ShadowStackGC.cpp revision baf3c404409d5e47b13984a7f95bfbd6d1f2e79e
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/Compiler.h" 35#include "llvm/Support/IRBuilder.h" 36 37using namespace llvm; 38 39namespace { 40 41 class VISIBILITY_HIDDEN 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 const StructType *StackEntryTy; 49 50 /// Roots - GC roots in the current function. Each is a pair of the 51 /// intrinsic call and its corresponding alloca. 52 std::vector<std::pair<CallInst*,AllocaInst*> > Roots; 53 54 public: 55 ShadowStackGC(); 56 57 bool initializeCustomLowering(Module &M); 58 bool performCustomLowering(Function &F); 59 60 private: 61 bool IsNullValue(Value *V); 62 Constant *GetFrameMap(Function &F); 63 const Type* GetConcreteStackEntryType(Function &F); 64 void CollectRoots(Function &F); 65 static GetElementPtrInst *CreateGEP(LLVMContext &Context, 66 IRBuilder<> &B, Value *BasePtr, 67 int Idx1, const char *Name); 68 static GetElementPtrInst *CreateGEP(LLVMContext &Context, 69 IRBuilder<> &B, Value *BasePtr, 70 int Idx1, int Idx2, const char *Name); 71 }; 72 73} 74 75static GCRegistry::Add<ShadowStackGC> 76X("shadow-stack", "Very portable GC for uncooperative code generators"); 77 78namespace { 79 /// EscapeEnumerator - This is a little algorithm to find all escape points 80 /// from a function so that "finally"-style code can be inserted. In addition 81 /// to finding the existing return and unwind instructions, it also (if 82 /// necessary) transforms any call instructions into invokes and sends them to 83 /// a landing pad. 84 /// 85 /// It's wrapped up in a state machine using the same transform C# uses for 86 /// 'yield return' enumerators, This transform allows it to be non-allocating. 87 class VISIBILITY_HIDDEN EscapeEnumerator { 88 Function &F; 89 const char *CleanupBBName; 90 91 // State. 92 int State; 93 Function::iterator StateBB, StateE; 94 IRBuilder<> Builder; 95 96 public: 97 EscapeEnumerator(Function &F, const char *N = "cleanup") 98 : F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {} 99 100 IRBuilder<> *Next() { 101 switch (State) { 102 default: 103 return 0; 104 105 case 0: 106 StateBB = F.begin(); 107 StateE = F.end(); 108 State = 1; 109 110 case 1: 111 // Find all 'return' and 'unwind' instructions. 112 while (StateBB != StateE) { 113 BasicBlock *CurBB = StateBB++; 114 115 // Branches and invokes do not escape, only unwind and return do. 116 TerminatorInst *TI = CurBB->getTerminator(); 117 if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(TI)) 118 continue; 119 120 Builder.SetInsertPoint(TI->getParent(), TI); 121 return &Builder; 122 } 123 124 State = 2; 125 126 // Find all 'call' instructions. 127 SmallVector<Instruction*,16> Calls; 128 for (Function::iterator BB = F.begin(), 129 E = F.end(); BB != E; ++BB) 130 for (BasicBlock::iterator II = BB->begin(), 131 EE = BB->end(); II != EE; ++II) 132 if (CallInst *CI = dyn_cast<CallInst>(II)) 133 if (!CI->getCalledFunction() || 134 !CI->getCalledFunction()->getIntrinsicID()) 135 Calls.push_back(CI); 136 137 if (Calls.empty()) 138 return 0; 139 140 // Create a cleanup block. 141 BasicBlock *CleanupBB = BasicBlock::Create(CleanupBBName, &F); 142 UnwindInst *UI = new UnwindInst(CleanupBB); 143 144 // Transform the 'call' instructions into 'invoke's branching to the 145 // cleanup block. Go in reverse order to make prettier BB names. 146 SmallVector<Value*,16> Args; 147 for (unsigned I = Calls.size(); I != 0; ) { 148 CallInst *CI = cast<CallInst>(Calls[--I]); 149 150 // Split the basic block containing the function call. 151 BasicBlock *CallBB = CI->getParent(); 152 BasicBlock *NewBB = 153 CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont"); 154 155 // Remove the unconditional branch inserted at the end of CallBB. 156 CallBB->getInstList().pop_back(); 157 NewBB->getInstList().remove(CI); 158 159 // Create a new invoke instruction. 160 Args.clear(); 161 Args.append(CI->op_begin() + 1, CI->op_end()); 162 163 InvokeInst *II = InvokeInst::Create(CI->getOperand(0), 164 NewBB, CleanupBB, 165 Args.begin(), Args.end(), 166 CI->getName(), CallBB); 167 II->setCallingConv(CI->getCallingConv()); 168 II->setAttributes(CI->getAttributes()); 169 CI->replaceAllUsesWith(II); 170 delete CI; 171 } 172 173 Builder.SetInsertPoint(UI->getParent(), UI); 174 return &Builder; 175 } 176 } 177 }; 178} 179 180// ----------------------------------------------------------------------------- 181 182void llvm::linkShadowStackGC() { } 183 184ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) { 185 InitRoots = true; 186 CustomRoots = true; 187} 188 189Constant *ShadowStackGC::GetFrameMap(Function &F) { 190 // doInitialization creates the abstract type of this value. 191 LLVMContext &Context = F.getContext(); 192 193 Type *VoidPtr = PointerType::getUnqual(Type::Int8Ty); 194 195 // Truncate the ShadowStackDescriptor if some metadata is null. 196 unsigned NumMeta = 0; 197 SmallVector<Constant*,16> Metadata; 198 for (unsigned I = 0; I != Roots.size(); ++I) { 199 Constant *C = cast<Constant>(Roots[I].first->getOperand(2)); 200 if (!C->isNullValue()) 201 NumMeta = I + 1; 202 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); 203 } 204 205 Constant *BaseElts[] = { 206 ConstantInt::get(Type::Int32Ty, Roots.size(), false), 207 ConstantInt::get(Type::Int32Ty, NumMeta, false), 208 }; 209 210 Constant *DescriptorElts[] = { 211 ConstantStruct::get(BaseElts, 2), 212 ConstantArray::get(Context.getArrayType(VoidPtr, NumMeta), 213 Metadata.begin(), NumMeta) 214 }; 215 216 Constant *FrameMap = ConstantStruct::get(DescriptorElts, 2); 217 218 std::string TypeName("gc_map."); 219 TypeName += utostr(NumMeta); 220 F.getParent()->addTypeName(TypeName, FrameMap->getType()); 221 222 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems 223 // that, short of multithreaded LLVM, it should be safe; all that is 224 // necessary is that a simple Module::iterator loop not be invalidated. 225 // Appending to the GlobalVariable list is safe in that sense. 226 // 227 // All of the output passes emit globals last. The ExecutionEngine 228 // explicitly supports adding globals to the module after 229 // initialization. 230 // 231 // Still, if it isn't deemed acceptable, then this transformation needs 232 // to be a ModulePass (which means it cannot be in the 'llc' pipeline 233 // (which uses a FunctionPassManager (which segfaults (not asserts) if 234 // provided a ModulePass))). 235 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true, 236 GlobalVariable::InternalLinkage, 237 FrameMap, "__gc_" + F.getName()); 238 239 Constant *GEPIndices[2] = { ConstantInt::get(Type::Int32Ty, 0), 240 ConstantInt::get(Type::Int32Ty, 0) }; 241 return ConstantExpr::getGetElementPtr(GV, GEPIndices, 2); 242} 243 244const Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) { 245 // doInitialization creates the generic version of this type. 246 std::vector<const Type*> EltTys; 247 EltTys.push_back(StackEntryTy); 248 for (size_t I = 0; I != Roots.size(); I++) 249 EltTys.push_back(Roots[I].second->getAllocatedType()); 250 Type *Ty = StructType::get(EltTys); 251 252 std::string TypeName("gc_stackentry."); 253 TypeName += F.getName(); 254 F.getParent()->addTypeName(TypeName, Ty); 255 256 return Ty; 257} 258 259/// doInitialization - If this module uses the GC intrinsics, find them now. If 260/// not, exit fast. 261bool ShadowStackGC::initializeCustomLowering(Module &M) { 262 // struct FrameMap { 263 // int32_t NumRoots; // Number of roots in stack frame. 264 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots. 265 // void *Meta[]; // May be absent for roots without metadata. 266 // }; 267 std::vector<const Type*> EltTys; 268 EltTys.push_back(Type::Int32Ty); // 32 bits is ok up to a 32GB stack frame. :) 269 EltTys.push_back(Type::Int32Ty); // Specifies length of variable length array. 270 StructType *FrameMapTy = StructType::get(EltTys); 271 M.addTypeName("gc_map", FrameMapTy); 272 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy); 273 274 // struct StackEntry { 275 // ShadowStackEntry *Next; // Caller's stack entry. 276 // FrameMap *Map; // Pointer to constant FrameMap. 277 // void *Roots[]; // Stack roots (in-place array, so we pretend). 278 // }; 279 OpaqueType *RecursiveTy = OpaqueType::get(); 280 281 EltTys.clear(); 282 EltTys.push_back(PointerType::getUnqual(RecursiveTy)); 283 EltTys.push_back(FrameMapPtrTy); 284 PATypeHolder LinkTyH = StructType::get(EltTys); 285 286 RecursiveTy->refineAbstractTypeTo(LinkTyH.get()); 287 StackEntryTy = cast<StructType>(LinkTyH.get()); 288 const PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); 289 M.addTypeName("gc_stackentry", LinkTyH.get()); // FIXME: Is this safe from 290 // a FunctionPass? 291 292 // Get the root chain if it already exists. 293 Head = M.getGlobalVariable("llvm_gc_root_chain"); 294 if (!Head) { 295 // If the root chain does not exist, insert a new one with linkonce 296 // linkage! 297 Head = new GlobalVariable(M, StackEntryPtrTy, false, 298 GlobalValue::LinkOnceAnyLinkage, 299 M.getContext().getNullValue(StackEntryPtrTy), 300 "llvm_gc_root_chain"); 301 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { 302 Head->setInitializer(M.getContext().getNullValue(StackEntryPtrTy)); 303 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage); 304 } 305 306 return true; 307} 308 309bool ShadowStackGC::IsNullValue(Value *V) { 310 if (Constant *C = dyn_cast<Constant>(V)) 311 return C->isNullValue(); 312 return false; 313} 314 315void ShadowStackGC::CollectRoots(Function &F) { 316 // FIXME: Account for original alignment. Could fragment the root array. 317 // Approach 1: Null initialize empty slots at runtime. Yuck. 318 // Approach 2: Emit a map of the array instead of just a count. 319 320 assert(Roots.empty() && "Not cleaned up?"); 321 322 SmallVector<std::pair<CallInst*,AllocaInst*>,16> MetaRoots; 323 324 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 325 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 326 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 327 if (Function *F = CI->getCalledFunction()) 328 if (F->getIntrinsicID() == Intrinsic::gcroot) { 329 std::pair<CallInst*,AllocaInst*> Pair = std::make_pair( 330 CI, cast<AllocaInst>(CI->getOperand(1)->stripPointerCasts())); 331 if (IsNullValue(CI->getOperand(2))) 332 Roots.push_back(Pair); 333 else 334 MetaRoots.push_back(Pair); 335 } 336 337 // Number roots with metadata (usually empty) at the beginning, so that the 338 // FrameMap::Meta array can be elided. 339 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end()); 340} 341 342GetElementPtrInst * 343ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr, 344 int Idx, int Idx2, const char *Name) { 345 Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0), 346 ConstantInt::get(Type::Int32Ty, Idx), 347 ConstantInt::get(Type::Int32Ty, Idx2) }; 348 Value* Val = B.CreateGEP(BasePtr, Indices, Indices + 3, Name); 349 350 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 351 352 return dyn_cast<GetElementPtrInst>(Val); 353} 354 355GetElementPtrInst * 356ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr, 357 int Idx, const char *Name) { 358 Value *Indices[] = { ConstantInt::get(Type::Int32Ty, 0), 359 ConstantInt::get(Type::Int32Ty, Idx) }; 360 Value *Val = B.CreateGEP(BasePtr, Indices, Indices + 2, Name); 361 362 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 363 364 return dyn_cast<GetElementPtrInst>(Val); 365} 366 367/// runOnFunction - Insert code to maintain the shadow stack. 368bool ShadowStackGC::performCustomLowering(Function &F) { 369 LLVMContext &Context = F.getContext(); 370 371 // Find calls to llvm.gcroot. 372 CollectRoots(F); 373 374 // If there are no roots in this function, then there is no need to add a 375 // stack map entry for it. 376 if (Roots.empty()) 377 return false; 378 379 // Build the constant map and figure the type of the shadow stack entry. 380 Value *FrameMap = GetFrameMap(F); 381 const Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F); 382 383 // Build the shadow stack entry at the very start of the function. 384 BasicBlock::iterator IP = F.getEntryBlock().begin(); 385 IRBuilder<> AtEntry(IP->getParent(), IP); 386 387 Instruction *StackEntry = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0, 388 "gc_frame"); 389 390 while (isa<AllocaInst>(IP)) ++IP; 391 AtEntry.SetInsertPoint(IP->getParent(), IP); 392 393 // Initialize the map pointer and load the current head of the shadow stack. 394 Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead"); 395 Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, StackEntry, 396 0,1,"gc_frame.map"); 397 AtEntry.CreateStore(FrameMap, EntryMapPtr); 398 399 // After all the allocas... 400 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 401 // For each root, find the corresponding slot in the aggregate... 402 Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root"); 403 404 // And use it in lieu of the alloca. 405 AllocaInst *OriginalAlloca = Roots[I].second; 406 SlotPtr->takeName(OriginalAlloca); 407 OriginalAlloca->replaceAllUsesWith(SlotPtr); 408 } 409 410 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't 411 // really necessary (the collector would never see the intermediate state at 412 // runtime), but it's nicer not to push the half-initialized entry onto the 413 // shadow stack. 414 while (isa<StoreInst>(IP)) ++IP; 415 AtEntry.SetInsertPoint(IP->getParent(), IP); 416 417 // Push the entry onto the shadow stack. 418 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, 419 StackEntry,0,0,"gc_frame.next"); 420 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, 421 StackEntry, 0, "gc_newhead"); 422 AtEntry.CreateStore(CurrentHead, EntryNextPtr); 423 AtEntry.CreateStore(NewHeadVal, Head); 424 425 // For each instruction that escapes... 426 EscapeEnumerator EE(F, "gc_cleanup"); 427 while (IRBuilder<> *AtExit = EE.Next()) { 428 // Pop the entry from the shadow stack. Don't reuse CurrentHead from 429 // AtEntry, since that would make the value live for the entire function. 430 Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0, 431 "gc_frame.next"); 432 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead"); 433 AtExit->CreateStore(SavedHead, Head); 434 } 435 436 // Delete the original allocas (which are no longer used) and the intrinsic 437 // calls (which are no longer valid). Doing this last avoids invalidating 438 // iterators. 439 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 440 Roots[I].first->eraseFromParent(); 441 Roots[I].second->eraseFromParent(); 442 } 443 444 Roots.clear(); 445 return true; 446} 447