ShadowStackGC.cpp revision 0598866c052147c31b808391f58434ce3dbfb838
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/IRBuilder.h" 35 36using namespace llvm; 37 38namespace { 39 40 class VISIBILITY_HIDDEN ShadowStackGC : public GCStrategy { 41 /// RootChain - This is the global linked-list that contains the chain of GC 42 /// roots. 43 GlobalVariable *Head; 44 45 /// StackEntryTy - Abstract type of a link in the shadow stack. 46 /// 47 const StructType *StackEntryTy; 48 49 /// Roots - GC roots in the current function. Each is a pair of the 50 /// intrinsic call and its corresponding alloca. 51 std::vector<std::pair<CallInst*,AllocaInst*> > Roots; 52 53 public: 54 ShadowStackGC(); 55 56 bool initializeCustomLowering(Module &M); 57 bool performCustomLowering(Function &F); 58 59 private: 60 bool IsNullValue(Value *V); 61 Constant *GetFrameMap(Function &F); 62 const Type* GetConcreteStackEntryType(Function &F); 63 void CollectRoots(Function &F); 64 static GetElementPtrInst *CreateGEP(IRBuilder<> &B, Value *BasePtr, 65 int Idx1, const char *Name); 66 static GetElementPtrInst *CreateGEP(IRBuilder<> &B, Value *BasePtr, 67 int Idx1, int Idx2, const char *Name); 68 }; 69 70} 71 72static GCRegistry::Add<ShadowStackGC> 73X("shadow-stack", "Very portable GC for uncooperative code generators"); 74 75namespace { 76 /// EscapeEnumerator - This is a little algorithm to find all escape points 77 /// from a function so that "finally"-style code can be inserted. In addition 78 /// to finding the existing return and unwind instructions, it also (if 79 /// necessary) transforms any call instructions into invokes and sends them to 80 /// a landing pad. 81 /// 82 /// It's wrapped up in a state machine using the same transform C# uses for 83 /// 'yield return' enumerators, This transform allows it to be non-allocating. 84 class VISIBILITY_HIDDEN EscapeEnumerator { 85 Function &F; 86 const char *CleanupBBName; 87 88 // State. 89 int State; 90 Function::iterator StateBB, StateE; 91 IRBuilder<> Builder; 92 93 public: 94 EscapeEnumerator(Function &F, const char *N = "cleanup") 95 : F(F), CleanupBBName(N), State(0) {} 96 97 IRBuilder<> *Next() { 98 switch (State) { 99 default: 100 return 0; 101 102 case 0: 103 StateBB = F.begin(); 104 StateE = F.end(); 105 State = 1; 106 107 case 1: 108 // Find all 'return' and 'unwind' instructions. 109 while (StateBB != StateE) { 110 BasicBlock *CurBB = StateBB++; 111 112 // Branches and invokes do not escape, only unwind and return do. 113 TerminatorInst *TI = CurBB->getTerminator(); 114 if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(TI)) 115 continue; 116 117 Builder.SetInsertPoint(TI->getParent(), TI); 118 return &Builder; 119 } 120 121 State = 2; 122 123 // Find all 'call' instructions. 124 SmallVector<Instruction*,16> Calls; 125 for (Function::iterator BB = F.begin(), 126 E = F.end(); BB != E; ++BB) 127 for (BasicBlock::iterator II = BB->begin(), 128 EE = BB->end(); II != EE; ++II) 129 if (CallInst *CI = dyn_cast<CallInst>(II)) 130 if (!CI->getCalledFunction() || 131 !CI->getCalledFunction()->getIntrinsicID()) 132 Calls.push_back(CI); 133 134 if (Calls.empty()) 135 return 0; 136 137 // Create a cleanup block. 138 BasicBlock *CleanupBB = BasicBlock::Create(CleanupBBName, &F); 139 UnwindInst *UI = new UnwindInst(CleanupBB); 140 141 // Transform the 'call' instructions into 'invoke's branching to the 142 // cleanup block. Go in reverse order to make prettier BB names. 143 SmallVector<Value*,16> Args; 144 for (unsigned I = Calls.size(); I != 0; ) { 145 CallInst *CI = cast<CallInst>(Calls[--I]); 146 147 // Split the basic block containing the function call. 148 BasicBlock *CallBB = CI->getParent(); 149 BasicBlock *NewBB = 150 CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont"); 151 152 // Remove the unconditional branch inserted at the end of CallBB. 153 CallBB->getInstList().pop_back(); 154 NewBB->getInstList().remove(CI); 155 156 // Create a new invoke instruction. 157 Args.clear(); 158 Args.append(CI->op_begin() + 1, CI->op_end()); 159 160 InvokeInst *II = InvokeInst::Create(CI->getOperand(0), 161 NewBB, CleanupBB, 162 Args.begin(), Args.end(), 163 CI->getName(), CallBB); 164 II->setCallingConv(CI->getCallingConv()); 165 II->setAttributes(CI->getAttributes()); 166 CI->replaceAllUsesWith(II); 167 delete CI; 168 } 169 170 Builder.SetInsertPoint(UI->getParent(), UI); 171 return &Builder; 172 } 173 } 174 }; 175} 176 177// ----------------------------------------------------------------------------- 178 179void llvm::linkShadowStackGC() { } 180 181ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) { 182 InitRoots = true; 183 CustomRoots = true; 184} 185 186Constant *ShadowStackGC::GetFrameMap(Function &F) { 187 // doInitialization creates the abstract type of this value. 188 189 Type *VoidPtr = PointerType::getUnqual(Type::Int8Ty); 190 191 // Truncate the ShadowStackDescriptor if some metadata is null. 192 unsigned NumMeta = 0; 193 SmallVector<Constant*,16> Metadata; 194 for (unsigned I = 0; I != Roots.size(); ++I) { 195 Constant *C = cast<Constant>(Roots[I].first->getOperand(2)); 196 if (!C->isNullValue()) 197 NumMeta = I + 1; 198 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); 199 } 200 201 Constant *BaseElts[] = { 202 ConstantInt::get(Type::Int32Ty, Roots.size(), false), 203 ConstantInt::get(Type::Int32Ty, NumMeta, false), 204 }; 205 206 Constant *DescriptorElts[] = { 207 ConstantStruct::get(BaseElts, 2), 208 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), 209 Metadata.begin(), NumMeta) 210 }; 211 212 Constant *FrameMap = ConstantStruct::get(DescriptorElts, 2); 213 214 std::string TypeName("gc_map."); 215 TypeName += utostr(NumMeta); 216 F.getParent()->addTypeName(TypeName, FrameMap->getType()); 217 218 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems 219 // that, short of multithreaded LLVM, it should be safe; all that is 220 // necessary is that a simple Module::iterator loop not be invalidated. 221 // Appending to the GlobalVariable list is safe in that sense. 222 // 223 // All of the output passes emit globals last. The ExecutionEngine 224 // explicitly supports adding globals to the module after 225 // initialization. 226 // 227 // Still, if it isn't deemed acceptable, then this transformation needs 228 // to be a ModulePass (which means it cannot be in the 'llc' pipeline 229 // (which uses a FunctionPassManager (which segfaults (not asserts) if 230 // provided a ModulePass))). 231 Constant *GV = new GlobalVariable(FrameMap->getType(), true, 232 GlobalVariable::InternalLinkage, 233 FrameMap, "__gc_" + F.getName(), 234 F.getParent()); 235 236 Constant *GEPIndices[2] = { ConstantInt::get(Type::Int32Ty, 0), 237 ConstantInt::get(Type::Int32Ty, 0) }; 238 return ConstantExpr::getGetElementPtr(GV, GEPIndices, 2); 239} 240 241const Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) { 242 // doInitialization creates the generic version of this type. 243 std::vector<const Type*> EltTys; 244 EltTys.push_back(StackEntryTy); 245 for (size_t I = 0; I != Roots.size(); I++) 246 EltTys.push_back(Roots[I].second->getAllocatedType()); 247 Type *Ty = StructType::get(EltTys); 248 249 std::string TypeName("gc_stackentry."); 250 TypeName += F.getName(); 251 F.getParent()->addTypeName(TypeName, Ty); 252 253 return Ty; 254} 255 256/// doInitialization - If this module uses the GC intrinsics, find them now. If 257/// not, exit fast. 258bool ShadowStackGC::initializeCustomLowering(Module &M) { 259 // struct FrameMap { 260 // int32_t NumRoots; // Number of roots in stack frame. 261 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots. 262 // void *Meta[]; // May be absent for roots without metadata. 263 // }; 264 std::vector<const Type*> EltTys; 265 EltTys.push_back(Type::Int32Ty); // 32 bits is ok up to a 32GB stack frame. :) 266 EltTys.push_back(Type::Int32Ty); // Specifies length of variable length array. 267 StructType *FrameMapTy = StructType::get(EltTys); 268 M.addTypeName("gc_map", FrameMapTy); 269 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy); 270 271 // struct StackEntry { 272 // ShadowStackEntry *Next; // Caller's stack entry. 273 // FrameMap *Map; // Pointer to constant FrameMap. 274 // void *Roots[]; // Stack roots (in-place array, so we pretend). 275 // }; 276 OpaqueType *RecursiveTy = OpaqueType::get(); 277 278 EltTys.clear(); 279 EltTys.push_back(PointerType::getUnqual(RecursiveTy)); 280 EltTys.push_back(FrameMapPtrTy); 281 PATypeHolder LinkTyH = StructType::get(EltTys); 282 283 RecursiveTy->refineAbstractTypeTo(LinkTyH.get()); 284 StackEntryTy = cast<StructType>(LinkTyH.get()); 285 const PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); 286 M.addTypeName("gc_stackentry", LinkTyH.get()); // FIXME: Is this safe from 287 // a FunctionPass? 288 289 // Get the root chain if it already exists. 290 Head = M.getGlobalVariable("llvm_gc_root_chain"); 291 if (!Head) { 292 // If the root chain does not exist, insert a new one with linkonce 293 // linkage! 294 Head = new GlobalVariable(StackEntryPtrTy, false, 295 GlobalValue::LinkOnceLinkage, 296 Constant::getNullValue(StackEntryPtrTy), 297 "llvm_gc_root_chain", &M); 298 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { 299 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy)); 300 Head->setLinkage(GlobalValue::LinkOnceLinkage); 301 } 302 303 return true; 304} 305 306bool ShadowStackGC::IsNullValue(Value *V) { 307 if (Constant *C = dyn_cast<Constant>(V)) 308 return C->isNullValue(); 309 return false; 310} 311 312void ShadowStackGC::CollectRoots(Function &F) { 313 // FIXME: Account for original alignment. Could fragment the root array. 314 // Approach 1: Null initialize empty slots at runtime. Yuck. 315 // Approach 2: Emit a map of the array instead of just a count. 316 317 assert(Roots.empty() && "Not cleaned up?"); 318 319 SmallVector<std::pair<CallInst*,AllocaInst*>,16> MetaRoots; 320 321 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 322 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 323 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 324 if (Function *F = CI->getCalledFunction()) 325 if (F->getIntrinsicID() == Intrinsic::gcroot) { 326 std::pair<CallInst*,AllocaInst*> Pair = std::make_pair( 327 CI, cast<AllocaInst>(CI->getOperand(1)->stripPointerCasts())); 328 if (IsNullValue(CI->getOperand(2))) 329 Roots.push_back(Pair); 330 else 331 MetaRoots.push_back(Pair); 332 } 333 334 // Number roots with metadata (usually empty) at the beginning, so that the 335 // FrameMap::Meta array can be elided. 336 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end()); 337} 338 339GetElementPtrInst * 340ShadowStackGC::CreateGEP(IRBuilder<> &B, Value *BasePtr, 341 int Idx, int Idx2, const char *Name) { 342 Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0), 343 ConstantInt::get(Type::Int32Ty, Idx), 344 ConstantInt::get(Type::Int32Ty, Idx2) }; 345 Value* Val = B.CreateGEP(BasePtr, Indices, Indices + 3, Name); 346 347 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 348 349 return dyn_cast<GetElementPtrInst>(Val); 350} 351 352GetElementPtrInst * 353ShadowStackGC::CreateGEP(IRBuilder<> &B, Value *BasePtr, 354 int Idx, const char *Name) { 355 Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0), 356 ConstantInt::get(Type::Int32Ty, Idx) }; 357 Value *Val = B.CreateGEP(BasePtr, Indices, Indices + 2, Name); 358 359 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 360 361 return dyn_cast<GetElementPtrInst>(Val); 362} 363 364/// runOnFunction - Insert code to maintain the shadow stack. 365bool ShadowStackGC::performCustomLowering(Function &F) { 366 // Find calls to llvm.gcroot. 367 CollectRoots(F); 368 369 // If there are no roots in this function, then there is no need to add a 370 // stack map entry for it. 371 if (Roots.empty()) 372 return false; 373 374 // Build the constant map and figure the type of the shadow stack entry. 375 Value *FrameMap = GetFrameMap(F); 376 const Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F); 377 378 // Build the shadow stack entry at the very start of the function. 379 BasicBlock::iterator IP = F.getEntryBlock().begin(); 380 IRBuilder<> AtEntry(IP->getParent(), IP); 381 382 Instruction *StackEntry = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0, 383 "gc_frame"); 384 385 while (isa<AllocaInst>(IP)) ++IP; 386 AtEntry.SetInsertPoint(IP->getParent(), IP); 387 388 // Initialize the map pointer and load the current head of the shadow stack. 389 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead"); 390 Instruction *EntryMapPtr = CreateGEP(AtEntry, StackEntry,0,1,"gc_frame.map"); 391 AtEntry.CreateStore(FrameMap, EntryMapPtr); 392 393 // After all the allocas... 394 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 395 // For each root, find the corresponding slot in the aggregate... 396 Value *SlotPtr = CreateGEP(AtEntry, StackEntry, 1 + I, "gc_root"); 397 398 // And use it in lieu of the alloca. 399 AllocaInst *OriginalAlloca = Roots[I].second; 400 SlotPtr->takeName(OriginalAlloca); 401 OriginalAlloca->replaceAllUsesWith(SlotPtr); 402 } 403 404 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't 405 // really necessary (the collector would never see the intermediate state at 406 // runtime), but it's nicer not to push the half-initialized entry onto the 407 // shadow stack. 408 while (isa<StoreInst>(IP)) ++IP; 409 AtEntry.SetInsertPoint(IP->getParent(), IP); 410 411 // Push the entry onto the shadow stack. 412 Instruction *EntryNextPtr = CreateGEP(AtEntry,StackEntry,0,0,"gc_frame.next"); 413 Instruction *NewHeadVal = CreateGEP(AtEntry,StackEntry, 0, "gc_newhead"); 414 AtEntry.CreateStore(CurrentHead, EntryNextPtr); 415 AtEntry.CreateStore(NewHeadVal, Head); 416 417 // For each instruction that escapes... 418 EscapeEnumerator EE(F, "gc_cleanup"); 419 while (IRBuilder<> *AtExit = EE.Next()) { 420 // Pop the entry from the shadow stack. Don't reuse CurrentHead from 421 // AtEntry, since that would make the value live for the entire function. 422 Instruction *EntryNextPtr2 = CreateGEP(*AtExit, StackEntry, 0, 0, 423 "gc_frame.next"); 424 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead"); 425 AtExit->CreateStore(SavedHead, Head); 426 } 427 428 // Delete the original allocas (which are no longer used) and the intrinsic 429 // calls (which are no longer valid). Doing this last avoids invalidating 430 // iterators. 431 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 432 Roots[I].first->eraseFromParent(); 433 Roots[I].second->eraseFromParent(); 434 } 435 436 Roots.clear(); 437 return true; 438} 439