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Diffstat (limited to 'contrib/llvm/lib/CodeGen/ShadowStackGCLowering.cpp')
| -rw-r--r-- | contrib/llvm/lib/CodeGen/ShadowStackGCLowering.cpp | 354 |
1 files changed, 354 insertions, 0 deletions
diff --git a/contrib/llvm/lib/CodeGen/ShadowStackGCLowering.cpp b/contrib/llvm/lib/CodeGen/ShadowStackGCLowering.cpp new file mode 100644 index 000000000000..7b60d22c7ace --- /dev/null +++ b/contrib/llvm/lib/CodeGen/ShadowStackGCLowering.cpp @@ -0,0 +1,354 @@ +//===-- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ---===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file contains the custom lowering code required by the shadow-stack GC +// strategy. +// +// This pass implements the code transformation described in this paper: +// "Accurate Garbage Collection in an Uncooperative Environment" +// Fergus Henderson, ISMM, 2002 +// +//===----------------------------------------------------------------------===// + +#include "llvm/ADT/StringExtras.h" +#include "llvm/CodeGen/GCStrategy.h" +#include "llvm/CodeGen/Passes.h" +#include "llvm/IR/CallSite.h" +#include "llvm/IR/IRBuilder.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/Module.h" +#include "llvm/Transforms/Utils/EscapeEnumerator.h" + +using namespace llvm; + +#define DEBUG_TYPE "shadow-stack-gc-lowering" + +namespace { + +class ShadowStackGCLowering : public FunctionPass { + /// 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. + /// + StructType *StackEntryTy; + StructType *FrameMapTy; + + /// Roots - GC roots in the current function. Each is a pair of the + /// intrinsic call and its corresponding alloca. + std::vector<std::pair<CallInst *, AllocaInst *>> Roots; + +public: + static char ID; + ShadowStackGCLowering(); + + bool doInitialization(Module &M) override; + bool runOnFunction(Function &F) override; + +private: + bool IsNullValue(Value *V); + Constant *GetFrameMap(Function &F); + Type *GetConcreteStackEntryType(Function &F); + void CollectRoots(Function &F); + static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B, + Type *Ty, Value *BasePtr, int Idx1, + const char *Name); + static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B, + Type *Ty, Value *BasePtr, int Idx1, int Idx2, + const char *Name); +}; +} + +INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE, + "Shadow Stack GC Lowering", false, false) +INITIALIZE_PASS_DEPENDENCY(GCModuleInfo) +INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE, + "Shadow Stack GC Lowering", false, false) + +FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); } + +char ShadowStackGCLowering::ID = 0; + +ShadowStackGCLowering::ShadowStackGCLowering() + : FunctionPass(ID), Head(nullptr), StackEntryTy(nullptr), + FrameMapTy(nullptr) { + initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry()); +} + +Constant *ShadowStackGCLowering::GetFrameMap(Function &F) { + // doInitialization creates the abstract type of this value. + Type *VoidPtr = Type::getInt8PtrTy(F.getContext()); + + // Truncate the ShadowStackDescriptor if some metadata is null. + unsigned NumMeta = 0; + SmallVector<Constant *, 16> Metadata; + for (unsigned I = 0; I != Roots.size(); ++I) { + Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1)); + if (!C->isNullValue()) + NumMeta = I + 1; + Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); + } + Metadata.resize(NumMeta); + + Type *Int32Ty = Type::getInt32Ty(F.getContext()); + + Constant *BaseElts[] = { + ConstantInt::get(Int32Ty, Roots.size(), false), + ConstantInt::get(Int32Ty, NumMeta, false), + }; + + Constant *DescriptorElts[] = { + ConstantStruct::get(FrameMapTy, BaseElts), + ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)}; + + Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()}; + StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta)); + + Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts); + + // 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(*F.getParent(), FrameMap->getType(), true, + GlobalVariable::InternalLinkage, FrameMap, + "__gc_" + F.getName()); + + Constant *GEPIndices[2] = { + ConstantInt::get(Type::getInt32Ty(F.getContext()), 0), + ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)}; + return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices); +} + +Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) { + // doInitialization creates the generic version of this type. + std::vector<Type *> EltTys; + EltTys.push_back(StackEntryTy); + for (size_t I = 0; I != Roots.size(); I++) + EltTys.push_back(Roots[I].second->getAllocatedType()); + + return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str()); +} + +/// doInitialization - If this module uses the GC intrinsics, find them now. If +/// not, exit fast. +bool ShadowStackGCLowering::doInitialization(Module &M) { + bool Active = false; + for (Function &F : M) { + if (F.hasGC() && F.getGC() == std::string("shadow-stack")) { + Active = true; + break; + } + } + if (!Active) + return false; + + // 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<Type *> EltTys; + // 32 bits is ok up to a 32GB stack frame. :) + EltTys.push_back(Type::getInt32Ty(M.getContext())); + // Specifies length of variable length array. + EltTys.push_back(Type::getInt32Ty(M.getContext())); + FrameMapTy = StructType::create(EltTys, "gc_map"); + 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). + // }; + + StackEntryTy = StructType::create(M.getContext(), "gc_stackentry"); + + EltTys.clear(); + EltTys.push_back(PointerType::getUnqual(StackEntryTy)); + EltTys.push_back(FrameMapPtrTy); + StackEntryTy->setBody(EltTys); + PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); + + // 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( + M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage, + Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain"); + } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { + Head->setInitializer(Constant::getNullValue(StackEntryPtrTy)); + Head->setLinkage(GlobalValue::LinkOnceAnyLinkage); + } + + return true; +} + +bool ShadowStackGCLowering::IsNullValue(Value *V) { + if (Constant *C = dyn_cast<Constant>(V)) + return C->isNullValue(); + return false; +} + +void ShadowStackGCLowering::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<std::pair<CallInst *, AllocaInst *>, 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<IntrinsicInst>(II++)) + if (Function *F = CI->getCalledFunction()) + if (F->getIntrinsicID() == Intrinsic::gcroot) { + std::pair<CallInst *, AllocaInst *> Pair = std::make_pair( + CI, + cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts())); + if (IsNullValue(CI->getArgOperand(1))) + 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 *ShadowStackGCLowering::CreateGEP(LLVMContext &Context, + IRBuilder<> &B, Type *Ty, + Value *BasePtr, int Idx, + int Idx2, + const char *Name) { + Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0), + ConstantInt::get(Type::getInt32Ty(Context), Idx), + ConstantInt::get(Type::getInt32Ty(Context), Idx2)}; + Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name); + + assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); + + return dyn_cast<GetElementPtrInst>(Val); +} + +GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context, + IRBuilder<> &B, Type *Ty, Value *BasePtr, + int Idx, const char *Name) { + Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0), + ConstantInt::get(Type::getInt32Ty(Context), Idx)}; + Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name); + + assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); + + return dyn_cast<GetElementPtrInst>(Val); +} + +/// runOnFunction - Insert code to maintain the shadow stack. +bool ShadowStackGCLowering::runOnFunction(Function &F) { + // Quick exit for functions that do not use the shadow stack GC. + if (!F.hasGC() || + F.getGC() != std::string("shadow-stack")) + return false; + + LLVMContext &Context = F.getContext(); + + // 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); + 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, nullptr, "gc_frame"); + + while (isa<AllocaInst>(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(Context, AtEntry, ConcreteStackEntryTy, + 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(Context, AtEntry, ConcreteStackEntryTy, + 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 GCStrategy::InitRoots. This isn't + // really necessary (the collector would never see the intermediate state at + // runtime), but it's nicer not to push the half-initialized entry onto the + // shadow stack. + while (isa<StoreInst>(IP)) + ++IP; + AtEntry.SetInsertPoint(IP->getParent(), IP); + + // Push the entry onto the shadow stack. + Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, + StackEntry, 0, 0, "gc_frame.next"); + Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, + 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(Context, *AtExit, ConcreteStackEntryTy, 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; +} |