llvm/lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp

   1 //===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 ///
   9 /// \file
  10 /// Fix bitcasted functions.
  11 ///
  12 /// WebAssembly requires caller and callee signatures to match, however in LLVM,
  13 /// some amount of slop is vaguely permitted. Detect mismatch by looking for
  14 /// bitcasts of functions and rewrite them to use wrapper functions instead.
  15 ///
  16 /// This doesn't catch all cases, such as when a function's address is taken in
  17 /// one place and casted in another, but it works for many common cases.
  18 ///
  19 /// Note that LLVM already optimizes away function bitcasts in common cases by
  20 /// dropping arguments as needed, so this pass only ends up getting used in less
  21 /// common cases.
  22 ///
  23 //===----------------------------------------------------------------------===//
  24
  25 #include "WebAssembly.h"
  26 #include "llvm/IR/Constants.h"
  27 #include "llvm/IR/Instructions.h"
  28 #include "llvm/IR/Module.h"
  29 #include "llvm/IR/Operator.h"
  30 #include "llvm/Pass.h"
  31 #include "llvm/Support/Debug.h"
  32 #include "llvm/Support/raw_ostream.h"
  33 using namespace llvm;
  34
  35 #define DEBUG_TYPE "wasm-fix-function-bitcasts"
  36
  37 namespace {
  38 class FixFunctionBitcasts final : public ModulePass {
  39   StringRef getPassName() const override {
  40     return "WebAssembly Fix Function Bitcasts";
  41   }
  42
  43   void getAnalysisUsage(AnalysisUsage &AU) const override {
  44     AU.setPreservesCFG();
  45     ModulePass::getAnalysisUsage(AU);
  46   }
  47
  48   bool runOnModule(Module &M) override;
  49
  50 public:
  51   static char ID;
  52   FixFunctionBitcasts() : ModulePass(ID) {}
  53 };
  54 } // End anonymous namespace
  55
  56 char FixFunctionBitcasts::ID = 0;
  57 INITIALIZE_PASS(FixFunctionBitcasts, DEBUG_TYPE,
  58                 "Fix mismatching bitcasts for WebAssembly", false, false)
  59
  60 ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() {
  61   return new FixFunctionBitcasts();
  62 }
  63
  64 // Recursively descend the def-use lists from V to find non-bitcast users of
  65 // bitcasts of V.
  66 static void findUses(Value *V, Function &F,
  67                      SmallVectorImpl<std::pair<Use *, Function *>> &Uses,
  68                      SmallPtrSetImpl<Constant *> &ConstantBCs) {
  69   for (Use &U : V->uses()) {
  70     if (auto *BC = dyn_cast<BitCastOperator>(U.getUser()))
  71       findUses(BC, F, Uses, ConstantBCs);
  72     else if (auto *A = dyn_cast<GlobalAlias>(U.getUser()))
  73       findUses(A, F, Uses, ConstantBCs);
  74     else if (U.get()->getType() != F.getType()) {
  75       CallBase *CB = dyn_cast<CallBase>(U.getUser());
  76       if (!CB)
  77         // Skip uses that aren't immediately called
  78         continue;
  79       Value *Callee = CB->getCalledOperand();
  80       if (Callee != V)
  81         // Skip calls where the function isn't the callee
  82         continue;
  83       if (isa<Constant>(U.get())) {
  84         // Only add constant bitcasts to the list once; they get RAUW'd
  85         auto C = ConstantBCs.insert(cast<Constant>(U.get()));
  86         if (!C.second)
  87           continue;
  88       }
  89       Uses.push_back(std::make_pair(&U, &F));
  90     }
  91   }
  92 }
  93
  94 // Create a wrapper function with type Ty that calls F (which may have a
  95 // different type). Attempt to support common bitcasted function idioms:
  96 //  - Call with more arguments than needed: arguments are dropped
  97 //  - Call with fewer arguments than needed: arguments are filled in with undef
  98 //  - Return value is not needed: drop it
  99 //  - Return value needed but not present: supply an undef
 100 //
 101 // If the all the argument types of trivially castable to one another (i.e.
 102 // I32 vs pointer type) then we don't create a wrapper at all (return nullptr
 103 // instead).
 104 //
 105 // If there is a type mismatch that we know would result in an invalid wasm
 106 // module then generate wrapper that contains unreachable (i.e. abort at
 107 // runtime).  Such programs are deep into undefined behaviour territory,
 108 // but we choose to fail at runtime rather than generate and invalid module
 109 // or fail at compiler time.  The reason we delay the error is that we want
 110 // to support the CMake which expects to be able to compile and link programs
 111 // that refer to functions with entirely incorrect signatures (this is how
 112 // CMake detects the existence of a function in a toolchain).
 113 //
 114 // For bitcasts that involve struct types we don't know at this stage if they
 115 // would be equivalent at the wasm level and so we can't know if we need to
 116 // generate a wrapper.
 117 static Function *createWrapper(Function *F, FunctionType *Ty) {
 118   Module *M = F->getParent();
 119
 120   Function *Wrapper = Function::Create(Ty, Function::PrivateLinkage,
 121                                        F->getName() + "_bitcast", M);
 122   BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
 123   const DataLayout &DL = BB->getModule()->getDataLayout();
 124
 125   // Determine what arguments to pass.
 126   SmallVector<Value *, 4> Args;
 127   Function::arg_iterator AI = Wrapper->arg_begin();
 128   Function::arg_iterator AE = Wrapper->arg_end();
 129   FunctionType::param_iterator PI = F->getFunctionType()->param_begin();
 130   FunctionType::param_iterator PE = F->getFunctionType()->param_end();
 131   bool TypeMismatch = false;
 132   bool WrapperNeeded = false;
 133
 134   Type *ExpectedRtnType = F->getFunctionType()->getReturnType();
 135   Type *RtnType = Ty->getReturnType();
 136
 137   if ((F->getFunctionType()->getNumParams() != Ty->getNumParams()) ||
 138       (F->getFunctionType()->isVarArg() != Ty->isVarArg()) ||
 139       (ExpectedRtnType != RtnType))
 140     WrapperNeeded = true;
 141
 142   for (; AI != AE && PI != PE; ++AI, ++PI) {
 143     Type *ArgType = AI->getType();
 144     Type *ParamType = *PI;
 145
 146     if (ArgType == ParamType) {
 147       Args.push_back(&*AI);
 148     } else {
 149       if (CastInst::isBitOrNoopPointerCastable(ArgType, ParamType, DL)) {
 150         Instruction *PtrCast =
 151             CastInst::CreateBitOrPointerCast(AI, ParamType, "cast");
 152         BB->getInstList().push_back(PtrCast);
 153         Args.push_back(PtrCast);
 154       } else if (ArgType->isStructTy() || ParamType->isStructTy()) {
 155         LLVM_DEBUG(dbgs() << "createWrapper: struct param type in bitcast: "
 156                           << F->getName() << "\n");
 157         WrapperNeeded = false;
 158       } else {
 159         LLVM_DEBUG(dbgs() << "createWrapper: arg type mismatch calling: "
 160                           << F->getName() << "\n");
 161         LLVM_DEBUG(dbgs() << "Arg[" << Args.size() << "] Expected: "
 162                           << *ParamType << " Got: " << *ArgType << "\n");
 163         TypeMismatch = true;
 164         break;
 165       }
 166     }
 167   }
 168
 169   if (WrapperNeeded && !TypeMismatch) {
 170     for (; PI != PE; ++PI)
 171       Args.push_back(UndefValue::get(*PI));
 172     if (F->isVarArg())
 173       for (; AI != AE; ++AI)
 174         Args.push_back(&*AI);
 175
 176     CallInst *Call = CallInst::Create(F, Args, "", BB);
 177
 178     Type *ExpectedRtnType = F->getFunctionType()->getReturnType();
 179     Type *RtnType = Ty->getReturnType();
 180     // Determine what value to return.
 181     if (RtnType->isVoidTy()) {
 182       ReturnInst::Create(M->getContext(), BB);
 183     } else if (ExpectedRtnType->isVoidTy()) {
 184       LLVM_DEBUG(dbgs() << "Creating dummy return: " << *RtnType << "\n");
 185       ReturnInst::Create(M->getContext(), UndefValue::get(RtnType), BB);
 186     } else if (RtnType == ExpectedRtnType) {
 187       ReturnInst::Create(M->getContext(), Call, BB);
 188     } else if (CastInst::isBitOrNoopPointerCastable(ExpectedRtnType, RtnType,
 189                                                     DL)) {
 190       Instruction *Cast =
 191           CastInst::CreateBitOrPointerCast(Call, RtnType, "cast");
 192       BB->getInstList().push_back(Cast);
 193       ReturnInst::Create(M->getContext(), Cast, BB);
 194     } else if (RtnType->isStructTy() || ExpectedRtnType->isStructTy()) {
 195       LLVM_DEBUG(dbgs() << "createWrapper: struct return type in bitcast: "
 196                         << F->getName() << "\n");
 197       WrapperNeeded = false;
 198     } else {
 199       LLVM_DEBUG(dbgs() << "createWrapper: return type mismatch calling: "
 200                         << F->getName() << "\n");
 201       LLVM_DEBUG(dbgs() << "Expected: " << *ExpectedRtnType
 202                         << " Got: " << *RtnType << "\n");
 203       TypeMismatch = true;
 204     }
 205   }
 206
 207   if (TypeMismatch) {
 208     // Create a new wrapper that simply contains `unreachable`.
 209     Wrapper->eraseFromParent();
 210     Wrapper = Function::Create(Ty, Function::PrivateLinkage,
 211                                F->getName() + "_bitcast_invalid", M);
 212     BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
 213     new UnreachableInst(M->getContext(), BB);
 214     Wrapper->setName(F->getName() + "_bitcast_invalid");
 215   } else if (!WrapperNeeded) {
 216     LLVM_DEBUG(dbgs() << "createWrapper: no wrapper needed: " << F->getName()
 217                       << "\n");
 218     Wrapper->eraseFromParent();
 219     return nullptr;
 220   }
 221   LLVM_DEBUG(dbgs() << "createWrapper: " << F->getName() << "\n");
 222   return Wrapper;
 223 }
 224
 225 // Test whether a main function with type FuncTy should be rewritten to have
 226 // type MainTy.
 227 static bool shouldFixMainFunction(FunctionType *FuncTy, FunctionType *MainTy) {
 228   // Only fix the main function if it's the standard zero-arg form. That way,
 229   // the standard cases will work as expected, and users will see signature
 230   // mismatches from the linker for non-standard cases.
 231   return FuncTy->getReturnType() == MainTy->getReturnType() &&
 232          FuncTy->getNumParams() == 0 &&
 233          !FuncTy->isVarArg();
 234 }
 235
 236 bool FixFunctionBitcasts::runOnModule(Module &M) {
 237   LLVM_DEBUG(dbgs() << "********** Fix Function Bitcasts **********\n");
 238
 239   Function *Main = nullptr;
 240   CallInst *CallMain = nullptr;
 241   SmallVector<std::pair<Use *, Function *>, 0> Uses;
 242   SmallPtrSet<Constant *, 2> ConstantBCs;
 243
 244   // Collect all the places that need wrappers.
 245   for (Function &F : M) {
 246     // Skip to fix when the function is swiftcc because swiftcc allows
 247     // bitcast type difference for swiftself and swifterror.
 248     if (F.getCallingConv() == CallingConv::Swift)
 249       continue;
 250     findUses(&F, F, Uses, ConstantBCs);
 251
 252     // If we have a "main" function, and its type isn't
 253     // "int main(int argc, char *argv[])", create an artificial call with it
 254     // bitcasted to that type so that we generate a wrapper for it, so that
 255     // the C runtime can call it.
 256     if (F.getName() == "main") {
 257       Main = &F;
 258       LLVMContext &C = M.getContext();
 259       Type *MainArgTys[] = {Type::getInt32Ty(C),
 260                             PointerType::get(Type::getInt8PtrTy(C), 0)};
 261       FunctionType *MainTy = FunctionType::get(Type::getInt32Ty(C), MainArgTys,
 262                                                /*isVarArg=*/false);
 263       if (shouldFixMainFunction(F.getFunctionType(), MainTy)) {
 264         LLVM_DEBUG(dbgs() << "Found `main` function with incorrect type: "
 265                           << *F.getFunctionType() << "\n");
 266         Value *Args[] = {UndefValue::get(MainArgTys[0]),
 267                          UndefValue::get(MainArgTys[1])};
 268         Value *Casted =
 269             ConstantExpr::getBitCast(Main, PointerType::get(MainTy, 0));
 270         CallMain = CallInst::Create(MainTy, Casted, Args, "call_main");
 271         Use *UseMain = &CallMain->getOperandUse(2);
 272         Uses.push_back(std::make_pair(UseMain, &F));
 273       }
 274     }
 275   }
 276
 277   DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers;
 278
 279   for (auto &UseFunc : Uses) {
 280     Use *U = UseFunc.first;
 281     Function *F = UseFunc.second;
 282     auto *PTy = cast<PointerType>(U->get()->getType());
 283     auto *Ty = dyn_cast<FunctionType>(PTy->getElementType());
 284
 285     // If the function is casted to something like i8* as a "generic pointer"
 286     // to be later casted to something else, we can't generate a wrapper for it.
 287     // Just ignore such casts for now.
 288     if (!Ty)
 289       continue;
 290
 291     auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
 292     if (Pair.second)
 293       Pair.first->second = createWrapper(F, Ty);
 294
 295     Function *Wrapper = Pair.first->second;
 296     if (!Wrapper)
 297       continue;
 298
 299     if (isa<Constant>(U->get()))
 300       U->get()->replaceAllUsesWith(Wrapper);
 301     else
 302       U->set(Wrapper);
 303   }
 304
 305   // If we created a wrapper for main, rename the wrapper so that it's the
 306   // one that gets called from startup.
 307   if (CallMain) {
 308     Main->setName("__original_main");
 309     auto *MainWrapper =
 310         cast<Function>(CallMain->getCalledOperand()->stripPointerCasts());
 311     delete CallMain;
 312     if (Main->isDeclaration()) {
 313       // The wrapper is not needed in this case as we don't need to export
 314       // it to anyone else.
 315       MainWrapper->eraseFromParent();
 316     } else {
 317       // Otherwise give the wrapper the same linkage as the original main
 318       // function, so that it can be called from the same places.
 319       MainWrapper->setName("main");
 320       MainWrapper->setLinkage(Main->getLinkage());
 321       MainWrapper->setVisibility(Main->getVisibility());
 322     }
 323   }
 324
 325   return true;
 326 }