1 //===-- StackProtector.cpp - Stack Protector Insertion --------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
15 //===----------------------------------------------------------------------===//
17 #define DEBUG_TYPE "stack-protector"
18 #include "llvm/CodeGen/StackProtector.h"
19 #include "llvm/CodeGen/Analysis.h"
20 #include "llvm/CodeGen/Passes.h"
21 #include "llvm/ADT/SmallPtrSet.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/Analysis/Dominators.h"
24 #include "llvm/Analysis/ValueTracking.h"
25 #include "llvm/IR/Attributes.h"
26 #include "llvm/IR/Constants.h"
27 #include "llvm/IR/DataLayout.h"
28 #include "llvm/IR/DerivedTypes.h"
29 #include "llvm/IR/Function.h"
30 #include "llvm/IR/GlobalValue.h"
31 #include "llvm/IR/GlobalVariable.h"
32 #include "llvm/IR/IRBuilder.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/Intrinsics.h"
36 #include "llvm/IR/Module.h"
37 #include "llvm/Support/CommandLine.h"
41 STATISTIC(NumFunProtected, "Number of functions protected");
42 STATISTIC(NumAddrTaken, "Number of local variables that have their address"
45 static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp",
46 cl::init(true), cl::Hidden);
48 char StackProtector::ID = 0;
49 INITIALIZE_PASS(StackProtector, "stack-protector", "Insert stack protectors",
52 FunctionPass *llvm::createStackProtectorPass(const TargetMachine *TM) {
53 return new StackProtector(TM);
56 StackProtector::SSPLayoutKind
57 StackProtector::getSSPLayout(const AllocaInst *AI) const {
58 return AI ? Layout.lookup(AI) : SSPLK_None;
61 bool StackProtector::runOnFunction(Function &Fn) {
64 DT = getAnalysisIfAvailable<DominatorTree>();
65 TLI = TM->getTargetLowering();
67 if (!RequiresStackProtector())
70 Attribute Attr = Fn.getAttributes().getAttribute(
71 AttributeSet::FunctionIndex, "stack-protector-buffer-size");
72 if (Attr.isStringAttribute())
73 Attr.getValueAsString().getAsInteger(10, SSPBufferSize);
76 return InsertStackProtectors();
79 /// \param [out] IsLarge is set to true if a protectable array is found and
80 /// it is "large" ( >= ssp-buffer-size). In the case of a structure with
81 /// multiple arrays, this gets set if any of them is large.
82 bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
84 bool InStruct) const {
87 if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
88 if (!AT->getElementType()->isIntegerTy(8)) {
89 // If we're on a non-Darwin platform or we're inside of a structure, don't
90 // add stack protectors unless the array is a character array.
91 // However, in strong mode any array, regardless of type and size,
92 // triggers a protector.
93 if (!Strong && (InStruct || !Trip.isOSDarwin()))
97 // If an array has more than SSPBufferSize bytes of allocated space, then we
98 // emit stack protectors.
99 if (SSPBufferSize <= TLI->getDataLayout()->getTypeAllocSize(AT)) {
105 // Require a protector for all arrays in strong mode
109 const StructType *ST = dyn_cast<StructType>(Ty);
113 bool NeedsProtector = false;
114 for (StructType::element_iterator I = ST->element_begin(),
115 E = ST->element_end();
117 if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
118 // If the element is a protectable array and is large (>= SSPBufferSize)
119 // then we are done. If the protectable array is not large, then
120 // keep looking in case a subsequent element is a large array.
123 NeedsProtector = true;
126 return NeedsProtector;
129 bool StackProtector::HasAddressTaken(const Instruction *AI) {
130 for (Value::const_use_iterator UI = AI->use_begin(), UE = AI->use_end();
133 if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
134 if (AI == SI->getValueOperand())
136 } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
137 if (AI == SI->getOperand(0))
139 } else if (isa<CallInst>(U)) {
141 } else if (isa<InvokeInst>(U)) {
143 } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
144 if (HasAddressTaken(SI))
146 } else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
147 // Keep track of what PHI nodes we have already visited to ensure
148 // they are only visited once.
149 if (VisitedPHIs.insert(PN))
150 if (HasAddressTaken(PN))
152 } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
153 if (HasAddressTaken(GEP))
155 } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
156 if (HasAddressTaken(BI))
163 /// \brief Check whether or not this function needs a stack protector based
164 /// upon the stack protector level.
166 /// We use two heuristics: a standard (ssp) and strong (sspstrong).
167 /// The standard heuristic which will add a guard variable to functions that
168 /// call alloca with a either a variable size or a size >= SSPBufferSize,
169 /// functions with character buffers larger than SSPBufferSize, and functions
170 /// with aggregates containing character buffers larger than SSPBufferSize. The
171 /// strong heuristic will add a guard variables to functions that call alloca
172 /// regardless of size, functions with any buffer regardless of type and size,
173 /// functions with aggregates that contain any buffer regardless of type and
174 /// size, and functions that contain stack-based variables that have had their
176 bool StackProtector::RequiresStackProtector() {
178 bool NeedsProtector = false;
179 if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
180 Attribute::StackProtectReq)) {
181 NeedsProtector = true;
182 Strong = true; // Use the same heuristic as strong to determine SSPLayout
183 } else if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
184 Attribute::StackProtectStrong))
186 else if (!F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
187 Attribute::StackProtect))
190 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
193 for (BasicBlock::iterator II = BB->begin(), IE = BB->end(); II != IE;
195 if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
196 if (AI->isArrayAllocation()) {
197 // SSP-Strong: Enable protectors for any call to alloca, regardless
202 if (const ConstantInt *CI =
203 dyn_cast<ConstantInt>(AI->getArraySize())) {
204 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
205 // A call to alloca with size >= SSPBufferSize requires
207 Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
208 NeedsProtector = true;
210 // Require protectors for all alloca calls in strong mode.
211 Layout.insert(std::make_pair(AI, SSPLK_SmallArray));
212 NeedsProtector = true;
215 // A call to alloca with a variable size requires protectors.
216 Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
217 NeedsProtector = true;
222 bool IsLarge = false;
223 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
224 Layout.insert(std::make_pair(AI, IsLarge ? SSPLK_LargeArray
225 : SSPLK_SmallArray));
226 NeedsProtector = true;
230 if (Strong && HasAddressTaken(AI)) {
232 Layout.insert(std::make_pair(AI, SSPLK_AddrOf));
233 NeedsProtector = true;
239 return NeedsProtector;
242 static bool InstructionWillNotHaveChain(const Instruction *I) {
243 return !I->mayHaveSideEffects() && !I->mayReadFromMemory() &&
244 isSafeToSpeculativelyExecute(I);
247 /// Identify if RI has a previous instruction in the "Tail Position" and return
248 /// it. Otherwise return 0.
250 /// This is based off of the code in llvm::isInTailCallPosition. The difference
251 /// is that it inverts the first part of llvm::isInTailCallPosition since
252 /// isInTailCallPosition is checking if a call is in a tail call position, and
253 /// we are searching for an unknown tail call that might be in the tail call
254 /// position. Once we find the call though, the code uses the same refactored
255 /// code, returnTypeIsEligibleForTailCall.
256 static CallInst *FindPotentialTailCall(BasicBlock *BB, ReturnInst *RI,
257 const TargetLoweringBase *TLI) {
258 // Establish a reasonable upper bound on the maximum amount of instructions we
259 // will look through to find a tail call.
260 unsigned SearchCounter = 0;
261 const unsigned MaxSearch = 4;
262 bool NoInterposingChain = true;
264 for (BasicBlock::reverse_iterator I = llvm::next(BB->rbegin()),
266 I != E && SearchCounter < MaxSearch; ++I) {
267 Instruction *Inst = &*I;
269 // Skip over debug intrinsics and do not allow them to affect our MaxSearch
271 if (isa<DbgInfoIntrinsic>(Inst))
274 // If we find a call and the following conditions are satisifed, then we
275 // have found a tail call that satisfies at least the target independent
276 // requirements of a tail call:
278 // 1. The call site has the tail marker.
280 // 2. The call site either will not cause the creation of a chain or if a
281 // chain is necessary there are no instructions in between the callsite and
282 // the call which would create an interposing chain.
284 // 3. The return type of the function does not impede tail call
286 if (CallInst *CI = dyn_cast<CallInst>(Inst)) {
287 if (CI->isTailCall() &&
288 (InstructionWillNotHaveChain(CI) || NoInterposingChain) &&
289 returnTypeIsEligibleForTailCall(BB->getParent(), CI, RI, *TLI))
293 // If we did not find a call see if we have an instruction that may create
294 // an interposing chain.
296 NoInterposingChain && InstructionWillNotHaveChain(Inst);
298 // Increment max search.
305 /// Insert code into the entry block that stores the __stack_chk_guard
306 /// variable onto the stack:
309 /// StackGuardSlot = alloca i8*
310 /// StackGuard = load __stack_chk_guard
311 /// call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
313 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo
315 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
316 const TargetLoweringBase *TLI, const Triple &Trip,
317 AllocaInst *&AI, Value *&StackGuardVar) {
318 bool SupportsSelectionDAGSP = false;
319 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
320 unsigned AddressSpace, Offset;
321 if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
322 Constant *OffsetVal =
323 ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
325 StackGuardVar = ConstantExpr::getIntToPtr(
326 OffsetVal, PointerType::get(PtrTy, AddressSpace));
327 } else if (Trip.getOS() == llvm::Triple::OpenBSD) {
328 StackGuardVar = M->getOrInsertGlobal("__guard_local", PtrTy);
329 cast<GlobalValue>(StackGuardVar)
330 ->setVisibility(GlobalValue::HiddenVisibility);
332 SupportsSelectionDAGSP = true;
333 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
336 IRBuilder<> B(&F->getEntryBlock().front());
337 AI = B.CreateAlloca(PtrTy, 0, "StackGuardSlot");
338 LoadInst *LI = B.CreateLoad(StackGuardVar, "StackGuard");
339 B.CreateCall2(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), LI,
342 return SupportsSelectionDAGSP;
345 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
348 /// - The prologue code loads and stores the stack guard onto the stack.
349 /// - The epilogue checks the value stored in the prologue against the original
350 /// value. It calls __stack_chk_fail if they differ.
351 bool StackProtector::InsertStackProtectors() {
352 bool HasPrologue = false;
353 bool SupportsSelectionDAGSP =
354 EnableSelectionDAGSP && !TM->Options.EnableFastISel;
355 AllocaInst *AI = 0; // Place on stack that stores the stack guard.
356 Value *StackGuardVar = 0; // The stack guard variable.
358 for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
359 BasicBlock *BB = I++;
360 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
366 SupportsSelectionDAGSP &=
367 CreatePrologue(F, M, RI, TLI, Trip, AI, StackGuardVar);
370 if (SupportsSelectionDAGSP) {
371 // Since we have a potential tail call, insert the special stack check
373 Instruction *InsertionPt = 0;
374 if (CallInst *CI = FindPotentialTailCall(BB, RI, TLI)) {
378 // At this point we know that BB has a return statement so it *DOES*
379 // have a terminator.
380 assert(InsertionPt != 0 && "BB must have a terminator instruction at "
384 Function *Intrinsic =
385 Intrinsic::getDeclaration(M, Intrinsic::stackprotectorcheck);
386 CallInst::Create(Intrinsic, StackGuardVar, "", InsertionPt);
389 // If we do not support SelectionDAG based tail calls, generate IR level
392 // For each block with a return instruction, convert this:
402 // %1 = load __stack_chk_guard
403 // %2 = load StackGuardSlot
404 // %3 = cmp i1 %1, %2
405 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk
410 // CallStackCheckFailBlk:
411 // call void @__stack_chk_fail()
414 // Create the FailBB. We duplicate the BB every time since the MI tail
415 // merge pass will merge together all of the various BB into one including
416 // fail BB generated by the stack protector pseudo instruction.
417 BasicBlock *FailBB = CreateFailBB();
419 // Split the basic block before the return instruction.
420 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
422 // Update the dominator tree if we need to.
423 if (DT && DT->isReachableFromEntry(BB)) {
424 DT->addNewBlock(NewBB, BB);
425 DT->addNewBlock(FailBB, BB);
428 // Remove default branch instruction to the new BB.
429 BB->getTerminator()->eraseFromParent();
431 // Move the newly created basic block to the point right after the old
432 // basic block so that it's in the "fall through" position.
433 NewBB->moveAfter(BB);
435 // Generate the stack protector instructions in the old basic block.
437 LoadInst *LI1 = B.CreateLoad(StackGuardVar);
438 LoadInst *LI2 = B.CreateLoad(AI);
439 Value *Cmp = B.CreateICmpEQ(LI1, LI2);
440 B.CreateCondBr(Cmp, NewBB, FailBB);
444 // Return if we didn't modify any basic blocks. I.e., there are no return
445 // statements in the function.
452 /// CreateFailBB - Create a basic block to jump to when the stack protector
454 BasicBlock *StackProtector::CreateFailBB() {
455 LLVMContext &Context = F->getContext();
456 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
457 IRBuilder<> B(FailBB);
458 if (Trip.getOS() == llvm::Triple::OpenBSD) {
459 Constant *StackChkFail = M->getOrInsertFunction(
460 "__stack_smash_handler", Type::getVoidTy(Context),
461 Type::getInt8PtrTy(Context), NULL);
463 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
465 Constant *StackChkFail = M->getOrInsertFunction(
466 "__stack_chk_fail", Type::getVoidTy(Context), NULL);
467 B.CreateCall(StackChkFail);
469 B.CreateUnreachable();