1 //===-- ShadowStackGC.cpp - GC support for uncooperative targets ----------===// 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 implements lowering for the llvm.gc* intrinsics for targets that do 11 // not natively support them (which includes the C backend). Note that the code 12 // generated is not quite as efficient as algorithms which generate stack maps 13 // to identify roots. 14 // 15 // This pass implements the code transformation described in this paper: 16 // "Accurate Garbage Collection in an Uncooperative Environment" 17 // Fergus Henderson, ISMM, 2002 18 // 19 // In runtime/GC/SemiSpace.cpp is a prototype runtime which is compatible with 20 // ShadowStackGC. 21 // 22 // In order to support this particular transformation, all stack roots are 23 // coallocated in the stack. This allows a fully target-independent stack map 24 // while introducing only minor runtime overhead. 25 // 26 //===----------------------------------------------------------------------===// 27 28 #define DEBUG_TYPE "shadowstackgc" 29 #include "llvm/CodeGen/GCs.h" 30 #include "llvm/ADT/StringExtras.h" 31 #include "llvm/CodeGen/GCStrategy.h" 32 #include "llvm/IntrinsicInst.h" 33 #include "llvm/Module.h" 34 #include "llvm/Support/CallSite.h" 35 #include "llvm/Support/IRBuilder.h" 36 37 using namespace llvm; 38 39 namespace { 40 41 class ShadowStackGC : public GCStrategy { 42 /// RootChain - This is the global linked-list that contains the chain of GC 43 /// roots. 44 GlobalVariable *Head; 45 46 /// StackEntryTy - Abstract type of a link in the shadow stack. 47 /// 48 StructType *StackEntryTy; 49 StructType *FrameMapTy; 50 51 /// Roots - GC roots in the current function. Each is a pair of the 52 /// intrinsic call and its corresponding alloca. 53 std::vector<std::pair<CallInst*,AllocaInst*> > Roots; 54 55 public: 56 ShadowStackGC(); 57 58 bool initializeCustomLowering(Module &M); 59 bool performCustomLowering(Function &F); 60 61 private: 62 bool IsNullValue(Value *V); 63 Constant *GetFrameMap(Function &F); 64 Type* GetConcreteStackEntryType(Function &F); 65 void CollectRoots(Function &F); 66 static GetElementPtrInst *CreateGEP(LLVMContext &Context, 67 IRBuilder<> &B, Value *BasePtr, 68 int Idx1, const char *Name); 69 static GetElementPtrInst *CreateGEP(LLVMContext &Context, 70 IRBuilder<> &B, Value *BasePtr, 71 int Idx1, int Idx2, const char *Name); 72 }; 73 74 } 75 76 static GCRegistry::Add<ShadowStackGC> 77 X("shadow-stack", "Very portable GC for uncooperative code generators"); 78 79 namespace { 80 /// EscapeEnumerator - This is a little algorithm to find all escape points 81 /// from a function so that "finally"-style code can be inserted. In addition 82 /// to finding the existing return and unwind instructions, it also (if 83 /// necessary) transforms any call instructions into invokes and sends them to 84 /// a landing pad. 85 /// 86 /// It's wrapped up in a state machine using the same transform C# uses for 87 /// 'yield return' enumerators, This transform allows it to be non-allocating. 88 class EscapeEnumerator { 89 Function &F; 90 const char *CleanupBBName; 91 92 // State. 93 int State; 94 Function::iterator StateBB, StateE; 95 IRBuilder<> Builder; 96 97 public: 98 EscapeEnumerator(Function &F, const char *N = "cleanup") 99 : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {} 100 101 IRBuilder<> *Next() { 102 switch (State) { 103 default: 104 return 0; 105 106 case 0: 107 StateBB = F.begin(); 108 StateE = F.end(); 109 State = 1; 110 111 case 1: 112 // Find all 'return', 'resume', and 'unwind' instructions. 113 while (StateBB != StateE) { 114 BasicBlock *CurBB = StateBB++; 115 116 // Branches and invokes do not escape, only unwind, resume, and return 117 // do. 118 TerminatorInst *TI = CurBB->getTerminator(); 119 if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(TI) && 120 !isa<ResumeInst>(TI)) 121 continue; 122 123 Builder.SetInsertPoint(TI->getParent(), TI); 124 return &Builder; 125 } 126 127 State = 2; 128 129 // Find all 'call' instructions. 130 SmallVector<Instruction*,16> Calls; 131 for (Function::iterator BB = F.begin(), 132 E = F.end(); BB != E; ++BB) 133 for (BasicBlock::iterator II = BB->begin(), 134 EE = BB->end(); II != EE; ++II) 135 if (CallInst *CI = dyn_cast<CallInst>(II)) 136 if (!CI->getCalledFunction() || 137 !CI->getCalledFunction()->getIntrinsicID()) 138 Calls.push_back(CI); 139 140 if (Calls.empty()) 141 return 0; 142 143 // Create a cleanup block. 144 LLVMContext &C = F.getContext(); 145 BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F); 146 Type *ExnTy = StructType::get(Type::getInt8PtrTy(C), 147 Type::getInt32Ty(C), NULL); 148 Constant *PersFn = 149 F.getParent()-> 150 getOrInsertFunction("__gcc_personality_v0", 151 FunctionType::get(Type::getInt32Ty(C), true)); 152 LandingPadInst *LPad = LandingPadInst::Create(ExnTy, PersFn, 1, 153 "cleanup.lpad", 154 CleanupBB); 155 LPad->setCleanup(true); 156 ResumeInst *RI = ResumeInst::Create(LPad, CleanupBB); 157 158 // Transform the 'call' instructions into 'invoke's branching to the 159 // cleanup block. Go in reverse order to make prettier BB names. 160 SmallVector<Value*,16> Args; 161 for (unsigned I = Calls.size(); I != 0; ) { 162 CallInst *CI = cast<CallInst>(Calls[--I]); 163 164 // Split the basic block containing the function call. 165 BasicBlock *CallBB = CI->getParent(); 166 BasicBlock *NewBB = 167 CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont"); 168 169 // Remove the unconditional branch inserted at the end of CallBB. 170 CallBB->getInstList().pop_back(); 171 NewBB->getInstList().remove(CI); 172 173 // Create a new invoke instruction. 174 Args.clear(); 175 CallSite CS(CI); 176 Args.append(CS.arg_begin(), CS.arg_end()); 177 178 InvokeInst *II = InvokeInst::Create(CI->getCalledValue(), 179 NewBB, CleanupBB, 180 Args, CI->getName(), CallBB); 181 II->setCallingConv(CI->getCallingConv()); 182 II->setAttributes(CI->getAttributes()); 183 CI->replaceAllUsesWith(II); 184 delete CI; 185 } 186 187 Builder.SetInsertPoint(RI->getParent(), RI); 188 return &Builder; 189 } 190 } 191 }; 192 } 193 194 // ----------------------------------------------------------------------------- 195 196 void llvm::linkShadowStackGC() { } 197 198 ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) { 199 InitRoots = true; 200 CustomRoots = true; 201 } 202 203 Constant *ShadowStackGC::GetFrameMap(Function &F) { 204 // doInitialization creates the abstract type of this value. 205 Type *VoidPtr = Type::getInt8PtrTy(F.getContext()); 206 207 // Truncate the ShadowStackDescriptor if some metadata is null. 208 unsigned NumMeta = 0; 209 SmallVector<Constant*, 16> Metadata; 210 for (unsigned I = 0; I != Roots.size(); ++I) { 211 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1)); 212 if (!C->isNullValue()) 213 NumMeta = I + 1; 214 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); 215 } 216 Metadata.resize(NumMeta); 217 218 Type *Int32Ty = Type::getInt32Ty(F.getContext()); 219 220 Constant *BaseElts[] = { 221 ConstantInt::get(Int32Ty, Roots.size(), false), 222 ConstantInt::get(Int32Ty, NumMeta, false), 223 }; 224 225 Constant *DescriptorElts[] = { 226 ConstantStruct::get(FrameMapTy, BaseElts), 227 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata) 228 }; 229 230 Type *EltTys[] = { DescriptorElts[0]->getType(),DescriptorElts[1]->getType()}; 231 StructType *STy = StructType::create(EltTys, "gc_map."+utostr(NumMeta)); 232 233 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts); 234 235 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems 236 // that, short of multithreaded LLVM, it should be safe; all that is 237 // necessary is that a simple Module::iterator loop not be invalidated. 238 // Appending to the GlobalVariable list is safe in that sense. 239 // 240 // All of the output passes emit globals last. The ExecutionEngine 241 // explicitly supports adding globals to the module after 242 // initialization. 243 // 244 // Still, if it isn't deemed acceptable, then this transformation needs 245 // to be a ModulePass (which means it cannot be in the 'llc' pipeline 246 // (which uses a FunctionPassManager (which segfaults (not asserts) if 247 // provided a ModulePass))). 248 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true, 249 GlobalVariable::InternalLinkage, 250 FrameMap, "__gc_" + F.getName()); 251 252 Constant *GEPIndices[2] = { 253 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0), 254 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0) 255 }; 256 return ConstantExpr::getGetElementPtr(GV, GEPIndices); 257 } 258 259 Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) { 260 // doInitialization creates the generic version of this type. 261 std::vector<Type*> EltTys; 262 EltTys.push_back(StackEntryTy); 263 for (size_t I = 0; I != Roots.size(); I++) 264 EltTys.push_back(Roots[I].second->getAllocatedType()); 265 266 return StructType::create(EltTys, "gc_stackentry."+F.getName().str()); 267 } 268 269 /// doInitialization - If this module uses the GC intrinsics, find them now. If 270 /// not, exit fast. 271 bool ShadowStackGC::initializeCustomLowering(Module &M) { 272 // struct FrameMap { 273 // int32_t NumRoots; // Number of roots in stack frame. 274 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots. 275 // void *Meta[]; // May be absent for roots without metadata. 276 // }; 277 std::vector<Type*> EltTys; 278 // 32 bits is ok up to a 32GB stack frame. :) 279 EltTys.push_back(Type::getInt32Ty(M.getContext())); 280 // Specifies length of variable length array. 281 EltTys.push_back(Type::getInt32Ty(M.getContext())); 282 FrameMapTy = StructType::create(EltTys, "gc_map"); 283 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy); 284 285 // struct StackEntry { 286 // ShadowStackEntry *Next; // Caller's stack entry. 287 // FrameMap *Map; // Pointer to constant FrameMap. 288 // void *Roots[]; // Stack roots (in-place array, so we pretend). 289 // }; 290 291 StackEntryTy = StructType::create(M.getContext(), "gc_stackentry"); 292 293 EltTys.clear(); 294 EltTys.push_back(PointerType::getUnqual(StackEntryTy)); 295 EltTys.push_back(FrameMapPtrTy); 296 StackEntryTy->setBody(EltTys); 297 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); 298 299 // Get the root chain if it already exists. 300 Head = M.getGlobalVariable("llvm_gc_root_chain"); 301 if (!Head) { 302 // If the root chain does not exist, insert a new one with linkonce 303 // linkage! 304 Head = new GlobalVariable(M, StackEntryPtrTy, false, 305 GlobalValue::LinkOnceAnyLinkage, 306 Constant::getNullValue(StackEntryPtrTy), 307 "llvm_gc_root_chain"); 308 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { 309 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy)); 310 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage); 311 } 312 313 return true; 314 } 315 316 bool ShadowStackGC::IsNullValue(Value *V) { 317 if (Constant *C = dyn_cast<Constant>(V)) 318 return C->isNullValue(); 319 return false; 320 } 321 322 void ShadowStackGC::CollectRoots(Function &F) { 323 // FIXME: Account for original alignment. Could fragment the root array. 324 // Approach 1: Null initialize empty slots at runtime. Yuck. 325 // Approach 2: Emit a map of the array instead of just a count. 326 327 assert(Roots.empty() && "Not cleaned up?"); 328 329 SmallVector<std::pair<CallInst*, AllocaInst*>, 16> MetaRoots; 330 331 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 332 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 333 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 334 if (Function *F = CI->getCalledFunction()) 335 if (F->getIntrinsicID() == Intrinsic::gcroot) { 336 std::pair<CallInst*, AllocaInst*> Pair = std::make_pair( 337 CI, cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts())); 338 if (IsNullValue(CI->getArgOperand(1))) 339 Roots.push_back(Pair); 340 else 341 MetaRoots.push_back(Pair); 342 } 343 344 // Number roots with metadata (usually empty) at the beginning, so that the 345 // FrameMap::Meta array can be elided. 346 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end()); 347 } 348 349 GetElementPtrInst * 350 ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr, 351 int Idx, int Idx2, const char *Name) { 352 Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0), 353 ConstantInt::get(Type::getInt32Ty(Context), Idx), 354 ConstantInt::get(Type::getInt32Ty(Context), Idx2) }; 355 Value* Val = B.CreateGEP(BasePtr, Indices, Name); 356 357 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 358 359 return dyn_cast<GetElementPtrInst>(Val); 360 } 361 362 GetElementPtrInst * 363 ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr, 364 int Idx, const char *Name) { 365 Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0), 366 ConstantInt::get(Type::getInt32Ty(Context), Idx) }; 367 Value *Val = B.CreateGEP(BasePtr, Indices, Name); 368 369 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 370 371 return dyn_cast<GetElementPtrInst>(Val); 372 } 373 374 /// runOnFunction - Insert code to maintain the shadow stack. 375 bool ShadowStackGC::performCustomLowering(Function &F) { 376 LLVMContext &Context = F.getContext(); 377 378 // Find calls to llvm.gcroot. 379 CollectRoots(F); 380 381 // If there are no roots in this function, then there is no need to add a 382 // stack map entry for it. 383 if (Roots.empty()) 384 return false; 385 386 // Build the constant map and figure the type of the shadow stack entry. 387 Value *FrameMap = GetFrameMap(F); 388 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F); 389 390 // Build the shadow stack entry at the very start of the function. 391 BasicBlock::iterator IP = F.getEntryBlock().begin(); 392 IRBuilder<> AtEntry(IP->getParent(), IP); 393 394 Instruction *StackEntry = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0, 395 "gc_frame"); 396 397 while (isa<AllocaInst>(IP)) ++IP; 398 AtEntry.SetInsertPoint(IP->getParent(), IP); 399 400 // Initialize the map pointer and load the current head of the shadow stack. 401 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead"); 402 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, StackEntry, 403 0,1,"gc_frame.map"); 404 AtEntry.CreateStore(FrameMap, EntryMapPtr); 405 406 // After all the allocas... 407 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 408 // For each root, find the corresponding slot in the aggregate... 409 Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root"); 410 411 // And use it in lieu of the alloca. 412 AllocaInst *OriginalAlloca = Roots[I].second; 413 SlotPtr->takeName(OriginalAlloca); 414 OriginalAlloca->replaceAllUsesWith(SlotPtr); 415 } 416 417 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't 418 // really necessary (the collector would never see the intermediate state at 419 // runtime), but it's nicer not to push the half-initialized entry onto the 420 // shadow stack. 421 while (isa<StoreInst>(IP)) ++IP; 422 AtEntry.SetInsertPoint(IP->getParent(), IP); 423 424 // Push the entry onto the shadow stack. 425 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, 426 StackEntry,0,0,"gc_frame.next"); 427 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, 428 StackEntry, 0, "gc_newhead"); 429 AtEntry.CreateStore(CurrentHead, EntryNextPtr); 430 AtEntry.CreateStore(NewHeadVal, Head); 431 432 // For each instruction that escapes... 433 EscapeEnumerator EE(F, "gc_cleanup"); 434 while (IRBuilder<> *AtExit = EE.Next()) { 435 // Pop the entry from the shadow stack. Don't reuse CurrentHead from 436 // AtEntry, since that would make the value live for the entire function. 437 Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0, 438 "gc_frame.next"); 439 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead"); 440 AtExit->CreateStore(SavedHead, Head); 441 } 442 443 // Delete the original allocas (which are no longer used) and the intrinsic 444 // calls (which are no longer valid). Doing this last avoids invalidating 445 // iterators. 446 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 447 Roots[I].first->eraseFromParent(); 448 Roots[I].second->eraseFromParent(); 449 } 450 451 Roots.clear(); 452 return true; 453 } 454