| //===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===// |
| // |
| // 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 |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // 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/CodeGen/ShadowStackGCLowering.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/StringExtras.h" |
| #include "llvm/Analysis/DomTreeUpdater.h" |
| #include "llvm/CodeGen/GCMetadata.h" |
| #include "llvm/CodeGen/Passes.h" |
| #include "llvm/IR/BasicBlock.h" |
| #include "llvm/IR/Constant.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/DerivedTypes.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/GlobalValue.h" |
| #include "llvm/IR/GlobalVariable.h" |
| #include "llvm/IR/IRBuilder.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/IntrinsicInst.h" |
| #include "llvm/IR/Intrinsics.h" |
| #include "llvm/IR/Module.h" |
| #include "llvm/IR/Type.h" |
| #include "llvm/IR/Value.h" |
| #include "llvm/InitializePasses.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/Casting.h" |
| #include "llvm/Transforms/Utils/EscapeEnumerator.h" |
| #include <cassert> |
| #include <optional> |
| #include <string> |
| #include <utility> |
| #include <vector> |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "shadow-stack-gc-lowering" |
| |
| namespace { |
| |
| class ShadowStackGCLoweringImpl { |
| /// RootChain - This is the global linked-list that contains the chain of GC |
| /// roots. |
| GlobalVariable *Head = nullptr; |
| |
| /// StackEntryTy - Abstract type of a link in the shadow stack. |
| StructType *StackEntryTy = nullptr; |
| StructType *FrameMapTy = nullptr; |
| |
| /// 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: |
| ShadowStackGCLoweringImpl() = default; |
| |
| bool doInitialization(Module &M); |
| bool runOnFunction(Function &F, DomTreeUpdater *DTU); |
| |
| 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); |
| }; |
| |
| class ShadowStackGCLowering : public FunctionPass { |
| ShadowStackGCLoweringImpl Impl; |
| |
| public: |
| static char ID; |
| |
| ShadowStackGCLowering(); |
| |
| bool doInitialization(Module &M) override { return Impl.doInitialization(M); } |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.addPreserved<DominatorTreeWrapperPass>(); |
| } |
| bool runOnFunction(Function &F) override { |
| std::optional<DomTreeUpdater> DTU; |
| if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>()) |
| DTU.emplace(DTWP->getDomTree(), DomTreeUpdater::UpdateStrategy::Lazy); |
| return Impl.runOnFunction(F, DTU ? &*DTU : nullptr); |
| } |
| }; |
| |
| } // end anonymous namespace |
| |
| PreservedAnalyses ShadowStackGCLoweringPass::run(Module &M, |
| ModuleAnalysisManager &MAM) { |
| auto &Map = MAM.getResult<CollectorMetadataAnalysis>(M); |
| if (Map.StrategyMap.contains("shadow-stack")) |
| return PreservedAnalyses::all(); |
| |
| ShadowStackGCLoweringImpl Impl; |
| bool Changed = Impl.doInitialization(M); |
| for (auto &F : M) { |
| auto &FAM = |
| MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager(); |
| auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F); |
| DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy); |
| Changed |= Impl.runOnFunction(F, DT ? &DTU : nullptr); |
| } |
| |
| if (!Changed) |
| return PreservedAnalyses::all(); |
| PreservedAnalyses PA; |
| PA.preserve<DominatorTreeAnalysis>(); |
| return PA; |
| } |
| |
| char ShadowStackGCLowering::ID = 0; |
| char &llvm::ShadowStackGCLoweringID = ShadowStackGCLowering::ID; |
| |
| INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE, |
| "Shadow Stack GC Lowering", false, false) |
| INITIALIZE_PASS_DEPENDENCY(GCModuleInfo) |
| INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) |
| INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE, |
| "Shadow Stack GC Lowering", false, false) |
| |
| FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); } |
| |
| ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) { |
| initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| Constant *ShadowStackGCLoweringImpl::GetFrameMap(Function &F) { |
| // doInitialization creates the abstract type of this value. |
| Type *VoidPtr = PointerType::getUnqual(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(C); |
| } |
| 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 *ShadowStackGCLoweringImpl::GetConcreteStackEntryType(Function &F) { |
| // doInitialization creates the generic version of this type. |
| std::vector<Type *> EltTys; |
| EltTys.push_back(StackEntryTy); |
| for (const std::pair<CallInst *, AllocaInst *> &Root : Roots) |
| EltTys.push_back(Root.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 ShadowStackGCLoweringImpl::doInitialization(Module &M) { |
| bool Active = false; |
| for (Function &F : M) { |
| if (F.hasGC() && F.getGC() == "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). |
| // }; |
| |
| PointerType *StackEntryPtrTy = PointerType::getUnqual(M.getContext()); |
| |
| EltTys.clear(); |
| EltTys.push_back(StackEntryPtrTy); |
| EltTys.push_back(FrameMapPtrTy); |
| StackEntryTy = StructType::create(EltTys, "gc_stackentry"); |
| |
| // 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 ShadowStackGCLoweringImpl::IsNullValue(Value *V) { |
| if (Constant *C = dyn_cast<Constant>(V)) |
| return C->isNullValue(); |
| return false; |
| } |
| |
| void ShadowStackGCLoweringImpl::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 (BasicBlock &BB : F) |
| for (Instruction &I : BB) |
| if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I)) |
| 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 * |
| ShadowStackGCLoweringImpl::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 *ShadowStackGCLoweringImpl::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 ShadowStackGCLoweringImpl::runOnFunction(Function &F, |
| DomTreeUpdater *DTU) { |
| // Quick exit for functions that do not use the shadow stack GC. |
| if (!F.hasGC() || F.getGC() != "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"); |
| |
| AtEntry.SetInsertPointPastAllocas(&F); |
| IP = AtEntry.GetInsertPoint(); |
| |
| // Initialize the map pointer and load the current head of the shadow stack. |
| Instruction *CurrentHead = |
| AtEntry.CreateLoad(AtEntry.getPtrTy(), 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", /*HandleExceptions=*/true, DTU); |
| 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(AtExit->getPtrTy(), 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 (std::pair<CallInst *, AllocaInst *> &Root : Roots) { |
| Root.first->eraseFromParent(); |
| Root.second->eraseFromParent(); |
| } |
| |
| Roots.clear(); |
| return true; |
| } |