419 lines · cpp
1//===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===//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-exception6//7//===----------------------------------------------------------------------===//8//9// This file contains the custom lowering code required by the shadow-stack GC10// strategy.11//12// This pass implements the code transformation described in this paper:13// "Accurate Garbage Collection in an Uncooperative Environment"14// Fergus Henderson, ISMM, 200215//16//===----------------------------------------------------------------------===//17 18#include "llvm/CodeGen/ShadowStackGCLowering.h"19#include "llvm/ADT/SmallVector.h"20#include "llvm/ADT/StringExtras.h"21#include "llvm/Analysis/DomTreeUpdater.h"22#include "llvm/CodeGen/GCMetadata.h"23#include "llvm/CodeGen/Passes.h"24#include "llvm/IR/BasicBlock.h"25#include "llvm/IR/Constant.h"26#include "llvm/IR/Constants.h"27#include "llvm/IR/DerivedTypes.h"28#include "llvm/IR/Dominators.h"29#include "llvm/IR/Function.h"30#include "llvm/IR/GlobalValue.h"31#include "llvm/IR/GlobalVariable.h"32#include "llvm/IR/IRBuilder.h"33#include "llvm/IR/Instructions.h"34#include "llvm/IR/IntrinsicInst.h"35#include "llvm/IR/Intrinsics.h"36#include "llvm/IR/Module.h"37#include "llvm/IR/Type.h"38#include "llvm/IR/Value.h"39#include "llvm/InitializePasses.h"40#include "llvm/Pass.h"41#include "llvm/Support/Casting.h"42#include "llvm/Transforms/Utils/EscapeEnumerator.h"43#include <cassert>44#include <optional>45#include <utility>46#include <vector>47 48using namespace llvm;49 50#define DEBUG_TYPE "shadow-stack-gc-lowering"51 52namespace {53 54class ShadowStackGCLoweringImpl {55 /// RootChain - This is the global linked-list that contains the chain of GC56 /// roots.57 GlobalVariable *Head = nullptr;58 59 /// StackEntryTy - Abstract type of a link in the shadow stack.60 StructType *StackEntryTy = nullptr;61 StructType *FrameMapTy = nullptr;62 63 /// Roots - GC roots in the current function. Each is a pair of the64 /// intrinsic call and its corresponding alloca.65 std::vector<std::pair<CallInst *, AllocaInst *>> Roots;66 67public:68 ShadowStackGCLoweringImpl() = default;69 70 bool doInitialization(Module &M);71 bool runOnFunction(Function &F, DomTreeUpdater *DTU);72 73private:74 bool IsNullValue(Value *V);75 Constant *GetFrameMap(Function &F);76 Type *GetConcreteStackEntryType(Function &F);77 void CollectRoots(Function &F);78 79 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,80 Type *Ty, Value *BasePtr, int Idx1,81 const char *Name);82 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,83 Type *Ty, Value *BasePtr, int Idx1, int Idx2,84 const char *Name);85};86 87class ShadowStackGCLowering : public FunctionPass {88 ShadowStackGCLoweringImpl Impl;89 90public:91 static char ID;92 93 ShadowStackGCLowering();94 95 bool doInitialization(Module &M) override { return Impl.doInitialization(M); }96 void getAnalysisUsage(AnalysisUsage &AU) const override {97 AU.addPreserved<DominatorTreeWrapperPass>();98 }99 bool runOnFunction(Function &F) override {100 std::optional<DomTreeUpdater> DTU;101 if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())102 DTU.emplace(DTWP->getDomTree(), DomTreeUpdater::UpdateStrategy::Lazy);103 return Impl.runOnFunction(F, DTU ? &*DTU : nullptr);104 }105};106 107} // end anonymous namespace108 109PreservedAnalyses ShadowStackGCLoweringPass::run(Module &M,110 ModuleAnalysisManager &MAM) {111 auto &Map = MAM.getResult<CollectorMetadataAnalysis>(M);112 if (!Map.contains("shadow-stack"))113 return PreservedAnalyses::all();114 115 ShadowStackGCLoweringImpl Impl;116 bool Changed = Impl.doInitialization(M);117 for (auto &F : M) {118 auto &FAM =119 MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();120 auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F);121 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);122 Changed |= Impl.runOnFunction(F, DT ? &DTU : nullptr);123 }124 125 if (!Changed)126 return PreservedAnalyses::all();127 PreservedAnalyses PA;128 PA.preserve<DominatorTreeAnalysis>();129 return PA;130}131 132char ShadowStackGCLowering::ID = 0;133char &llvm::ShadowStackGCLoweringID = ShadowStackGCLowering::ID;134 135INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,136 "Shadow Stack GC Lowering", false, false)137INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)138INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)139INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,140 "Shadow Stack GC Lowering", false, false)141 142FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }143 144ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {145 initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());146}147 148Constant *ShadowStackGCLoweringImpl::GetFrameMap(Function &F) {149 // doInitialization creates the abstract type of this value.150 Type *VoidPtr = PointerType::getUnqual(F.getContext());151 152 // Truncate the ShadowStackDescriptor if some metadata is null.153 unsigned NumMeta = 0;154 SmallVector<Constant *, 16> Metadata;155 for (unsigned I = 0; I != Roots.size(); ++I) {156 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));157 if (!C->isNullValue())158 NumMeta = I + 1;159 Metadata.push_back(C);160 }161 Metadata.resize(NumMeta);162 163 Type *Int32Ty = Type::getInt32Ty(F.getContext());164 165 Constant *BaseElts[] = {166 ConstantInt::get(Int32Ty, Roots.size(), false),167 ConstantInt::get(Int32Ty, NumMeta, false),168 };169 170 Constant *DescriptorElts[] = {171 ConstantStruct::get(FrameMapTy, BaseElts),172 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};173 174 Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};175 StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));176 177 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);178 179 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems180 // that, short of multithreaded LLVM, it should be safe; all that is181 // necessary is that a simple Module::iterator loop not be invalidated.182 // Appending to the GlobalVariable list is safe in that sense.183 //184 // All of the output passes emit globals last. The ExecutionEngine185 // explicitly supports adding globals to the module after186 // initialization.187 //188 // Still, if it isn't deemed acceptable, then this transformation needs189 // to be a ModulePass (which means it cannot be in the 'llc' pipeline190 // (which uses a FunctionPassManager (which segfaults (not asserts) if191 // provided a ModulePass))).192 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,193 GlobalVariable::InternalLinkage, FrameMap,194 "__gc_" + F.getName());195 196 Constant *GEPIndices[2] = {197 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),198 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};199 return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);200}201 202Type *ShadowStackGCLoweringImpl::GetConcreteStackEntryType(Function &F) {203 // doInitialization creates the generic version of this type.204 std::vector<Type *> EltTys;205 EltTys.push_back(StackEntryTy);206 for (const std::pair<CallInst *, AllocaInst *> &Root : Roots)207 EltTys.push_back(Root.second->getAllocatedType());208 209 return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());210}211 212/// doInitialization - If this module uses the GC intrinsics, find them now. If213/// not, exit fast.214bool ShadowStackGCLoweringImpl::doInitialization(Module &M) {215 bool Active = false;216 for (Function &F : M) {217 if (F.hasGC() && F.getGC() == "shadow-stack") {218 Active = true;219 break;220 }221 }222 if (!Active)223 return false;224 225 // struct FrameMap {226 // int32_t NumRoots; // Number of roots in stack frame.227 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.228 // void *Meta[]; // May be absent for roots without metadata.229 // };230 std::vector<Type *> EltTys;231 // 32 bits is ok up to a 32GB stack frame. :)232 EltTys.push_back(Type::getInt32Ty(M.getContext()));233 // Specifies length of variable length array.234 EltTys.push_back(Type::getInt32Ty(M.getContext()));235 FrameMapTy = StructType::create(EltTys, "gc_map");236 PointerType *FrameMapPtrTy = PointerType::getUnqual(M.getContext());237 238 // struct StackEntry {239 // ShadowStackEntry *Next; // Caller's stack entry.240 // FrameMap *Map; // Pointer to constant FrameMap.241 // void *Roots[]; // Stack roots (in-place array, so we pretend).242 // };243 244 PointerType *StackEntryPtrTy = PointerType::getUnqual(M.getContext());245 246 EltTys.clear();247 EltTys.push_back(StackEntryPtrTy);248 EltTys.push_back(FrameMapPtrTy);249 StackEntryTy = StructType::create(EltTys, "gc_stackentry");250 251 // Get the root chain if it already exists.252 Head = M.getGlobalVariable("llvm_gc_root_chain");253 if (!Head) {254 // If the root chain does not exist, insert a new one with linkonce255 // linkage!256 Head = new GlobalVariable(257 M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,258 Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");259 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {260 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));261 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);262 }263 264 return true;265}266 267bool ShadowStackGCLoweringImpl::IsNullValue(Value *V) {268 if (Constant *C = dyn_cast<Constant>(V))269 return C->isNullValue();270 return false;271}272 273void ShadowStackGCLoweringImpl::CollectRoots(Function &F) {274 // FIXME: Account for original alignment. Could fragment the root array.275 // Approach 1: Null initialize empty slots at runtime. Yuck.276 // Approach 2: Emit a map of the array instead of just a count.277 278 assert(Roots.empty() && "Not cleaned up?");279 280 SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;281 282 for (BasicBlock &BB : F)283 for (Instruction &I : BB)284 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I))285 if (Function *F = CI->getCalledFunction())286 if (F->getIntrinsicID() == Intrinsic::gcroot) {287 std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(288 CI,289 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));290 if (IsNullValue(CI->getArgOperand(1)))291 Roots.push_back(Pair);292 else293 MetaRoots.push_back(Pair);294 }295 296 // Number roots with metadata (usually empty) at the beginning, so that the297 // FrameMap::Meta array can be elided.298 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());299}300 301GetElementPtrInst *302ShadowStackGCLoweringImpl::CreateGEP(LLVMContext &Context, IRBuilder<> &B,303 Type *Ty, Value *BasePtr, int Idx,304 int Idx2, const char *Name) {305 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),306 ConstantInt::get(Type::getInt32Ty(Context), Idx),307 ConstantInt::get(Type::getInt32Ty(Context), Idx2)};308 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);309 310 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");311 312 return dyn_cast<GetElementPtrInst>(Val);313}314 315GetElementPtrInst *ShadowStackGCLoweringImpl::CreateGEP(LLVMContext &Context,316 IRBuilder<> &B,317 Type *Ty,318 Value *BasePtr, int Idx,319 const char *Name) {320 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),321 ConstantInt::get(Type::getInt32Ty(Context), Idx)};322 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);323 324 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");325 326 return dyn_cast<GetElementPtrInst>(Val);327}328 329/// runOnFunction - Insert code to maintain the shadow stack.330bool ShadowStackGCLoweringImpl::runOnFunction(Function &F,331 DomTreeUpdater *DTU) {332 // Quick exit for functions that do not use the shadow stack GC.333 if (!F.hasGC() || F.getGC() != "shadow-stack")334 return false;335 336 LLVMContext &Context = F.getContext();337 338 // Find calls to llvm.gcroot.339 CollectRoots(F);340 341 // If there are no roots in this function, then there is no need to add a342 // stack map entry for it.343 if (Roots.empty())344 return false;345 346 // Build the constant map and figure the type of the shadow stack entry.347 Value *FrameMap = GetFrameMap(F);348 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);349 350 // Build the shadow stack entry at the very start of the function.351 BasicBlock::iterator IP = F.getEntryBlock().begin();352 IRBuilder<> AtEntry(IP->getParent(), IP);353 354 Instruction *StackEntry =355 AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");356 357 AtEntry.SetInsertPointPastAllocas(&F);358 IP = AtEntry.GetInsertPoint();359 360 // Initialize the map pointer and load the current head of the shadow stack.361 Instruction *CurrentHead =362 AtEntry.CreateLoad(AtEntry.getPtrTy(), Head, "gc_currhead");363 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,364 StackEntry, 0, 1, "gc_frame.map");365 AtEntry.CreateStore(FrameMap, EntryMapPtr);366 367 // After all the allocas...368 for (unsigned I = 0, E = Roots.size(); I != E; ++I) {369 // For each root, find the corresponding slot in the aggregate...370 Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,371 StackEntry, 1 + I, "gc_root");372 373 // And use it in lieu of the alloca.374 AllocaInst *OriginalAlloca = Roots[I].second;375 SlotPtr->takeName(OriginalAlloca);376 OriginalAlloca->replaceAllUsesWith(SlotPtr);377 }378 379 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't380 // really necessary (the collector would never see the intermediate state at381 // runtime), but it's nicer not to push the half-initialized entry onto the382 // shadow stack.383 while (isa<StoreInst>(IP))384 ++IP;385 AtEntry.SetInsertPoint(IP->getParent(), IP);386 387 // Push the entry onto the shadow stack.388 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,389 StackEntry, 0, 0, "gc_frame.next");390 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,391 StackEntry, 0, "gc_newhead");392 AtEntry.CreateStore(CurrentHead, EntryNextPtr);393 AtEntry.CreateStore(NewHeadVal, Head);394 395 // For each instruction that escapes...396 EscapeEnumerator EE(F, "gc_cleanup", /*HandleExceptions=*/true, DTU);397 while (IRBuilder<> *AtExit = EE.Next()) {398 // Pop the entry from the shadow stack. Don't reuse CurrentHead from399 // AtEntry, since that would make the value live for the entire function.400 Instruction *EntryNextPtr2 =401 CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,402 "gc_frame.next");403 Value *SavedHead =404 AtExit->CreateLoad(AtExit->getPtrTy(), EntryNextPtr2, "gc_savedhead");405 AtExit->CreateStore(SavedHead, Head);406 }407 408 // Delete the original allocas (which are no longer used) and the intrinsic409 // calls (which are no longer valid). Doing this last avoids invalidating410 // iterators.411 for (std::pair<CallInst *, AllocaInst *> &Root : Roots) {412 Root.first->eraseFromParent();413 Root.second->eraseFromParent();414 }415 416 Roots.clear();417 return true;418}419