ShadowStackGC.cpp revision db125cfaf57cc83e7dd7453de2d509bc8efd0e5e
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 37using namespace llvm; 38 39namespace { 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 76static GCRegistry::Add<ShadowStackGC> 77X("shadow-stack", "Very portable GC for uncooperative code generators"); 78 79namespace { 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' and 'unwind' instructions. 113 while (StateBB != StateE) { 114 BasicBlock *CurBB = StateBB++; 115 116 // Branches and invokes do not escape, only unwind and return do. 117 TerminatorInst *TI = CurBB->getTerminator(); 118 if (!isa<UnwindInst>(TI) && !isa<ReturnInst>(TI)) 119 continue; 120 121 Builder.SetInsertPoint(TI->getParent(), TI); 122 return &Builder; 123 } 124 125 State = 2; 126 127 // Find all 'call' instructions. 128 SmallVector<Instruction*,16> Calls; 129 for (Function::iterator BB = F.begin(), 130 E = F.end(); BB != E; ++BB) 131 for (BasicBlock::iterator II = BB->begin(), 132 EE = BB->end(); II != EE; ++II) 133 if (CallInst *CI = dyn_cast<CallInst>(II)) 134 if (!CI->getCalledFunction() || 135 !CI->getCalledFunction()->getIntrinsicID()) 136 Calls.push_back(CI); 137 138 if (Calls.empty()) 139 return 0; 140 141 // Create a cleanup block. 142 BasicBlock *CleanupBB = BasicBlock::Create(F.getContext(), 143 CleanupBBName, &F); 144 UnwindInst *UI = new UnwindInst(F.getContext(), CleanupBB); 145 146 // Transform the 'call' instructions into 'invoke's branching to the 147 // cleanup block. Go in reverse order to make prettier BB names. 148 SmallVector<Value*,16> Args; 149 for (unsigned I = Calls.size(); I != 0; ) { 150 CallInst *CI = cast<CallInst>(Calls[--I]); 151 152 // Split the basic block containing the function call. 153 BasicBlock *CallBB = CI->getParent(); 154 BasicBlock *NewBB = 155 CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont"); 156 157 // Remove the unconditional branch inserted at the end of CallBB. 158 CallBB->getInstList().pop_back(); 159 NewBB->getInstList().remove(CI); 160 161 // Create a new invoke instruction. 162 Args.clear(); 163 CallSite CS(CI); 164 Args.append(CS.arg_begin(), CS.arg_end()); 165 166 InvokeInst *II = InvokeInst::Create(CI->getCalledValue(), 167 NewBB, CleanupBB, 168 Args, CI->getName(), CallBB); 169 II->setCallingConv(CI->getCallingConv()); 170 II->setAttributes(CI->getAttributes()); 171 CI->replaceAllUsesWith(II); 172 delete CI; 173 } 174 175 Builder.SetInsertPoint(UI->getParent(), UI); 176 return &Builder; 177 } 178 } 179 }; 180} 181 182// ----------------------------------------------------------------------------- 183 184void llvm::linkShadowStackGC() { } 185 186ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) { 187 InitRoots = true; 188 CustomRoots = true; 189} 190 191Constant *ShadowStackGC::GetFrameMap(Function &F) { 192 // doInitialization creates the abstract type of this value. 193 Type *VoidPtr = Type::getInt8PtrTy(F.getContext()); 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->getArgOperand(1)); 200 if (!C->isNullValue()) 201 NumMeta = I + 1; 202 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); 203 } 204 Metadata.resize(NumMeta); 205 206 Type *Int32Ty = Type::getInt32Ty(F.getContext()); 207 208 Constant *BaseElts[] = { 209 ConstantInt::get(Int32Ty, Roots.size(), false), 210 ConstantInt::get(Int32Ty, NumMeta, false), 211 }; 212 213 Constant *DescriptorElts[] = { 214 ConstantStruct::get(FrameMapTy, BaseElts), 215 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata) 216 }; 217 218 Type *EltTys[] = { DescriptorElts[0]->getType(),DescriptorElts[1]->getType()}; 219 StructType *STy = StructType::createNamed("gc_map."+utostr(NumMeta), EltTys); 220 221 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts); 222 223 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems 224 // that, short of multithreaded LLVM, it should be safe; all that is 225 // necessary is that a simple Module::iterator loop not be invalidated. 226 // Appending to the GlobalVariable list is safe in that sense. 227 // 228 // All of the output passes emit globals last. The ExecutionEngine 229 // explicitly supports adding globals to the module after 230 // initialization. 231 // 232 // Still, if it isn't deemed acceptable, then this transformation needs 233 // to be a ModulePass (which means it cannot be in the 'llc' pipeline 234 // (which uses a FunctionPassManager (which segfaults (not asserts) if 235 // provided a ModulePass))). 236 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true, 237 GlobalVariable::InternalLinkage, 238 FrameMap, "__gc_" + F.getName()); 239 240 Constant *GEPIndices[2] = { 241 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0), 242 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0) 243 }; 244 return ConstantExpr::getGetElementPtr(GV, GEPIndices, 2); 245} 246 247Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) { 248 // doInitialization creates the generic version of this type. 249 std::vector<Type*> EltTys; 250 EltTys.push_back(StackEntryTy); 251 for (size_t I = 0; I != Roots.size(); I++) 252 EltTys.push_back(Roots[I].second->getAllocatedType()); 253 254 return StructType::createNamed("gc_stackentry."+F.getName().str(), EltTys); 255} 256 257/// doInitialization - If this module uses the GC intrinsics, find them now. If 258/// not, exit fast. 259bool ShadowStackGC::initializeCustomLowering(Module &M) { 260 // struct FrameMap { 261 // int32_t NumRoots; // Number of roots in stack frame. 262 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots. 263 // void *Meta[]; // May be absent for roots without metadata. 264 // }; 265 std::vector<Type*> EltTys; 266 // 32 bits is ok up to a 32GB stack frame. :) 267 EltTys.push_back(Type::getInt32Ty(M.getContext())); 268 // Specifies length of variable length array. 269 EltTys.push_back(Type::getInt32Ty(M.getContext())); 270 FrameMapTy = StructType::createNamed("gc_map", EltTys); 271 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy); 272 273 // struct StackEntry { 274 // ShadowStackEntry *Next; // Caller's stack entry. 275 // FrameMap *Map; // Pointer to constant FrameMap. 276 // void *Roots[]; // Stack roots (in-place array, so we pretend). 277 // }; 278 279 StackEntryTy = StructType::createNamed(M.getContext(), "gc_stackentry"); 280 281 EltTys.clear(); 282 EltTys.push_back(PointerType::getUnqual(StackEntryTy)); 283 EltTys.push_back(FrameMapPtrTy); 284 StackEntryTy->setBody(EltTys); 285 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); 286 287 // Get the root chain if it already exists. 288 Head = M.getGlobalVariable("llvm_gc_root_chain"); 289 if (!Head) { 290 // If the root chain does not exist, insert a new one with linkonce 291 // linkage! 292 Head = new GlobalVariable(M, StackEntryPtrTy, false, 293 GlobalValue::LinkOnceAnyLinkage, 294 Constant::getNullValue(StackEntryPtrTy), 295 "llvm_gc_root_chain"); 296 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { 297 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy)); 298 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage); 299 } 300 301 return true; 302} 303 304bool ShadowStackGC::IsNullValue(Value *V) { 305 if (Constant *C = dyn_cast<Constant>(V)) 306 return C->isNullValue(); 307 return false; 308} 309 310void ShadowStackGC::CollectRoots(Function &F) { 311 // FIXME: Account for original alignment. Could fragment the root array. 312 // Approach 1: Null initialize empty slots at runtime. Yuck. 313 // Approach 2: Emit a map of the array instead of just a count. 314 315 assert(Roots.empty() && "Not cleaned up?"); 316 317 SmallVector<std::pair<CallInst*, AllocaInst*>, 16> MetaRoots; 318 319 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 320 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 321 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 322 if (Function *F = CI->getCalledFunction()) 323 if (F->getIntrinsicID() == Intrinsic::gcroot) { 324 std::pair<CallInst*, AllocaInst*> Pair = std::make_pair( 325 CI, cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts())); 326 if (IsNullValue(CI->getArgOperand(1))) 327 Roots.push_back(Pair); 328 else 329 MetaRoots.push_back(Pair); 330 } 331 332 // Number roots with metadata (usually empty) at the beginning, so that the 333 // FrameMap::Meta array can be elided. 334 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end()); 335} 336 337GetElementPtrInst * 338ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr, 339 int Idx, int Idx2, const char *Name) { 340 Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0), 341 ConstantInt::get(Type::getInt32Ty(Context), Idx), 342 ConstantInt::get(Type::getInt32Ty(Context), Idx2) }; 343 Value* Val = B.CreateGEP(BasePtr, Indices, Indices + 3, Name); 344 345 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 346 347 return dyn_cast<GetElementPtrInst>(Val); 348} 349 350GetElementPtrInst * 351ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr, 352 int Idx, const char *Name) { 353 Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0), 354 ConstantInt::get(Type::getInt32Ty(Context), Idx) }; 355 Value *Val = B.CreateGEP(BasePtr, Indices, Indices + 2, Name); 356 357 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 358 359 return dyn_cast<GetElementPtrInst>(Val); 360} 361 362/// runOnFunction - Insert code to maintain the shadow stack. 363bool ShadowStackGC::performCustomLowering(Function &F) { 364 LLVMContext &Context = F.getContext(); 365 366 // 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 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(Context, AtEntry, StackEntry, 391 0,1,"gc_frame.map"); 392 AtEntry.CreateStore(FrameMap, EntryMapPtr); 393 394 // After all the allocas... 395 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 396 // For each root, find the corresponding slot in the aggregate... 397 Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root"); 398 399 // And use it in lieu of the alloca. 400 AllocaInst *OriginalAlloca = Roots[I].second; 401 SlotPtr->takeName(OriginalAlloca); 402 OriginalAlloca->replaceAllUsesWith(SlotPtr); 403 } 404 405 // Move past the original stores inserted by GCStrategy::InitRoots. This isn't 406 // really necessary (the collector would never see the intermediate state at 407 // runtime), but it's nicer not to push the half-initialized entry onto the 408 // shadow stack. 409 while (isa<StoreInst>(IP)) ++IP; 410 AtEntry.SetInsertPoint(IP->getParent(), IP); 411 412 // Push the entry onto the shadow stack. 413 Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, 414 StackEntry,0,0,"gc_frame.next"); 415 Instruction *NewHeadVal = CreateGEP(Context, AtEntry, 416 StackEntry, 0, "gc_newhead"); 417 AtEntry.CreateStore(CurrentHead, EntryNextPtr); 418 AtEntry.CreateStore(NewHeadVal, Head); 419 420 // For each instruction that escapes... 421 EscapeEnumerator EE(F, "gc_cleanup"); 422 while (IRBuilder<> *AtExit = EE.Next()) { 423 // Pop the entry from the shadow stack. Don't reuse CurrentHead from 424 // AtEntry, since that would make the value live for the entire function. 425 Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0, 426 "gc_frame.next"); 427 Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead"); 428 AtExit->CreateStore(SavedHead, Head); 429 } 430 431 // Delete the original allocas (which are no longer used) and the intrinsic 432 // calls (which are no longer valid). Doing this last avoids invalidating 433 // iterators. 434 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 435 Roots[I].first->eraseFromParent(); 436 Roots[I].second->eraseFromParent(); 437 } 438 439 Roots.clear(); 440 return true; 441} 442