1 //===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file contains the custom lowering code required by the shadow-stack GC 10 // strategy. 11 // 12 // This pass implements the code transformation described in this paper: 13 // "Accurate Garbage Collection in an Uncooperative Environment" 14 // Fergus Henderson, ISMM, 2002 15 // 16 //===----------------------------------------------------------------------===// 17 18 #include "llvm/ADT/SmallVector.h" 19 #include "llvm/ADT/StringExtras.h" 20 #include "llvm/CodeGen/Passes.h" 21 #include "llvm/IR/BasicBlock.h" 22 #include "llvm/IR/Constant.h" 23 #include "llvm/IR/Constants.h" 24 #include "llvm/IR/DerivedTypes.h" 25 #include "llvm/IR/Function.h" 26 #include "llvm/IR/GlobalValue.h" 27 #include "llvm/IR/GlobalVariable.h" 28 #include "llvm/IR/IRBuilder.h" 29 #include "llvm/IR/Instructions.h" 30 #include "llvm/IR/IntrinsicInst.h" 31 #include "llvm/IR/Intrinsics.h" 32 #include "llvm/IR/Module.h" 33 #include "llvm/IR/Type.h" 34 #include "llvm/IR/Value.h" 35 #include "llvm/Pass.h" 36 #include "llvm/Support/Casting.h" 37 #include "llvm/Transforms/Utils/EscapeEnumerator.h" 38 #include <cassert> 39 #include <cstddef> 40 #include <string> 41 #include <utility> 42 #include <vector> 43 44 using namespace llvm; 45 46 #define DEBUG_TYPE "shadow-stack-gc-lowering" 47 48 namespace { 49 50 class ShadowStackGCLowering : public FunctionPass { 51 /// RootChain - This is the global linked-list that contains the chain of GC 52 /// roots. 53 GlobalVariable *Head = nullptr; 54 55 /// StackEntryTy - Abstract type of a link in the shadow stack. 56 StructType *StackEntryTy = nullptr; 57 StructType *FrameMapTy = nullptr; 58 59 /// Roots - GC roots in the current function. Each is a pair of the 60 /// intrinsic call and its corresponding alloca. 61 std::vector<std::pair<CallInst *, AllocaInst *>> Roots; 62 63 public: 64 static char ID; 65 66 ShadowStackGCLowering(); 67 68 bool doInitialization(Module &M) override; 69 bool runOnFunction(Function &F) override; 70 71 private: 72 bool IsNullValue(Value *V); 73 Constant *GetFrameMap(Function &F); 74 Type *GetConcreteStackEntryType(Function &F); 75 void CollectRoots(Function &F); 76 77 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B, 78 Type *Ty, Value *BasePtr, int Idx1, 79 const char *Name); 80 static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B, 81 Type *Ty, Value *BasePtr, int Idx1, int Idx2, 82 const char *Name); 83 }; 84 85 } // end anonymous namespace 86 87 char ShadowStackGCLowering::ID = 0; 88 89 INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE, 90 "Shadow Stack GC Lowering", false, false) 91 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo) 92 INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE, 93 "Shadow Stack GC Lowering", false, false) 94 95 FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); } 96 97 ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) { 98 initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry()); 99 } 100 101 Constant *ShadowStackGCLowering::GetFrameMap(Function &F) { 102 // doInitialization creates the abstract type of this value. 103 Type *VoidPtr = Type::getInt8PtrTy(F.getContext()); 104 105 // Truncate the ShadowStackDescriptor if some metadata is null. 106 unsigned NumMeta = 0; 107 SmallVector<Constant *, 16> Metadata; 108 for (unsigned I = 0; I != Roots.size(); ++I) { 109 Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1)); 110 if (!C->isNullValue()) 111 NumMeta = I + 1; 112 Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr)); 113 } 114 Metadata.resize(NumMeta); 115 116 Type *Int32Ty = Type::getInt32Ty(F.getContext()); 117 118 Constant *BaseElts[] = { 119 ConstantInt::get(Int32Ty, Roots.size(), false), 120 ConstantInt::get(Int32Ty, NumMeta, false), 121 }; 122 123 Constant *DescriptorElts[] = { 124 ConstantStruct::get(FrameMapTy, BaseElts), 125 ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)}; 126 127 Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()}; 128 StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta)); 129 130 Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts); 131 132 // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems 133 // that, short of multithreaded LLVM, it should be safe; all that is 134 // necessary is that a simple Module::iterator loop not be invalidated. 135 // Appending to the GlobalVariable list is safe in that sense. 136 // 137 // All of the output passes emit globals last. The ExecutionEngine 138 // explicitly supports adding globals to the module after 139 // initialization. 140 // 141 // Still, if it isn't deemed acceptable, then this transformation needs 142 // to be a ModulePass (which means it cannot be in the 'llc' pipeline 143 // (which uses a FunctionPassManager (which segfaults (not asserts) if 144 // provided a ModulePass))). 145 Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true, 146 GlobalVariable::InternalLinkage, FrameMap, 147 "__gc_" + F.getName()); 148 149 Constant *GEPIndices[2] = { 150 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0), 151 ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)}; 152 return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices); 153 } 154 155 Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) { 156 // doInitialization creates the generic version of this type. 157 std::vector<Type *> EltTys; 158 EltTys.push_back(StackEntryTy); 159 for (size_t I = 0; I != Roots.size(); I++) 160 EltTys.push_back(Roots[I].second->getAllocatedType()); 161 162 return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str()); 163 } 164 165 /// doInitialization - If this module uses the GC intrinsics, find them now. If 166 /// not, exit fast. 167 bool ShadowStackGCLowering::doInitialization(Module &M) { 168 bool Active = false; 169 for (Function &F : M) { 170 if (F.hasGC() && F.getGC() == std::string("shadow-stack")) { 171 Active = true; 172 break; 173 } 174 } 175 if (!Active) 176 return false; 177 178 // struct FrameMap { 179 // int32_t NumRoots; // Number of roots in stack frame. 180 // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots. 181 // void *Meta[]; // May be absent for roots without metadata. 182 // }; 183 std::vector<Type *> EltTys; 184 // 32 bits is ok up to a 32GB stack frame. :) 185 EltTys.push_back(Type::getInt32Ty(M.getContext())); 186 // Specifies length of variable length array. 187 EltTys.push_back(Type::getInt32Ty(M.getContext())); 188 FrameMapTy = StructType::create(EltTys, "gc_map"); 189 PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy); 190 191 // struct StackEntry { 192 // ShadowStackEntry *Next; // Caller's stack entry. 193 // FrameMap *Map; // Pointer to constant FrameMap. 194 // void *Roots[]; // Stack roots (in-place array, so we pretend). 195 // }; 196 197 StackEntryTy = StructType::create(M.getContext(), "gc_stackentry"); 198 199 EltTys.clear(); 200 EltTys.push_back(PointerType::getUnqual(StackEntryTy)); 201 EltTys.push_back(FrameMapPtrTy); 202 StackEntryTy->setBody(EltTys); 203 PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy); 204 205 // Get the root chain if it already exists. 206 Head = M.getGlobalVariable("llvm_gc_root_chain"); 207 if (!Head) { 208 // If the root chain does not exist, insert a new one with linkonce 209 // linkage! 210 Head = new GlobalVariable( 211 M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage, 212 Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain"); 213 } else if (Head->hasExternalLinkage() && Head->isDeclaration()) { 214 Head->setInitializer(Constant::getNullValue(StackEntryPtrTy)); 215 Head->setLinkage(GlobalValue::LinkOnceAnyLinkage); 216 } 217 218 return true; 219 } 220 221 bool ShadowStackGCLowering::IsNullValue(Value *V) { 222 if (Constant *C = dyn_cast<Constant>(V)) 223 return C->isNullValue(); 224 return false; 225 } 226 227 void ShadowStackGCLowering::CollectRoots(Function &F) { 228 // FIXME: Account for original alignment. Could fragment the root array. 229 // Approach 1: Null initialize empty slots at runtime. Yuck. 230 // Approach 2: Emit a map of the array instead of just a count. 231 232 assert(Roots.empty() && "Not cleaned up?"); 233 234 SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots; 235 236 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 237 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) 238 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) 239 if (Function *F = CI->getCalledFunction()) 240 if (F->getIntrinsicID() == Intrinsic::gcroot) { 241 std::pair<CallInst *, AllocaInst *> Pair = std::make_pair( 242 CI, 243 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts())); 244 if (IsNullValue(CI->getArgOperand(1))) 245 Roots.push_back(Pair); 246 else 247 MetaRoots.push_back(Pair); 248 } 249 250 // Number roots with metadata (usually empty) at the beginning, so that the 251 // FrameMap::Meta array can be elided. 252 Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end()); 253 } 254 255 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context, 256 IRBuilder<> &B, Type *Ty, 257 Value *BasePtr, int Idx, 258 int Idx2, 259 const char *Name) { 260 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0), 261 ConstantInt::get(Type::getInt32Ty(Context), Idx), 262 ConstantInt::get(Type::getInt32Ty(Context), Idx2)}; 263 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name); 264 265 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 266 267 return dyn_cast<GetElementPtrInst>(Val); 268 } 269 270 GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context, 271 IRBuilder<> &B, Type *Ty, Value *BasePtr, 272 int Idx, const char *Name) { 273 Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0), 274 ConstantInt::get(Type::getInt32Ty(Context), Idx)}; 275 Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name); 276 277 assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant"); 278 279 return dyn_cast<GetElementPtrInst>(Val); 280 } 281 282 /// runOnFunction - Insert code to maintain the shadow stack. 283 bool ShadowStackGCLowering::runOnFunction(Function &F) { 284 // Quick exit for functions that do not use the shadow stack GC. 285 if (!F.hasGC() || 286 F.getGC() != std::string("shadow-stack")) 287 return false; 288 289 LLVMContext &Context = F.getContext(); 290 291 // Find calls to llvm.gcroot. 292 CollectRoots(F); 293 294 // If there are no roots in this function, then there is no need to add a 295 // stack map entry for it. 296 if (Roots.empty()) 297 return false; 298 299 // Build the constant map and figure the type of the shadow stack entry. 300 Value *FrameMap = GetFrameMap(F); 301 Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F); 302 303 // Build the shadow stack entry at the very start of the function. 304 BasicBlock::iterator IP = F.getEntryBlock().begin(); 305 IRBuilder<> AtEntry(IP->getParent(), IP); 306 307 Instruction *StackEntry = 308 AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame"); 309 310 while (isa<AllocaInst>(IP)) 311 ++IP; 312 AtEntry.SetInsertPoint(IP->getParent(), IP); 313 314 // Initialize the map pointer and load the current head of the shadow stack. 315 Instruction *CurrentHead = 316 AtEntry.CreateLoad(StackEntryTy->getPointerTo(), 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(StackEntryTy->getPointerTo(), 358 EntryNextPtr2, "gc_savedhead"); 359 AtExit->CreateStore(SavedHead, Head); 360 } 361 362 // Delete the original allocas (which are no longer used) and the intrinsic 363 // calls (which are no longer valid). Doing this last avoids invalidating 364 // iterators. 365 for (unsigned I = 0, E = Roots.size(); I != E; ++I) { 366 Roots[I].first->eraseFromParent(); 367 Roots[I].second->eraseFromParent(); 368 } 369 370 Roots.clear(); 371 return true; 372 } 373