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/ADT/SmallPtrSet.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Analysis/ValueTracking.h"
22 #include "llvm/CodeGen/Analysis.h"
23 #include "llvm/CodeGen/Passes.h"
24 #include "llvm/IR/Attributes.h"
25 #include "llvm/IR/Constants.h"
26 #include "llvm/IR/DataLayout.h"
27 #include "llvm/IR/DerivedTypes.h"
28 #include "llvm/IR/Function.h"
29 #include "llvm/IR/GlobalValue.h"
30 #include "llvm/IR/GlobalVariable.h"
31 #include "llvm/IR/IRBuilder.h"
32 #include "llvm/IR/Instructions.h"
33 #include "llvm/IR/IntrinsicInst.h"
34 #include "llvm/IR/Intrinsics.h"
35 #include "llvm/IR/Module.h"
36 #include "llvm/Support/CommandLine.h"
40 STATISTIC(NumFunProtected, "Number of functions protected");
41 STATISTIC(NumAddrTaken, "Number of local variables that have their address"
44 static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp",
45 cl::init(true), cl::Hidden);
47 char StackProtector::ID = 0;
48 INITIALIZE_PASS(StackProtector, "stack-protector", "Insert stack protectors",
51 FunctionPass *llvm::createStackProtectorPass(const TargetMachine *TM) {
52 return new StackProtector(TM);
55 StackProtector::SSPLayoutKind
56 StackProtector::getSSPLayout(const AllocaInst *AI) const {
57 return AI ? Layout.lookup(AI) : SSPLK_None;
60 bool StackProtector::runOnFunction(Function &Fn) {
63 DT = getAnalysisIfAvailable<DominatorTree>();
64 TLI = TM->getTargetLowering();
66 if (!RequiresStackProtector())
69 Attribute Attr = Fn.getAttributes().getAttribute(
70 AttributeSet::FunctionIndex, "stack-protector-buffer-size");
71 if (Attr.isStringAttribute())
72 Attr.getValueAsString().getAsInteger(10, SSPBufferSize);
75 return InsertStackProtectors();
78 /// \param [out] IsLarge is set to true if a protectable array is found and
79 /// it is "large" ( >= ssp-buffer-size). In the case of a structure with
80 /// multiple arrays, this gets set if any of them is large.
81 bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
83 bool InStruct) const {
86 if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
87 if (!AT->getElementType()->isIntegerTy(8)) {
88 // If we're on a non-Darwin platform or we're inside of a structure, don't
89 // add stack protectors unless the array is a character array.
90 // However, in strong mode any array, regardless of type and size,
91 // triggers a protector.
92 if (!Strong && (InStruct || !Trip.isOSDarwin()))
96 // If an array has more than SSPBufferSize bytes of allocated space, then we
97 // emit stack protectors.
98 if (SSPBufferSize <= TLI->getDataLayout()->getTypeAllocSize(AT)) {
104 // Require a protector for all arrays in strong mode
108 const StructType *ST = dyn_cast<StructType>(Ty);
112 bool NeedsProtector = false;
113 for (StructType::element_iterator I = ST->element_begin(),
114 E = ST->element_end();
116 if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
117 // If the element is a protectable array and is large (>= SSPBufferSize)
118 // then we are done. If the protectable array is not large, then
119 // keep looking in case a subsequent element is a large array.
122 NeedsProtector = true;
125 return NeedsProtector;
128 bool StackProtector::HasAddressTaken(const Instruction *AI) {
129 for (Value::const_use_iterator UI = AI->use_begin(), UE = AI->use_end();
132 if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
133 if (AI == SI->getValueOperand())
135 } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
136 if (AI == SI->getOperand(0))
138 } else if (isa<CallInst>(U)) {
140 } else if (isa<InvokeInst>(U)) {
142 } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
143 if (HasAddressTaken(SI))
145 } else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
146 // Keep track of what PHI nodes we have already visited to ensure
147 // they are only visited once.
148 if (VisitedPHIs.insert(PN))
149 if (HasAddressTaken(PN))
151 } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
152 if (HasAddressTaken(GEP))
154 } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
155 if (HasAddressTaken(BI))
162 /// \brief Check whether or not this function needs a stack protector based
163 /// upon the stack protector level.
165 /// We use two heuristics: a standard (ssp) and strong (sspstrong).
166 /// The standard heuristic which will add a guard variable to functions that
167 /// call alloca with a either a variable size or a size >= SSPBufferSize,
168 /// functions with character buffers larger than SSPBufferSize, and functions
169 /// with aggregates containing character buffers larger than SSPBufferSize. The
170 /// strong heuristic will add a guard variables to functions that call alloca
171 /// regardless of size, functions with any buffer regardless of type and size,
172 /// functions with aggregates that contain any buffer regardless of type and
173 /// size, and functions that contain stack-based variables that have had their
175 bool StackProtector::RequiresStackProtector() {
177 bool NeedsProtector = false;
178 if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
179 Attribute::StackProtectReq)) {
180 NeedsProtector = true;
181 Strong = true; // Use the same heuristic as strong to determine SSPLayout
182 } else if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
183 Attribute::StackProtectStrong))
185 else if (!F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
186 Attribute::StackProtect))
189 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
192 for (BasicBlock::iterator II = BB->begin(), IE = BB->end(); II != IE;
194 if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
195 if (AI->isArrayAllocation()) {
196 // SSP-Strong: Enable protectors for any call to alloca, regardless
201 if (const ConstantInt *CI =
202 dyn_cast<ConstantInt>(AI->getArraySize())) {
203 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
204 // A call to alloca with size >= SSPBufferSize requires
206 Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
207 NeedsProtector = true;
209 // Require protectors for all alloca calls in strong mode.
210 Layout.insert(std::make_pair(AI, SSPLK_SmallArray));
211 NeedsProtector = true;
214 // A call to alloca with a variable size requires protectors.
215 Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
216 NeedsProtector = true;
221 bool IsLarge = false;
222 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
223 Layout.insert(std::make_pair(AI, IsLarge ? SSPLK_LargeArray
224 : SSPLK_SmallArray));
225 NeedsProtector = true;
229 if (Strong && HasAddressTaken(AI)) {
231 Layout.insert(std::make_pair(AI, SSPLK_AddrOf));
232 NeedsProtector = true;
238 return NeedsProtector;
241 static bool InstructionWillNotHaveChain(const Instruction *I) {
242 return !I->mayHaveSideEffects() && !I->mayReadFromMemory() &&
243 isSafeToSpeculativelyExecute(I);
246 /// Identify if RI has a previous instruction in the "Tail Position" and return
247 /// it. Otherwise return 0.
249 /// This is based off of the code in llvm::isInTailCallPosition. The difference
250 /// is that it inverts the first part of llvm::isInTailCallPosition since
251 /// isInTailCallPosition is checking if a call is in a tail call position, and
252 /// we are searching for an unknown tail call that might be in the tail call
253 /// position. Once we find the call though, the code uses the same refactored
254 /// code, returnTypeIsEligibleForTailCall.
255 static CallInst *FindPotentialTailCall(BasicBlock *BB, ReturnInst *RI,
256 const TargetLoweringBase *TLI) {
257 // Establish a reasonable upper bound on the maximum amount of instructions we
258 // will look through to find a tail call.
259 unsigned SearchCounter = 0;
260 const unsigned MaxSearch = 4;
261 bool NoInterposingChain = true;
263 for (BasicBlock::reverse_iterator I = llvm::next(BB->rbegin()),
265 I != E && SearchCounter < MaxSearch; ++I) {
266 Instruction *Inst = &*I;
268 // Skip over debug intrinsics and do not allow them to affect our MaxSearch
270 if (isa<DbgInfoIntrinsic>(Inst))
273 // If we find a call and the following conditions are satisifed, then we
274 // have found a tail call that satisfies at least the target independent
275 // requirements of a tail call:
277 // 1. The call site has the tail marker.
279 // 2. The call site either will not cause the creation of a chain or if a
280 // chain is necessary there are no instructions in between the callsite and
281 // the call which would create an interposing chain.
283 // 3. The return type of the function does not impede tail call
285 if (CallInst *CI = dyn_cast<CallInst>(Inst)) {
286 if (CI->isTailCall() &&
287 (InstructionWillNotHaveChain(CI) || NoInterposingChain) &&
288 returnTypeIsEligibleForTailCall(BB->getParent(), CI, RI, *TLI))
292 // If we did not find a call see if we have an instruction that may create
293 // an interposing chain.
295 NoInterposingChain && InstructionWillNotHaveChain(Inst);
297 // Increment max search.
304 /// Insert code into the entry block that stores the __stack_chk_guard
305 /// variable onto the stack:
308 /// StackGuardSlot = alloca i8*
309 /// StackGuard = load __stack_chk_guard
310 /// call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
312 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo
314 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
315 const TargetLoweringBase *TLI, const Triple &Trip,
316 AllocaInst *&AI, Value *&StackGuardVar) {
317 bool SupportsSelectionDAGSP = false;
318 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
319 unsigned AddressSpace, Offset;
320 if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
321 Constant *OffsetVal =
322 ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
324 StackGuardVar = ConstantExpr::getIntToPtr(
325 OffsetVal, PointerType::get(PtrTy, AddressSpace));
326 } else if (Trip.getOS() == llvm::Triple::OpenBSD) {
327 StackGuardVar = M->getOrInsertGlobal("__guard_local", PtrTy);
328 cast<GlobalValue>(StackGuardVar)
329 ->setVisibility(GlobalValue::HiddenVisibility);
331 SupportsSelectionDAGSP = true;
332 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
335 IRBuilder<> B(&F->getEntryBlock().front());
336 AI = B.CreateAlloca(PtrTy, 0, "StackGuardSlot");
337 LoadInst *LI = B.CreateLoad(StackGuardVar, "StackGuard");
338 B.CreateCall2(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), LI,
341 return SupportsSelectionDAGSP;
344 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
347 /// - The prologue code loads and stores the stack guard onto the stack.
348 /// - The epilogue checks the value stored in the prologue against the original
349 /// value. It calls __stack_chk_fail if they differ.
350 bool StackProtector::InsertStackProtectors() {
351 bool HasPrologue = false;
352 bool SupportsSelectionDAGSP =
353 EnableSelectionDAGSP && !TM->Options.EnableFastISel;
354 AllocaInst *AI = 0; // Place on stack that stores the stack guard.
355 Value *StackGuardVar = 0; // The stack guard variable.
357 for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
358 BasicBlock *BB = I++;
359 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
365 SupportsSelectionDAGSP &=
366 CreatePrologue(F, M, RI, TLI, Trip, AI, StackGuardVar);
369 if (SupportsSelectionDAGSP) {
370 // Since we have a potential tail call, insert the special stack check
372 Instruction *InsertionPt = 0;
373 if (CallInst *CI = FindPotentialTailCall(BB, RI, TLI)) {
377 // At this point we know that BB has a return statement so it *DOES*
378 // have a terminator.
379 assert(InsertionPt != 0 && "BB must have a terminator instruction at "
383 Function *Intrinsic =
384 Intrinsic::getDeclaration(M, Intrinsic::stackprotectorcheck);
385 CallInst::Create(Intrinsic, StackGuardVar, "", InsertionPt);
388 // If we do not support SelectionDAG based tail calls, generate IR level
391 // For each block with a return instruction, convert this:
401 // %1 = load __stack_chk_guard
402 // %2 = load StackGuardSlot
403 // %3 = cmp i1 %1, %2
404 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk
409 // CallStackCheckFailBlk:
410 // call void @__stack_chk_fail()
413 // Create the FailBB. We duplicate the BB every time since the MI tail
414 // merge pass will merge together all of the various BB into one including
415 // fail BB generated by the stack protector pseudo instruction.
416 BasicBlock *FailBB = CreateFailBB();
418 // Split the basic block before the return instruction.
419 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
421 // Update the dominator tree if we need to.
422 if (DT && DT->isReachableFromEntry(BB)) {
423 DT->addNewBlock(NewBB, BB);
424 DT->addNewBlock(FailBB, BB);
427 // Remove default branch instruction to the new BB.
428 BB->getTerminator()->eraseFromParent();
430 // Move the newly created basic block to the point right after the old
431 // basic block so that it's in the "fall through" position.
432 NewBB->moveAfter(BB);
434 // Generate the stack protector instructions in the old basic block.
436 LoadInst *LI1 = B.CreateLoad(StackGuardVar);
437 LoadInst *LI2 = B.CreateLoad(AI);
438 Value *Cmp = B.CreateICmpEQ(LI1, LI2);
439 B.CreateCondBr(Cmp, NewBB, FailBB);
443 // Return if we didn't modify any basic blocks. I.e., there are no return
444 // statements in the function.
451 /// CreateFailBB - Create a basic block to jump to when the stack protector
453 BasicBlock *StackProtector::CreateFailBB() {
454 LLVMContext &Context = F->getContext();
455 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
456 IRBuilder<> B(FailBB);
457 if (Trip.getOS() == llvm::Triple::OpenBSD) {
458 Constant *StackChkFail = M->getOrInsertFunction(
459 "__stack_smash_handler", Type::getVoidTy(Context),
460 Type::getInt8PtrTy(Context), NULL);
462 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
464 Constant *StackChkFail = M->getOrInsertFunction(
465 "__stack_chk_fail", Type::getVoidTy(Context), NULL);
466 B.CreateCall(StackChkFail);
468 B.CreateUnreachable();