//===-- ShadowStackCollector.cpp - GC support for uncooperative targets ---===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This file implements lowering for the llvm.gc* intrinsics for targets that do // not natively support them (which includes the C backend). Note that the code // generated is not quite as efficient as collectors which generate stack maps // to identify roots. // // This pass implements the code transformation described in this paper: // "Accurate Garbage Collection in an Uncooperative Environment" // Fergus Henderson, ISMM, 2002 // // In runtime/GC/SemiSpace.cpp is a prototype runtime which is compatible with // this collector. // // In order to support this particular transformation, all stack roots are // coallocated in the stack. This allows a fully target-independent stack map // while introducing only minor runtime overhead. // //===----------------------------------------------------------------------===// #define DEBUG_TYPE "shadowstackgc" #include "llvm/CodeGen/Collectors.h" #include "llvm/ADT/StringExtras.h" #include "llvm/CodeGen/Collector.h" #include "llvm/IntrinsicInst.h" #include "llvm/Module.h" #include "llvm/Support/IRBuilder.h" using namespace llvm; namespace { class VISIBILITY_HIDDEN ShadowStackCollector : public Collector { /// RootChain - This is the global linked-list that contains the chain of GC /// roots. GlobalVariable *Head; /// StackEntryTy - Abstract type of a link in the shadow stack. /// const StructType *StackEntryTy; /// Roots - GC roots in the current function. Each is a pair of the /// intrinsic call and its corresponding alloca. std::vector > Roots; public: ShadowStackCollector(); bool initializeCustomLowering(Module &M); bool performCustomLowering(Function &F); private: bool IsNullValue(Value *V); Constant *GetFrameMap(Function &F); const Type* GetConcreteStackEntryType(Function &F); void CollectRoots(Function &F); static GetElementPtrInst *CreateGEP(IRBuilder<> &B, Value *BasePtr, int Idx1, const char *Name); static GetElementPtrInst *CreateGEP(IRBuilder<> &B, Value *BasePtr, int Idx1, int Idx2, const char *Name); }; } static CollectorRegistry::Add Y("shadow-stack", "Very portable collector for uncooperative code generators"); namespace { /// EscapeEnumerator - This is a little algorithm to find all escape points /// from a function so that "finally"-style code can be inserted. In addition /// to finding the existing return and unwind instructions, it also (if /// necessary) transforms any call instructions into invokes and sends them to /// a landing pad. /// /// It's wrapped up in a state machine using the same transform C# uses for /// 'yield return' enumerators, This transform allows it to be non-allocating. class VISIBILITY_HIDDEN EscapeEnumerator { Function &F; const char *CleanupBBName; // State. int State; Function::iterator StateBB, StateE; IRBuilder<> Builder; public: EscapeEnumerator(Function &F, const char *N = "cleanup") : F(F), CleanupBBName(N), State(0) {} IRBuilder<> *Next() { switch (State) { default: return 0; case 0: StateBB = F.begin(); StateE = F.end(); State = 1; case 1: // Find all 'return' and 'unwind' instructions. while (StateBB != StateE) { BasicBlock *CurBB = StateBB++; // Branches and invokes do not escape, only unwind and return do. TerminatorInst *TI = CurBB->getTerminator(); if (!isa(TI) && !isa(TI)) continue; Builder.SetInsertPoint(TI->getParent(), TI); return &Builder; } State = 2; // Find all 'call' instructions. SmallVector Calls; for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) for (BasicBlock::iterator II = BB->begin(), EE = BB->end(); II != EE; ++II) if (CallInst *CI = dyn_cast(II)) if (!CI->getCalledFunction() || !CI->getCalledFunction()->getIntrinsicID()) Calls.push_back(CI); if (Calls.empty()) return 0; // Create a cleanup block. BasicBlock *CleanupBB = BasicBlock::Create(CleanupBBName, &F); UnwindInst *UI = new UnwindInst(CleanupBB); // Transform the 'call' instructions into 'invoke's branching to the // cleanup block. Go in reverse order to make prettier BB names. SmallVector Args; for (unsigned I = Calls.size(); I != 0; ) { CallInst *CI = cast(Calls[--I]); // Split the basic block containing the function call. BasicBlock *CallBB = CI->getParent(); BasicBlock *NewBB = CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont"); // Remove the unconditional branch inserted at the end of CallBB. CallBB->getInstList().pop_back(); NewBB->getInstList().remove(CI); // Create a new invoke instruction. Args.clear(); Args.append(CI->op_begin() + 1, CI->op_end()); InvokeInst *II = InvokeInst::Create(CI->getOperand(0), NewBB, CleanupBB, Args.begin(), Args.end(), CI->getName(), CallBB); II->setCallingConv(CI->getCallingConv()); II->setParamAttrs(CI->getParamAttrs()); CI->replaceAllUsesWith(II); delete CI; } Builder.SetInsertPoint(UI->getParent(), UI); return &Builder; } } }; } // ----------------------------------------------------------------------------- Collector *llvm::createShadowStackCollector() { return new ShadowStackCollector(); } ShadowStackCollector::ShadowStackCollector() : Head(0), StackEntryTy(0) { InitRoots = true; CustomRoots = true; } Constant *ShadowStackCollector::GetFrameMap(Function &F) { // doInitialization creates the abstract type of this value. Type *VoidPtr = PointerType::getUnqual(Type::Int8Ty); // Truncate the ShadowStackDescriptor if some metadata is null. unsigned NumMeta = 0; SmallVector Metadata; for (unsigned I = 0; I != Roots.size(); ++I) { Constant *C = cast(Roots[I].first->getOperand(2)); if (!C->isNullValue()) NumMeta = I + 1; Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); } Constant *BaseElts[] = { ConstantInt::get(Type::Int32Ty, Roots.size(), false), ConstantInt::get(Type::Int32Ty, NumMeta, false), }; Constant *DescriptorElts[] = { ConstantStruct::get(BaseElts, 2), ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata.begin(), NumMeta) }; Constant *FrameMap = ConstantStruct::get(DescriptorElts, 2); std::string TypeName("gc_map."); TypeName += utostr(NumMeta); F.getParent()->addTypeName(TypeName, FrameMap->getType()); // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems // that, short of multithreaded LLVM, it should be safe; all that is // necessary is that a simple Module::iterator loop not be invalidated. // Appending to the GlobalVariable list is safe in that sense. // // All of the output passes emit globals last. The ExecutionEngine // explicitly supports adding globals to the module after // initialization. // // Still, if it isn't deemed acceptable, then this transformation needs // to be a ModulePass (which means it cannot be in the 'llc' pipeline // (which uses a FunctionPassManager (which segfaults (not asserts) if // provided a ModulePass))). Constant *GV = new GlobalVariable(FrameMap->getType(), true, GlobalVariable::InternalLinkage, FrameMap, "__gc_" + F.getName(), F.getParent()); Constant *GEPIndices[2] = { ConstantInt::get(Type::Int32Ty, 0), ConstantInt::get(Type::Int32Ty, 0) }; return ConstantExpr::getGetElementPtr(GV, GEPIndices, 2); } const Type* ShadowStackCollector::GetConcreteStackEntryType(Function &F) { // doInitialization creates the generic version of this type. std::vector EltTys; EltTys.push_back(StackEntryTy); for (size_t I = 0; I != Roots.size(); I++) EltTys.push_back(Roots[I].second->getAllocatedType()); Type *Ty = StructType::get(EltTys); std::string TypeName("gc_stackentry."); TypeName += F.getName(); F.getParent()->addTypeName(TypeName, Ty); return Ty; } /// doInitialization - If this module uses the GC intrinsics, find them now. If /// not, exit fast. bool ShadowStackCollector::initializeCustomLowering(Module &M) { // struct FrameMap { // int32_t NumRoots; // Number of roots in stack frame. // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots. // void *Meta[]; // May be absent for roots without metadata. // }; std::vector EltTys; EltTys.push_back(Type::Int32Ty); // 32 bits is ok up to a 32GB stack frame. :) EltTys.push_back(Type::Int32Ty); // Specifies length of variable length array. StructType *FrameMapTy = StructType::get(EltTys); M.addTypeName("gc_map", FrameMapTy); PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy); // struct StackEntry { // ShadowStackEntry *Next; // Caller's stack entry. // FrameMap *Map; // Pointer to constant FrameMap. // void *Roots[]; // Stack roots (in-place array, so we pretend). // }; OpaqueType *RecursiveTy = OpaqueType::get(); EltTys.clear(); EltTys.push_back(PointerType::getUnqual(RecursiveTy)); EltTys.push_back(FrameMapPtrTy); PATypeHolder LinkTyH = StructType::get(EltTys); RecursiveTy->refineAbstractTypeTo(LinkTyH.get()); StackEntryTy = cast(LinkTyH.get()); const PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); M.addTypeName("gc_stackentry", LinkTyH.get()); // FIXME: Is this safe from // a FunctionPass? // Get the root chain if it already exists. Head = M.getGlobalVariable("llvm_gc_root_chain"); if (!Head) { // If the root chain does not exist, insert a new one with linkonce // linkage! Head = new GlobalVariable(StackEntryPtrTy, false, GlobalValue::LinkOnceLinkage, Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain", &M); } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { Head->setInitializer(Constant::getNullValue(StackEntryPtrTy)); Head->setLinkage(GlobalValue::LinkOnceLinkage); } return true; } bool ShadowStackCollector::IsNullValue(Value *V) { if (Constant *C = dyn_cast(V)) return C->isNullValue(); return false; } void ShadowStackCollector::CollectRoots(Function &F) { // FIXME: Account for original alignment. Could fragment the root array. // Approach 1: Null initialize empty slots at runtime. Yuck. // Approach 2: Emit a map of the array instead of just a count. assert(Roots.empty() && "Not cleaned up?"); SmallVector,16> MetaRoots; for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) if (IntrinsicInst *CI = dyn_cast(II++)) if (Function *F = CI->getCalledFunction()) if (F->getIntrinsicID() == Intrinsic::gcroot) { std::pair Pair = std::make_pair( CI, cast(CI->getOperand(1)->stripPointerCasts())); if (IsNullValue(CI->getOperand(2))) Roots.push_back(Pair); else MetaRoots.push_back(Pair); } // Number roots with metadata (usually empty) at the beginning, so that the // FrameMap::Meta array can be elided. Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end()); } GetElementPtrInst * ShadowStackCollector::CreateGEP(IRBuilder<> &B, Value *BasePtr, int Idx, int Idx2, const char *Name) { Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0), ConstantInt::get(Type::Int32Ty, Idx), ConstantInt::get(Type::Int32Ty, Idx2) }; Value* Val = B.CreateGEP(BasePtr, Indices, Indices + 3, Name); assert(isa(Val) && "Unexpected folded constant"); return dyn_cast(Val); } GetElementPtrInst * ShadowStackCollector::CreateGEP(IRBuilder<> &B, Value *BasePtr, int Idx, const char *Name) { Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0), ConstantInt::get(Type::Int32Ty, Idx) }; Value *Val = B.CreateGEP(BasePtr, Indices, Indices + 2, Name); assert(isa(Val) && "Unexpected folded constant"); return dyn_cast(Val); } /// runOnFunction - Insert code to maintain the shadow stack. bool ShadowStackCollector::performCustomLowering(Function &F) { // Find calls to llvm.gcroot. CollectRoots(F); // If there are no roots in this function, then there is no need to add a // stack map entry for it. if (Roots.empty()) return false; // Build the constant map and figure the type of the shadow stack entry. Value *FrameMap = GetFrameMap(F); const Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F); // Build the shadow stack entry at the very start of the function. BasicBlock::iterator IP = F.getEntryBlock().begin(); IRBuilder<> AtEntry(IP->getParent(), IP); Instruction *StackEntry = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0, "gc_frame"); while (isa(IP)) ++IP; AtEntry.SetInsertPoint(IP->getParent(), IP); // Initialize the map pointer and load the current head of the shadow stack. Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead"); Instruction *EntryMapPtr = CreateGEP(AtEntry, StackEntry,0,1,"gc_frame.map"); AtEntry.CreateStore(FrameMap, EntryMapPtr); // After all the allocas... for (unsigned I = 0, E = Roots.size(); I != E; ++I) { // For each root, find the corresponding slot in the aggregate... Value *SlotPtr = CreateGEP(AtEntry, StackEntry, 1 + I, "gc_root"); // And use it in lieu of the alloca. AllocaInst *OriginalAlloca = Roots[I].second; SlotPtr->takeName(OriginalAlloca); OriginalAlloca->replaceAllUsesWith(SlotPtr); } // Move past the original stores inserted by Collector::InitRoots. This isn't // really necessary (the collector would never see the intermediate state), // but it's nicer not to push the half-initialized entry onto the stack. while (isa(IP)) ++IP; AtEntry.SetInsertPoint(IP->getParent(), IP); // Push the entry onto the shadow stack. Instruction *EntryNextPtr = CreateGEP(AtEntry,StackEntry,0,0,"gc_frame.next"); Instruction *NewHeadVal = CreateGEP(AtEntry,StackEntry, 0, "gc_newhead"); AtEntry.CreateStore(CurrentHead, EntryNextPtr); AtEntry.CreateStore(NewHeadVal, Head); // For each instruction that escapes... EscapeEnumerator EE(F, "gc_cleanup"); while (IRBuilder<> *AtExit = EE.Next()) { // Pop the entry from the shadow stack. Don't reuse CurrentHead from // AtEntry, since that would make the value live for the entire function. Instruction *EntryNextPtr2 = CreateGEP(*AtExit, StackEntry, 0, 0, "gc_frame.next"); Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead"); AtExit->CreateStore(SavedHead, Head); } // Delete the original allocas (which are no longer used) and the intrinsic // calls (which are no longer valid). Doing this last avoids invalidating // iterators. for (unsigned I = 0, E = Roots.size(); I != E; ++I) { Roots[I].first->eraseFromParent(); Roots[I].second->eraseFromParent(); } Roots.clear(); return true; }