StackProtector.cpp revision e4957fb9b77a4fbdf711b9e5a722d107d86ccc50
1//===-- StackProtector.cpp - Stack Protector Insertion --------------------===// 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 pass inserts stack protectors into functions which need them. A variable 11// with a random value in it is stored onto the stack before the local variables 12// are allocated. Upon exiting the block, the stored value is checked. If it's 13// changed, then there was some sort of violation and the program aborts. 14// 15//===----------------------------------------------------------------------===// 16 17#define DEBUG_TYPE "stack-protector" 18#include "llvm/CodeGen/Passes.h" 19#include "llvm/ADT/SmallPtrSet.h" 20#include "llvm/ADT/Statistic.h" 21#include "llvm/ADT/Triple.h" 22#include "llvm/Analysis/Dominators.h" 23#include "llvm/IR/Attributes.h" 24#include "llvm/IR/Constants.h" 25#include "llvm/IR/DataLayout.h" 26#include "llvm/IR/DerivedTypes.h" 27#include "llvm/IR/Function.h" 28#include "llvm/IR/Instructions.h" 29#include "llvm/IR/Intrinsics.h" 30#include "llvm/IR/Module.h" 31#include "llvm/Pass.h" 32#include "llvm/Support/CommandLine.h" 33#include "llvm/Target/TargetLowering.h" 34#include "llvm/Target/TargetOptions.h" 35using namespace llvm; 36 37STATISTIC(NumFunProtected, "Number of functions protected"); 38STATISTIC(NumAddrTaken, "Number of local variables that have their address" 39 " taken."); 40 41namespace { 42 class StackProtector : public FunctionPass { 43 /// TLI - Keep a pointer of a TargetLowering to consult for determining 44 /// target type sizes. 45 const TargetLoweringBase *TLI; 46 47 Function *F; 48 Module *M; 49 50 DominatorTree *DT; 51 52 /// VisitedPHIs - The set of PHI nodes visited when determining 53 /// if a variable's reference has been taken. This set 54 /// is maintained to ensure we don't visit the same PHI node multiple 55 /// times. 56 SmallPtrSet<const PHINode*, 16> VisitedPHIs; 57 58 /// InsertStackProtectors - Insert code into the prologue and epilogue of 59 /// the function. 60 /// 61 /// - The prologue code loads and stores the stack guard onto the stack. 62 /// - The epilogue checks the value stored in the prologue against the 63 /// original value. It calls __stack_chk_fail if they differ. 64 bool InsertStackProtectors(); 65 66 /// CreateFailBB - Create a basic block to jump to when the stack protector 67 /// check fails. 68 BasicBlock *CreateFailBB(); 69 70 /// ContainsProtectableArray - Check whether the type either is an array or 71 /// contains an array of sufficient size so that we need stack protectors 72 /// for it. 73 bool ContainsProtectableArray(Type *Ty, bool Strong = false, 74 bool InStruct = false) const; 75 76 /// \brief Check whether a stack allocation has its address taken. 77 bool HasAddressTaken(const Instruction *AI); 78 79 /// RequiresStackProtector - Check whether or not this function needs a 80 /// stack protector based upon the stack protector level. 81 bool RequiresStackProtector(); 82 public: 83 static char ID; // Pass identification, replacement for typeid. 84 StackProtector() : FunctionPass(ID), TLI(0) { 85 initializeStackProtectorPass(*PassRegistry::getPassRegistry()); 86 } 87 StackProtector(const TargetLoweringBase *tli) 88 : FunctionPass(ID), TLI(tli) { 89 initializeStackProtectorPass(*PassRegistry::getPassRegistry()); 90 } 91 92 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 93 AU.addPreserved<DominatorTree>(); 94 } 95 96 virtual bool runOnFunction(Function &Fn); 97 }; 98} // end anonymous namespace 99 100char StackProtector::ID = 0; 101INITIALIZE_PASS(StackProtector, "stack-protector", 102 "Insert stack protectors", false, false) 103 104FunctionPass *llvm::createStackProtectorPass(const TargetLoweringBase *tli) { 105 return new StackProtector(tli); 106} 107 108bool StackProtector::runOnFunction(Function &Fn) { 109 F = &Fn; 110 M = F->getParent(); 111 DT = getAnalysisIfAvailable<DominatorTree>(); 112 113 if (!RequiresStackProtector()) return false; 114 115 ++NumFunProtected; 116 return InsertStackProtectors(); 117} 118 119/// ContainsProtectableArray - Check whether the type either is an array or 120/// contains a char array of sufficient size so that we need stack protectors 121/// for it. 122bool StackProtector::ContainsProtectableArray(Type *Ty, bool Strong, 123 bool InStruct) const { 124 if (!Ty) return false; 125 if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) { 126 // In strong mode any array, regardless of type and size, triggers a 127 // protector 128 if (Strong) 129 return true; 130 const TargetMachine &TM = TLI->getTargetMachine(); 131 if (!AT->getElementType()->isIntegerTy(8)) { 132 Triple Trip(TM.getTargetTriple()); 133 134 // If we're on a non-Darwin platform or we're inside of a structure, don't 135 // add stack protectors unless the array is a character array. 136 if (InStruct || !Trip.isOSDarwin()) 137 return false; 138 } 139 140 // If an array has more than SSPBufferSize bytes of allocated space, then we 141 // emit stack protectors. 142 if (TM.Options.SSPBufferSize <= TLI->getDataLayout()->getTypeAllocSize(AT)) 143 return true; 144 } 145 146 const StructType *ST = dyn_cast<StructType>(Ty); 147 if (!ST) return false; 148 149 for (StructType::element_iterator I = ST->element_begin(), 150 E = ST->element_end(); I != E; ++I) 151 if (ContainsProtectableArray(*I, Strong, true)) 152 return true; 153 154 return false; 155} 156 157bool StackProtector::HasAddressTaken(const Instruction *AI) { 158 for (Value::const_use_iterator UI = AI->use_begin(), UE = AI->use_end(); 159 UI != UE; ++UI) { 160 const User *U = *UI; 161 if (const StoreInst *SI = dyn_cast<StoreInst>(U)) { 162 if (AI == SI->getValueOperand()) 163 return true; 164 } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) { 165 if (AI == SI->getOperand(0)) 166 return true; 167 } else if (isa<CallInst>(U)) { 168 return true; 169 } else if (isa<InvokeInst>(U)) { 170 return true; 171 } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) { 172 if (HasAddressTaken(SI)) 173 return true; 174 } else if (const PHINode *PN = dyn_cast<PHINode>(U)) { 175 // Keep track of what PHI nodes we have already visited to ensure 176 // they are only visited once. 177 if (VisitedPHIs.insert(PN)) 178 if (HasAddressTaken(PN)) 179 return true; 180 } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) { 181 if (HasAddressTaken(GEP)) 182 return true; 183 } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) { 184 if (HasAddressTaken(BI)) 185 return true; 186 } 187 } 188 return false; 189} 190 191/// \brief Check whether or not this function needs a stack protector based 192/// upon the stack protector level. 193/// 194/// We use two heuristics: a standard (ssp) and strong (sspstrong). 195/// The standard heuristic which will add a guard variable to functions that 196/// call alloca with a either a variable size or a size >= SSPBufferSize, 197/// functions with character buffers larger than SSPBufferSize, and functions 198/// with aggregates containing character buffers larger than SSPBufferSize. The 199/// strong heuristic will add a guard variables to functions that call alloca 200/// regardless of size, functions with any buffer regardless of type and size, 201/// functions with aggregates that contain any buffer regardless of type and 202/// size, and functions that contain stack-based variables that have had their 203/// address taken. 204bool StackProtector::RequiresStackProtector() { 205 bool Strong = false; 206 if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex, 207 Attribute::StackProtectReq)) 208 return true; 209 else if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex, 210 Attribute::StackProtectStrong)) 211 Strong = true; 212 else if (!F->getAttributes().hasAttribute(AttributeSet::FunctionIndex, 213 Attribute::StackProtect)) 214 return false; 215 216 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) { 217 BasicBlock *BB = I; 218 219 for (BasicBlock::iterator 220 II = BB->begin(), IE = BB->end(); II != IE; ++II) { 221 if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) { 222 if (AI->isArrayAllocation()) { 223 // SSP-Strong: Enable protectors for any call to alloca, regardless 224 // of size. 225 if (Strong) 226 return true; 227 228 if (const ConstantInt *CI = 229 dyn_cast<ConstantInt>(AI->getArraySize())) { 230 unsigned BufferSize = TLI->getTargetMachine().Options.SSPBufferSize; 231 if (CI->getLimitedValue(BufferSize) >= BufferSize) 232 // A call to alloca with size >= SSPBufferSize requires 233 // stack protectors. 234 return true; 235 } else // A call to alloca with a variable size requires protectors. 236 return true; 237 } 238 239 if (ContainsProtectableArray(AI->getAllocatedType(), Strong)) 240 return true; 241 242 if (Strong && HasAddressTaken(AI)) { 243 ++NumAddrTaken; 244 return true; 245 } 246 } 247 } 248 } 249 250 return false; 251} 252 253/// InsertStackProtectors - Insert code into the prologue and epilogue of the 254/// function. 255/// 256/// - The prologue code loads and stores the stack guard onto the stack. 257/// - The epilogue checks the value stored in the prologue against the original 258/// value. It calls __stack_chk_fail if they differ. 259bool StackProtector::InsertStackProtectors() { 260 BasicBlock *FailBB = 0; // The basic block to jump to if check fails. 261 BasicBlock *FailBBDom = 0; // FailBB's dominator. 262 AllocaInst *AI = 0; // Place on stack that stores the stack guard. 263 Value *StackGuardVar = 0; // The stack guard variable. 264 265 for (Function::iterator I = F->begin(), E = F->end(); I != E; ) { 266 BasicBlock *BB = I++; 267 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()); 268 if (!RI) continue; 269 270 if (!FailBB) { 271 // Insert code into the entry block that stores the __stack_chk_guard 272 // variable onto the stack: 273 // 274 // entry: 275 // StackGuardSlot = alloca i8* 276 // StackGuard = load __stack_chk_guard 277 // call void @llvm.stackprotect.create(StackGuard, StackGuardSlot) 278 // 279 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext()); 280 unsigned AddressSpace, Offset; 281 if (TLI->getStackCookieLocation(AddressSpace, Offset)) { 282 Constant *OffsetVal = 283 ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset); 284 285 StackGuardVar = ConstantExpr::getIntToPtr(OffsetVal, 286 PointerType::get(PtrTy, AddressSpace)); 287 } else { 288 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy); 289 } 290 291 BasicBlock &Entry = F->getEntryBlock(); 292 Instruction *InsPt = &Entry.front(); 293 294 AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt); 295 LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt); 296 297 Value *Args[] = { LI, AI }; 298 CallInst:: 299 Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), 300 Args, "", InsPt); 301 302 // Create the basic block to jump to when the guard check fails. 303 FailBB = CreateFailBB(); 304 } 305 306 // For each block with a return instruction, convert this: 307 // 308 // return: 309 // ... 310 // ret ... 311 // 312 // into this: 313 // 314 // return: 315 // ... 316 // %1 = load __stack_chk_guard 317 // %2 = load StackGuardSlot 318 // %3 = cmp i1 %1, %2 319 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk 320 // 321 // SP_return: 322 // ret ... 323 // 324 // CallStackCheckFailBlk: 325 // call void @__stack_chk_fail() 326 // unreachable 327 328 // Split the basic block before the return instruction. 329 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return"); 330 331 if (DT && DT->isReachableFromEntry(BB)) { 332 DT->addNewBlock(NewBB, BB); 333 FailBBDom = FailBBDom ? DT->findNearestCommonDominator(FailBBDom, BB) :BB; 334 } 335 336 // Remove default branch instruction to the new BB. 337 BB->getTerminator()->eraseFromParent(); 338 339 // Move the newly created basic block to the point right after the old basic 340 // block so that it's in the "fall through" position. 341 NewBB->moveAfter(BB); 342 343 // Generate the stack protector instructions in the old basic block. 344 LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB); 345 LoadInst *LI2 = new LoadInst(AI, "", true, BB); 346 ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, ""); 347 BranchInst::Create(NewBB, FailBB, Cmp, BB); 348 } 349 350 // Return if we didn't modify any basic blocks. I.e., there are no return 351 // statements in the function. 352 if (!FailBB) return false; 353 354 if (DT && FailBBDom) 355 DT->addNewBlock(FailBB, FailBBDom); 356 357 return true; 358} 359 360/// CreateFailBB - Create a basic block to jump to when the stack protector 361/// check fails. 362BasicBlock *StackProtector::CreateFailBB() { 363 BasicBlock *FailBB = BasicBlock::Create(F->getContext(), 364 "CallStackCheckFailBlk", F); 365 Constant *StackChkFail = 366 M->getOrInsertFunction("__stack_chk_fail", 367 Type::getVoidTy(F->getContext()), NULL); 368 CallInst::Create(StackChkFail, "", FailBB); 369 new UnreachableInst(F->getContext(), FailBB); 370 return FailBB; 371} 372