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-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file contains the custom lowering code required by the shadow-stack GC 10 // 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, 2002 15 // 16 //===----------------------------------------------------------------------===// 17 18 #include "llvm/ADT/SmallVector.h" 19 #include "llvm/ADT/StringExtras.h" 20 #include "llvm/Analysis/DomTreeUpdater.h" 21 #include "llvm/CodeGen/Passes.h" 22 #include "llvm/IR/BasicBlock.h" 23 #include "llvm/IR/Constant.h" 24 #include "llvm/IR/Constants.h" 25 #include "llvm/IR/DerivedTypes.h" 26 #include "llvm/IR/Dominators.h" 27 #include "llvm/IR/Function.h" 28 #include "llvm/IR/GlobalValue.h" 29 #include "llvm/IR/GlobalVariable.h" 30 #include "llvm/IR/IRBuilder.h" 31 #include "llvm/IR/Instructions.h" 32 #include "llvm/IR/IntrinsicInst.h" 33 #include "llvm/IR/Intrinsics.h" 34 #include "llvm/IR/Module.h" 35 #include "llvm/IR/Type.h" 36 #include "llvm/IR/Value.h" 37 #include "llvm/InitializePasses.h" 38 #include "llvm/Pass.h" 39 #include "llvm/Support/Casting.h" 40 #include "llvm/Transforms/Utils/EscapeEnumerator.h" 41 #include <cassert> 42 #include <optional> 43 #include <string> 44 #include <utility> 45 #include <vector> 46 47 using namespace llvm; 48 49 #define DEBUG_TYPE "shadow-stack-gc-lowering" 50 51 namespace { 52 53 class ShadowStackGCLowering : public FunctionPass { 54 /// RootChain - This is the global linked-list that contains the chain of GC 55 /// roots. 56 GlobalVariable *Head = nullptr; 57 58 /// StackEntryTy - Abstract type of a link in the shadow stack. 59 StructType *StackEntryTy = nullptr; 60 StructType *FrameMapTy = nullptr; 61 62 /// Roots - GC roots in the current function. Each is a pair of the 63 /// intrinsic call and its corresponding alloca. 64 std::vector<std::pair<CallInst *, AllocaInst *>> Roots; 65 66 public: 67 static char ID; 68 69 ShadowStackGCLowering(); 70 71 bool doInitialization(Module &M) override; 72 void getAnalysisUsage(AnalysisUsage &AU) const override; 73 bool runOnFunction(Function &F) override; 74 75 private: 76 bool IsNullValue(Value *V); 77 Constant *GetFrameMap(Function &F); 78 Type *GetConcreteStackEntryType(Function &F); 79 void CollectRoots(Function &F); 80 81 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B, 82 Type *Ty, Value *BasePtr, int Idx1, 83 const char *Name); 84 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B, 85 Type *Ty, Value *BasePtr, int Idx1, int Idx2, 86 const char *Name); 87 }; 88 89 } // end anonymous namespace 90 91 char ShadowStackGCLowering::ID = 0; 92 char &llvm::ShadowStackGCLoweringID = ShadowStackGCLowering::ID; 93 94 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE, 95 "Shadow Stack GC Lowering", false, false) 96 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo) 97 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 98 INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE, 99 "Shadow Stack GC Lowering", false, false) 100 101 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); } 102 103 ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) { 104 initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry()); 105 } 106 107 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) { 108 // doInitialization creates the abstract type of this value. 109 Type *VoidPtr = Type::getInt8PtrTy(F.getContext()); 110 111 // Truncate the ShadowStackDescriptor if some metadata is null. 112 unsigned NumMeta = 0; 113 SmallVector<Constant *, 16> Metadata; 114 for (unsigned I = 0; I != Roots.size(); ++I) { 115 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1)); 116 if (!C->isNullValue()) 117 NumMeta = I + 1; 118 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); 119 } 120 Metadata.resize(NumMeta); 121 122 Type *Int32Ty = Type::getInt32Ty(F.getContext()); 123 124 Constant *BaseElts[] = { 125 ConstantInt::get(Int32Ty, Roots.size(), false), 126 ConstantInt::get(Int32Ty, NumMeta, false), 127 }; 128 129 Constant *DescriptorElts[] = { 130 ConstantStruct::get(FrameMapTy, BaseElts), 131 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)}; 132 133 Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()}; 134 StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta)); 135 136 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts); 137 138 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems 139 // that, short of multithreaded LLVM, it should be safe; all that is 140 // necessary is that a simple Module::iterator loop not be invalidated. 141 // Appending to the GlobalVariable list is safe in that sense. 142 // 143 // All of the output passes emit globals last. The ExecutionEngine 144 // explicitly supports adding globals to the module after 145 // initialization. 146 // 147 // Still, if it isn't deemed acceptable, then this transformation needs 148 // to be a ModulePass (which means it cannot be in the 'llc' pipeline 149 // (which uses a FunctionPassManager (which segfaults (not asserts) if 150 // provided a ModulePass))). 151 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true, 152 GlobalVariable::InternalLinkage, FrameMap, 153 "__gc_" + F.getName()); 154 155 Constant *GEPIndices[2] = { 156 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0), 157 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)}; 158 return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices); 159 } 160 161 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) { 162 // doInitialization creates the generic version of this type. 163 std::vector<Type *> EltTys; 164 EltTys.push_back(StackEntryTy); 165 for (const std::pair<CallInst *, AllocaInst *> &Root : Roots) 166 EltTys.push_back(Root.second->getAllocatedType()); 167 168 return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str()); 169 } 170 171 /// doInitialization - If this module uses the GC intrinsics, find them now. If 172 /// not, exit fast. 173 bool ShadowStackGCLowering::doInitialization(Module &M) { 174 bool Active = false; 175 for (Function &F : M) { 176 if (F.hasGC() && F.getGC() == std::string("shadow-stack")) { 177 Active = true; 178 break; 179 } 180 } 181 if (!Active) 182 return false; 183 184 // struct FrameMap { 185 // int32_t NumRoots; // Number of roots in stack frame. 186 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots. 187 // void *Meta[]; // May be absent for roots without metadata. 188 // }; 189 std::vector<Type *> EltTys; 190 // 32 bits is ok up to a 32GB stack frame. :) 191 EltTys.push_back(Type::getInt32Ty(M.getContext())); 192 // Specifies length of variable length array. 193 EltTys.push_back(Type::getInt32Ty(M.getContext())); 194 FrameMapTy = StructType::create(EltTys, "gc_map"); 195 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy); 196 197 // struct StackEntry { 198 // ShadowStackEntry *Next; // Caller's stack entry. 199 // FrameMap *Map; // Pointer to constant FrameMap. 200 // void *Roots[]; // Stack roots (in-place array, so we pretend). 201 // }; 202 203 StackEntryTy = StructType::create(M.getContext(), "gc_stackentry"); 204 205 EltTys.clear(); 206 EltTys.push_back(PointerType::getUnqual(StackEntryTy)); 207 EltTys.push_back(FrameMapPtrTy); 208 StackEntryTy->setBody(EltTys); 209 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); 210 211 // Get the root chain if it already exists. 212 Head = M.getGlobalVariable("llvm_gc_root_chain"); 213 if (!Head) { 214 // If the root chain does not exist, insert a new one with linkonce 215 // linkage! 216 Head = new GlobalVariable( 217 M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage, 218 Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain"); 219 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { 220 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy)); 221 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage); 222 } 223 224 return true; 225 } 226 227 bool ShadowStackGCLowering::IsNullValue(Value *V) { 228 if (Constant *C = dyn_cast<Constant>(V)) 229 return C->isNullValue(); 230 return false; 231 } 232 233 void ShadowStackGCLowering::CollectRoots(Function &F) { 234 // FIXME: Account for original alignment. Could fragment the root array. 235 // Approach 1: Null initialize empty slots at runtime. Yuck. 236 // Approach 2: Emit a map of the array instead of just a count. 237 238 assert(Roots.empty() && "Not cleaned up?"); 239 240 SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots; 241 242 for (BasicBlock &BB : F) 243 for (Instruction &I : BB) 244 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I)) 245 if (Function *F = CI->getCalledFunction()) 246 if (F->getIntrinsicID() == Intrinsic::gcroot) { 247 std::pair<CallInst *, AllocaInst *> Pair = std::make_pair( 248 CI, 249 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts())); 250 if (IsNullValue(CI->getArgOperand(1))) 251 Roots.push_back(Pair); 252 else 253 MetaRoots.push_back(Pair); 254 } 255 256 // Number roots with metadata (usually empty) at the beginning, so that the 257 // FrameMap::Meta array can be elided. 258 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end()); 259 } 260 261 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context, 262 IRBuilder<> &B, Type *Ty, 263 Value *BasePtr, int Idx, 264 int Idx2, 265 const char *Name) { 266 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0), 267 ConstantInt::get(Type::getInt32Ty(Context), Idx), 268 ConstantInt::get(Type::getInt32Ty(Context), Idx2)}; 269 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name); 270 271 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 272 273 return dyn_cast<GetElementPtrInst>(Val); 274 } 275 276 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context, 277 IRBuilder<> &B, Type *Ty, Value *BasePtr, 278 int Idx, const char *Name) { 279 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0), 280 ConstantInt::get(Type::getInt32Ty(Context), Idx)}; 281 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name); 282 283 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 284 285 return dyn_cast<GetElementPtrInst>(Val); 286 } 287 288 void ShadowStackGCLowering::getAnalysisUsage(AnalysisUsage &AU) const { 289 AU.addPreserved<DominatorTreeWrapperPass>(); 290 } 291 292 /// runOnFunction - Insert code to maintain the shadow stack. 293 bool ShadowStackGCLowering::runOnFunction(Function &F) { 294 // Quick exit for functions that do not use the shadow stack GC. 295 if (!F.hasGC() || 296 F.getGC() != std::string("shadow-stack")) 297 return false; 298 299 LLVMContext &Context = F.getContext(); 300 301 // Find calls to llvm.gcroot. 302 CollectRoots(F); 303 304 // If there are no roots in this function, then there is no need to add a 305 // stack map entry for it. 306 if (Roots.empty()) 307 return false; 308 309 std::optional<DomTreeUpdater> DTU; 310 if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>()) 311 DTU.emplace(DTWP->getDomTree(), DomTreeUpdater::UpdateStrategy::Lazy); 312 313 // Build the constant map and figure the type of the shadow stack entry. 314 Value *FrameMap = GetFrameMap(F); 315 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F); 316 317 // Build the shadow stack entry at the very start of the function. 318 BasicBlock::iterator IP = F.getEntryBlock().begin(); 319 IRBuilder<> AtEntry(IP->getParent(), IP); 320 321 Instruction *StackEntry = 322 AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame"); 323 324 AtEntry.SetInsertPointPastAllocas(&F); 325 IP = AtEntry.GetInsertPoint(); 326 327 // Initialize the map pointer and load the current head of the shadow stack. 328 Instruction *CurrentHead = 329 AtEntry.CreateLoad(StackEntryTy->getPointerTo(), Head, "gc_currhead"); 330 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, 331 StackEntry, 0, 1, "gc_frame.map"); 332 AtEntry.CreateStore(FrameMap, EntryMapPtr); 333 334 // After all the allocas... 335 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 336 // For each root, find the corresponding slot in the aggregate... 337 Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, 338 StackEntry, 1 + I, "gc_root"); 339 340 // And use it in lieu of the alloca. 341 AllocaInst *OriginalAlloca = Roots[I].second; 342 SlotPtr->takeName(OriginalAlloca); 343 OriginalAlloca->replaceAllUsesWith(SlotPtr); 344 } 345 346 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't 347 // really necessary (the collector would never see the intermediate state at 348 // runtime), but it's nicer not to push the half-initialized entry onto the 349 // shadow stack. 350 while (isa<StoreInst>(IP)) 351 ++IP; 352 AtEntry.SetInsertPoint(IP->getParent(), IP); 353 354 // Push the entry onto the shadow stack. 355 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, 356 StackEntry, 0, 0, "gc_frame.next"); 357 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, 358 StackEntry, 0, "gc_newhead"); 359 AtEntry.CreateStore(CurrentHead, EntryNextPtr); 360 AtEntry.CreateStore(NewHeadVal, Head); 361 362 // For each instruction that escapes... 363 EscapeEnumerator EE(F, "gc_cleanup", /*HandleExceptions=*/true, 364 DTU ? &*DTU : nullptr); 365 while (IRBuilder<> *AtExit = EE.Next()) { 366 // Pop the entry from the shadow stack. Don't reuse CurrentHead from 367 // AtEntry, since that would make the value live for the entire function. 368 Instruction *EntryNextPtr2 = 369 CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0, 370 "gc_frame.next"); 371 Value *SavedHead = AtExit->CreateLoad(StackEntryTy->getPointerTo(), 372 EntryNextPtr2, "gc_savedhead"); 373 AtExit->CreateStore(SavedHead, Head); 374 } 375 376 // Delete the original allocas (which are no longer used) and the intrinsic 377 // calls (which are no longer valid). Doing this last avoids invalidating 378 // iterators. 379 for (std::pair<CallInst *, AllocaInst *> &Root : Roots) { 380 Root.first->eraseFromParent(); 381 Root.second->eraseFromParent(); 382 } 383 384 Roots.clear(); 385 return true; 386 } 387