature
// mismatches from the linker for non-standard cases.
return FuncTy->getReturnType() == MainTy->getReturnType() &&
FuncTy->getNumParams() == 0 &&
!FuncTy->isVarArg();
}
bool FixFunctionBitcasts::runOnModule(Module &M) {
LLVM_DEBUG(dbgs() << "********** Fix Function Bitcasts **********\n");
Function *Main = nullptr;
CallInst *CallMain = nullptr;
SmallVector<std::pair<Use *, Function *>, 0> Uses;
SmallPtrSet<Constant *, 2> ConstantBCs;
// Collect all the places that need wrappers.
for (Function &F : M) {
findUses(&F, F, Uses, ConstantBCs);
// If we have a "main" function, and its type isn't
// "int main(int argc, char *argv[])", create an artificial call with it
// bitcasted to that type so that we generate a wrapper for it, so that
// the C runtime can call it.
if (F.getName() == "main") {
Main = &F;
LLVMContext &C = M.getContext();
Type *MainArgTys[] = {Type::getInt32Ty(C),
PointerType::get(Type::getInt8PtrTy(C), 0)};
FunctionType *MainTy = FunctionType::get(Type::getInt32Ty(C), MainArgTys,
/*isVarArg=*/false);
if (shouldFixMainFunction(F.getFunctionType(), MainTy)) {
LLVM_DEBUG(dbgs() << "Found `main` function with incorrect type: "
<< *F.getFunctionType() << "\n");
Value *Args[] = {UndefValue::get(MainArgTys[0]),
UndefValue::get(MainArgTys[1])};
Value *Casted =
ConstantExpr::getBitCast(Main, PointerType::get(MainTy, 0));
CallMain = CallInst::Create(MainTy, Casted, Args, "call_main");
Use *UseMain = &CallMain->getOperandUse(2);
Uses.push_back(std::make_pair(UseMain, &F));
}
}
}
DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers;
for (auto &UseFunc : Uses) {
Use *U = UseFunc.first;
Function *F = UseFunc.second;
auto *PTy = cast<PointerType>(U->get()->getType());
auto *Ty = dyn_cast<FunctionType>(PTy->getElementType());
// If the function is casted to something like i8* as a "generic pointer"
// to be later casted to something else, we can't generate a wrapper for it.
// Just ignore such casts for now.
if (!Ty)
continue;
auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
if (Pair.second)
Pair.first->second = createWrapper(F, Ty);
Function *Wrapper = Pair.first->second;
if (!Wrapper)
continue;
if (isa<Constant>(U->get()))
U->get()->replaceAllUsesWith(Wrapper);
else
U->set(Wrapper);
}
// If we created a wrapper for main, rename the wrapper so that it's the
// one that gets called from startup.
if (CallMain) {
Main->setName("__original_main");
auto *MainWrapper =
cast<Function>(CallMain->getCalledValue()->stripPointerCasts());
delete CallMain;
if (Main->isDeclaration()) {
// The wrapper is not needed in this case as we don't need to export
// it to anyone else.
MainWrapper->eraseFromParent();
} else {
// Otherwise give the wrapper the same linkage as the original main
// function, so that it can be called from the same places.
MainWrapper->setName("main");
MainWrapper->setLinkage(Main->getLinkage());
MainWrapper->setVisibility(Main->getVisibility());
}
}
return true;
}