1 //===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file contains the custom lowering code required by the shadow-stack GC 11 // strategy. 12 // 13 // This pass implements the code transformation described in this paper: 14 // "Accurate Garbage Collection in an Uncooperative Environment" 15 // Fergus Henderson, ISMM, 2002 16 // 17 //===----------------------------------------------------------------------===// 18 19 #include "llvm/ADT/SmallVector.h" 20 #include "llvm/ADT/StringExtras.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/Function.h" 27 #include "llvm/IR/GlobalValue.h" 28 #include "llvm/IR/GlobalVariable.h" 29 #include "llvm/IR/IRBuilder.h" 30 #include "llvm/IR/Instructions.h" 31 #include "llvm/IR/IntrinsicInst.h" 32 #include "llvm/IR/Intrinsics.h" 33 #include "llvm/IR/Module.h" 34 #include "llvm/IR/Type.h" 35 #include "llvm/IR/Value.h" 36 #include "llvm/Pass.h" 37 #include "llvm/Support/Casting.h" 38 #include "llvm/Transforms/Utils/EscapeEnumerator.h" 39 #include <cassert> 40 #include <cstddef> 41 #include <string> 42 #include <utility> 43 #include <vector> 44 45 using namespace llvm; 46 47 #define DEBUG_TYPE "shadow-stack-gc-lowering" 48 49 namespace { 50 51 class ShadowStackGCLowering : public FunctionPass { 52 /// RootChain - This is the global linked-list that contains the chain of GC 53 /// roots. 54 GlobalVariable *Head = nullptr; 55 56 /// StackEntryTy - Abstract type of a link in the shadow stack. 57 StructType *StackEntryTy = nullptr; 58 StructType *FrameMapTy = nullptr; 59 60 /// Roots - GC roots in the current function. Each is a pair of the 61 /// intrinsic call and its corresponding alloca. 62 std::vector<std::pair<CallInst *, AllocaInst *>> Roots; 63 64 public: 65 static char ID; 66 67 ShadowStackGCLowering(); 68 69 bool doInitialization(Module &M) override; 70 bool runOnFunction(Function &F) override; 71 72 private: 73 bool IsNullValue(Value *V); 74 Constant *GetFrameMap(Function &F); 75 Type *GetConcreteStackEntryType(Function &F); 76 void CollectRoots(Function &F); 77 78 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B, 79 Type *Ty, Value *BasePtr, int Idx1, 80 const char *Name); 81 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B, 82 Type *Ty, Value *BasePtr, int Idx1, int Idx2, 83 const char *Name); 84 }; 85 86 } // end anonymous namespace 87 88 char ShadowStackGCLowering::ID = 0; 89 90 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE, 91 "Shadow Stack GC Lowering", false, false) 92 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo) 93 INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE, 94 "Shadow Stack GC Lowering", false, false) 95 96 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); } 97 98 ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) { 99 initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry()); 100 } 101 102 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) { 103 // doInitialization creates the abstract type of this value. 104 Type *VoidPtr = Type::getInt8PtrTy(F.getContext()); 105 106 // Truncate the ShadowStackDescriptor if some metadata is null. 107 unsigned NumMeta = 0; 108 SmallVector<Constant *, 16> Metadata; 109 for (unsigned I = 0; I != Roots.size(); ++I) { 110 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1)); 111 if (!C->isNullValue()) 112 NumMeta = I + 1; 113 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); 114 } 115 Metadata.resize(NumMeta); 116 117 Type *Int32Ty = Type::getInt32Ty(F.getContext()); 118 119 Constant *BaseElts[] = { 120 ConstantInt::get(Int32Ty, Roots.size(), false), 121 ConstantInt::get(Int32Ty, NumMeta, false), 122 }; 123 124 Constant *DescriptorElts[] = { 125 ConstantStruct::get(FrameMapTy, BaseElts), 126 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)}; 127 128 Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()}; 129 StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta)); 130 131 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts); 132 133 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems 134 // that, short of multithreaded LLVM, it should be safe; all that is 135 // necessary is that a simple Module::iterator loop not be invalidated. 136 // Appending to the GlobalVariable list is safe in that sense. 137 // 138 // All of the output passes emit globals last. The ExecutionEngine 139 // explicitly supports adding globals to the module after 140 // initialization. 141 // 142 // Still, if it isn't deemed acceptable, then this transformation needs 143 // to be a ModulePass (which means it cannot be in the 'llc' pipeline 144 // (which uses a FunctionPassManager (which segfaults (not asserts) if 145 // provided a ModulePass))). 146 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true, 147 GlobalVariable::InternalLinkage, FrameMap, 148 "__gc_" + F.getName()); 149 150 Constant *GEPIndices[2] = { 151 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0), 152 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)}; 153 return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices); 154 } 155 156 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) { 157 // doInitialization creates the generic version of this type. 158 std::vector<Type *> EltTys; 159 EltTys.push_back(StackEntryTy); 160 for (size_t I = 0; I != Roots.size(); I++) 161 EltTys.push_back(Roots[I].second->getAllocatedType()); 162 163 return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str()); 164 } 165 166 /// doInitialization - If this module uses the GC intrinsics, find them now. If 167 /// not, exit fast. 168 bool ShadowStackGCLowering::doInitialization(Module &M) { 169 bool Active = false; 170 for (Function &F : M) { 171 if (F.hasGC() && F.getGC() == std::string("shadow-stack")) { 172 Active = true; 173 break; 174 } 175 } 176 if (!Active) 177 return false; 178 179 // struct FrameMap { 180 // int32_t NumRoots; // Number of roots in stack frame. 181 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots. 182 // void *Meta[]; // May be absent for roots without metadata. 183 // }; 184 std::vector<Type *> EltTys; 185 // 32 bits is ok up to a 32GB stack frame. :) 186 EltTys.push_back(Type::getInt32Ty(M.getContext())); 187 // Specifies length of variable length array. 188 EltTys.push_back(Type::getInt32Ty(M.getContext())); 189 FrameMapTy = StructType::create(EltTys, "gc_map"); 190 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy); 191 192 // struct StackEntry { 193 // ShadowStackEntry *Next; // Caller's stack entry. 194 // FrameMap *Map; // Pointer to constant FrameMap. 195 // void *Roots[]; // Stack roots (in-place array, so we pretend). 196 // }; 197 198 StackEntryTy = StructType::create(M.getContext(), "gc_stackentry"); 199 200 EltTys.clear(); 201 EltTys.push_back(PointerType::getUnqual(StackEntryTy)); 202 EltTys.push_back(FrameMapPtrTy); 203 StackEntryTy->setBody(EltTys); 204 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); 205 206 // Get the root chain if it already exists. 207 Head = M.getGlobalVariable("llvm_gc_root_chain"); 208 if (!Head) { 209 // If the root chain does not exist, insert a new one with linkonce 210 // linkage! 211 Head = new GlobalVariable( 212 M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage, 213 Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain"); 214 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { 215 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy)); 216 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage); 217 } 218 219 return true; 220 } 221 222 bool ShadowStackGCLowering::IsNullValue(Value *V) { 223 if (Constant *C = dyn_cast<Constant>(V)) 224 return C->isNullValue(); 225 return false; 226 } 227 228 void ShadowStackGCLowering::CollectRoots(Function &F) { 229 // FIXME: Account for original alignment. Could fragment the root array. 230 // Approach 1: Null initialize empty slots at runtime. Yuck. 231 // Approach 2: Emit a map of the array instead of just a count. 232 233 assert(Roots.empty() && "Not cleaned up?"); 234 235 SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots; 236 237 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 238 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 239 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 240 if (Function *F = CI->getCalledFunction()) 241 if (F->getIntrinsicID() == Intrinsic::gcroot) { 242 std::pair<CallInst *, AllocaInst *> Pair = std::make_pair( 243 CI, 244 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts())); 245 if (IsNullValue(CI->getArgOperand(1))) 246 Roots.push_back(Pair); 247 else 248 MetaRoots.push_back(Pair); 249 } 250 251 // Number roots with metadata (usually empty) at the beginning, so that the 252 // FrameMap::Meta array can be elided. 253 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end()); 254 } 255 256 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context, 257 IRBuilder<> &B, Type *Ty, 258 Value *BasePtr, int Idx, 259 int Idx2, 260 const char *Name) { 261 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0), 262 ConstantInt::get(Type::getInt32Ty(Context), Idx), 263 ConstantInt::get(Type::getInt32Ty(Context), Idx2)}; 264 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name); 265 266 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 267 268 return dyn_cast<GetElementPtrInst>(Val); 269 } 270 271 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context, 272 IRBuilder<> &B, Type *Ty, Value *BasePtr, 273 int Idx, const char *Name) { 274 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0), 275 ConstantInt::get(Type::getInt32Ty(Context), Idx)}; 276 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name); 277 278 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 279 280 return dyn_cast<GetElementPtrInst>(Val); 281 } 282 283 /// runOnFunction - Insert code to maintain the shadow stack. 284 bool ShadowStackGCLowering::runOnFunction(Function &F) { 285 // Quick exit for functions that do not use the shadow stack GC. 286 if (!F.hasGC() || 287 F.getGC() != std::string("shadow-stack")) 288 return false; 289 290 LLVMContext &Context = F.getContext(); 291 292 // Find calls to llvm.gcroot. 293 CollectRoots(F); 294 295 // If there are no roots in this function, then there is no need to add a 296 // stack map entry for it. 297 if (Roots.empty()) 298 return false; 299 300 // Build the constant map and figure the type of the shadow stack entry. 301 Value *FrameMap = GetFrameMap(F); 302 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F); 303 304 // Build the shadow stack entry at the very start of the function. 305 BasicBlock::iterator IP = F.getEntryBlock().begin(); 306 IRBuilder<> AtEntry(IP->getParent(), IP); 307 308 Instruction *StackEntry = 309 AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame"); 310 311 while (isa<AllocaInst>(IP)) 312 ++IP; 313 AtEntry.SetInsertPoint(IP->getParent(), IP); 314 315 // Initialize the map pointer and load the current head of the shadow stack. 316 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead"); 317 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, 318 StackEntry, 0, 1, "gc_frame.map"); 319 AtEntry.CreateStore(FrameMap, EntryMapPtr); 320 321 // After all the allocas... 322 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 323 // For each root, find the corresponding slot in the aggregate... 324 Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, 325 StackEntry, 1 + I, "gc_root"); 326 327 // And use it in lieu of the alloca. 328 AllocaInst *OriginalAlloca = Roots[I].second; 329 SlotPtr->takeName(OriginalAlloca); 330 OriginalAlloca->replaceAllUsesWith(SlotPtr); 331 } 332 333 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't 334 // really necessary (the collector would never see the intermediate state at 335 // runtime), but it's nicer not to push the half-initialized entry onto the 336 // shadow stack. 337 while (isa<StoreInst>(IP)) 338 ++IP; 339 AtEntry.SetInsertPoint(IP->getParent(), IP); 340 341 // Push the entry onto the shadow stack. 342 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, 343 StackEntry, 0, 0, "gc_frame.next"); 344 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy, 345 StackEntry, 0, "gc_newhead"); 346 AtEntry.CreateStore(CurrentHead, EntryNextPtr); 347 AtEntry.CreateStore(NewHeadVal, Head); 348 349 // For each instruction that escapes... 350 EscapeEnumerator EE(F, "gc_cleanup"); 351 while (IRBuilder<> *AtExit = EE.Next()) { 352 // Pop the entry from the shadow stack. Don't reuse CurrentHead from 353 // AtEntry, since that would make the value live for the entire function. 354 Instruction *EntryNextPtr2 = 355 CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0, 356 "gc_frame.next"); 357 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead"); 358 AtExit->CreateStore(SavedHead, Head); 359 } 360 361 // Delete the original allocas (which are no longer used) and the intrinsic 362 // calls (which are no longer valid). Doing this last avoids invalidating 363 // iterators. 364 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 365 Roots[I].first->eraseFromParent(); 366 Roots[I].second->eraseFromParent(); 367 } 368 369 Roots.clear(); 370 return true; 371 } 372
