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 #include "llvm/CodeGen/StackProtector.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/BranchProbabilityInfo.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/MDBuilder.h"
36 #include "llvm/IR/Module.h"
37 #include "llvm/Support/CommandLine.h"
38 #include "llvm/Target/TargetSubtargetInfo.h"
42 #define DEBUG_TYPE "stack-protector"
44 STATISTIC(NumFunProtected, "Number of functions protected");
45 STATISTIC(NumAddrTaken, "Number of local variables that have their address"
48 static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp",
49 cl::init(true), cl::Hidden);
51 char StackProtector::ID = 0;
52 INITIALIZE_PASS(StackProtector, "stack-protector", "Insert stack protectors",
55 FunctionPass *llvm::createStackProtectorPass(const TargetMachine *TM) {
56 return new StackProtector(TM);
59 StackProtector::SSPLayoutKind
60 StackProtector::getSSPLayout(const AllocaInst *AI) const {
61 return AI ? Layout.lookup(AI) : SSPLK_None;
64 void StackProtector::adjustForColoring(const AllocaInst *From,
65 const AllocaInst *To) {
66 // When coloring replaces one alloca with another, transfer the SSPLayoutKind
67 // tag from the remapped to the target alloca. The remapped alloca should
68 // have a size smaller than or equal to the replacement alloca.
69 SSPLayoutMap::iterator I = Layout.find(From);
70 if (I != Layout.end()) {
71 SSPLayoutKind Kind = I->second;
74 // Transfer the tag, but make sure that SSPLK_AddrOf does not overwrite
75 // SSPLK_SmallArray or SSPLK_LargeArray, and make sure that
76 // SSPLK_SmallArray does not overwrite SSPLK_LargeArray.
78 if (I == Layout.end())
79 Layout.insert(std::make_pair(To, Kind));
80 else if (I->second != SSPLK_LargeArray && Kind != SSPLK_AddrOf)
85 bool StackProtector::runOnFunction(Function &Fn) {
88 DominatorTreeWrapperPass *DTWP =
89 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
90 DT = DTWP ? &DTWP->getDomTree() : nullptr;
91 TLI = TM->getSubtargetImpl()->getTargetLowering();
93 Attribute Attr = Fn.getAttributes().getAttribute(
94 AttributeSet::FunctionIndex, "stack-protector-buffer-size");
95 if (Attr.isStringAttribute() &&
96 Attr.getValueAsString().getAsInteger(10, SSPBufferSize))
97 return false; // Invalid integer string
99 if (!RequiresStackProtector())
103 return InsertStackProtectors();
106 /// \param [out] IsLarge is set to true if a protectable array is found and
107 /// it is "large" ( >= ssp-buffer-size). In the case of a structure with
108 /// multiple arrays, this gets set if any of them is large.
109 bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
111 bool InStruct) const {
114 if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
115 if (!AT->getElementType()->isIntegerTy(8)) {
116 // If we're on a non-Darwin platform or we're inside of a structure, don't
117 // add stack protectors unless the array is a character array.
118 // However, in strong mode any array, regardless of type and size,
119 // triggers a protector.
120 if (!Strong && (InStruct || !Trip.isOSDarwin()))
124 // If an array has more than SSPBufferSize bytes of allocated space, then we
125 // emit stack protectors.
126 if (SSPBufferSize <= TLI->getDataLayout()->getTypeAllocSize(AT)) {
132 // Require a protector for all arrays in strong mode
136 const StructType *ST = dyn_cast<StructType>(Ty);
140 bool NeedsProtector = false;
141 for (StructType::element_iterator I = ST->element_begin(),
142 E = ST->element_end();
144 if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
145 // If the element is a protectable array and is large (>= SSPBufferSize)
146 // then we are done. If the protectable array is not large, then
147 // keep looking in case a subsequent element is a large array.
150 NeedsProtector = true;
153 return NeedsProtector;
156 bool StackProtector::HasAddressTaken(const Instruction *AI) {
157 for (const User *U : AI->users()) {
158 if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
159 if (AI == SI->getValueOperand())
161 } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
162 if (AI == SI->getOperand(0))
164 } else if (isa<CallInst>(U)) {
166 } else if (isa<InvokeInst>(U)) {
168 } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
169 if (HasAddressTaken(SI))
171 } else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
172 // Keep track of what PHI nodes we have already visited to ensure
173 // they are only visited once.
174 if (VisitedPHIs.insert(PN).second)
175 if (HasAddressTaken(PN))
177 } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
178 if (HasAddressTaken(GEP))
180 } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
181 if (HasAddressTaken(BI))
188 /// \brief Check whether or not this function needs a stack protector based
189 /// upon the stack protector level.
191 /// We use two heuristics: a standard (ssp) and strong (sspstrong).
192 /// The standard heuristic which will add a guard variable to functions that
193 /// call alloca with a either a variable size or a size >= SSPBufferSize,
194 /// functions with character buffers larger than SSPBufferSize, and functions
195 /// with aggregates containing character buffers larger than SSPBufferSize. The
196 /// strong heuristic will add a guard variables to functions that call alloca
197 /// regardless of size, functions with any buffer regardless of type and size,
198 /// functions with aggregates that contain any buffer regardless of type and
199 /// size, and functions that contain stack-based variables that have had their
201 bool StackProtector::RequiresStackProtector() {
203 bool NeedsProtector = false;
204 if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
205 Attribute::StackProtectReq)) {
206 NeedsProtector = true;
207 Strong = true; // Use the same heuristic as strong to determine SSPLayout
208 } else if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
209 Attribute::StackProtectStrong))
211 else if (!F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
212 Attribute::StackProtect))
215 for (const BasicBlock &BB : *F) {
216 for (const Instruction &I : BB) {
217 if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
218 if (AI->isArrayAllocation()) {
219 // SSP-Strong: Enable protectors for any call to alloca, regardless
224 if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) {
225 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
226 // A call to alloca with size >= SSPBufferSize requires
228 Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
229 NeedsProtector = true;
231 // Require protectors for all alloca calls in strong mode.
232 Layout.insert(std::make_pair(AI, SSPLK_SmallArray));
233 NeedsProtector = true;
236 // A call to alloca with a variable size requires protectors.
237 Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
238 NeedsProtector = true;
243 bool IsLarge = false;
244 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
245 Layout.insert(std::make_pair(AI, IsLarge ? SSPLK_LargeArray
246 : SSPLK_SmallArray));
247 NeedsProtector = true;
251 if (Strong && HasAddressTaken(AI)) {
253 Layout.insert(std::make_pair(AI, SSPLK_AddrOf));
254 NeedsProtector = true;
260 return NeedsProtector;
263 static bool InstructionWillNotHaveChain(const Instruction *I) {
264 return !I->mayHaveSideEffects() && !I->mayReadFromMemory() &&
265 isSafeToSpeculativelyExecute(I);
268 /// Identify if RI has a previous instruction in the "Tail Position" and return
269 /// it. Otherwise return 0.
271 /// This is based off of the code in llvm::isInTailCallPosition. The difference
272 /// is that it inverts the first part of llvm::isInTailCallPosition since
273 /// isInTailCallPosition is checking if a call is in a tail call position, and
274 /// we are searching for an unknown tail call that might be in the tail call
275 /// position. Once we find the call though, the code uses the same refactored
276 /// code, returnTypeIsEligibleForTailCall.
277 static CallInst *FindPotentialTailCall(BasicBlock *BB, ReturnInst *RI,
278 const TargetLoweringBase *TLI) {
279 // Establish a reasonable upper bound on the maximum amount of instructions we
280 // will look through to find a tail call.
281 unsigned SearchCounter = 0;
282 const unsigned MaxSearch = 4;
283 bool NoInterposingChain = true;
285 for (BasicBlock::reverse_iterator I = std::next(BB->rbegin()), E = BB->rend();
286 I != E && SearchCounter < MaxSearch; ++I) {
287 Instruction *Inst = &*I;
289 // Skip over debug intrinsics and do not allow them to affect our MaxSearch
291 if (isa<DbgInfoIntrinsic>(Inst))
294 // If we find a call and the following conditions are satisifed, then we
295 // have found a tail call that satisfies at least the target independent
296 // requirements of a tail call:
298 // 1. The call site has the tail marker.
300 // 2. The call site either will not cause the creation of a chain or if a
301 // chain is necessary there are no instructions in between the callsite and
302 // the call which would create an interposing chain.
304 // 3. The return type of the function does not impede tail call
306 if (CallInst *CI = dyn_cast<CallInst>(Inst)) {
307 if (CI->isTailCall() &&
308 (InstructionWillNotHaveChain(CI) || NoInterposingChain) &&
309 returnTypeIsEligibleForTailCall(BB->getParent(), CI, RI, *TLI))
313 // If we did not find a call see if we have an instruction that may create
314 // an interposing chain.
316 NoInterposingChain && InstructionWillNotHaveChain(Inst);
318 // Increment max search.
325 /// Insert code into the entry block that stores the __stack_chk_guard
326 /// variable onto the stack:
329 /// StackGuardSlot = alloca i8*
330 /// StackGuard = load __stack_chk_guard
331 /// call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
333 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo
335 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
336 const TargetLoweringBase *TLI, const Triple &TT,
337 AllocaInst *&AI, Value *&StackGuardVar) {
338 bool SupportsSelectionDAGSP = false;
339 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
340 unsigned AddressSpace, Offset;
341 if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
342 Constant *OffsetVal =
343 ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
346 ConstantExpr::getIntToPtr(OffsetVal, PointerType::get(PtrTy,
348 } else if (TT.isOSOpenBSD()) {
349 StackGuardVar = M->getOrInsertGlobal("__guard_local", PtrTy);
350 cast<GlobalValue>(StackGuardVar)
351 ->setVisibility(GlobalValue::HiddenVisibility);
353 SupportsSelectionDAGSP = true;
354 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
357 IRBuilder<> B(&F->getEntryBlock().front());
358 AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot");
359 LoadInst *LI = B.CreateLoad(StackGuardVar, "StackGuard");
360 B.CreateCall2(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), LI,
363 return SupportsSelectionDAGSP;
366 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
369 /// - The prologue code loads and stores the stack guard onto the stack.
370 /// - The epilogue checks the value stored in the prologue against the original
371 /// value. It calls __stack_chk_fail if they differ.
372 bool StackProtector::InsertStackProtectors() {
373 bool HasPrologue = false;
374 bool SupportsSelectionDAGSP =
375 EnableSelectionDAGSP && !TM->Options.EnableFastISel;
376 AllocaInst *AI = nullptr; // Place on stack that stores the stack guard.
377 Value *StackGuardVar = nullptr; // The stack guard variable.
379 for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
380 BasicBlock *BB = I++;
381 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
387 SupportsSelectionDAGSP &=
388 CreatePrologue(F, M, RI, TLI, Trip, AI, StackGuardVar);
391 if (SupportsSelectionDAGSP) {
392 // Since we have a potential tail call, insert the special stack check
394 Instruction *InsertionPt = nullptr;
395 if (CallInst *CI = FindPotentialTailCall(BB, RI, TLI)) {
399 // At this point we know that BB has a return statement so it *DOES*
400 // have a terminator.
401 assert(InsertionPt != nullptr &&
402 "BB must have a terminator instruction at this point.");
405 Function *Intrinsic =
406 Intrinsic::getDeclaration(M, Intrinsic::stackprotectorcheck);
407 CallInst::Create(Intrinsic, StackGuardVar, "", InsertionPt);
409 // If we do not support SelectionDAG based tail calls, generate IR level
412 // For each block with a return instruction, convert this:
422 // %1 = load __stack_chk_guard
423 // %2 = load StackGuardSlot
424 // %3 = cmp i1 %1, %2
425 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk
430 // CallStackCheckFailBlk:
431 // call void @__stack_chk_fail()
434 // Create the FailBB. We duplicate the BB every time since the MI tail
435 // merge pass will merge together all of the various BB into one including
436 // fail BB generated by the stack protector pseudo instruction.
437 BasicBlock *FailBB = CreateFailBB();
439 // Split the basic block before the return instruction.
440 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
442 // Update the dominator tree if we need to.
443 if (DT && DT->isReachableFromEntry(BB)) {
444 DT->addNewBlock(NewBB, BB);
445 DT->addNewBlock(FailBB, BB);
448 // Remove default branch instruction to the new BB.
449 BB->getTerminator()->eraseFromParent();
451 // Move the newly created basic block to the point right after the old
452 // basic block so that it's in the "fall through" position.
453 NewBB->moveAfter(BB);
455 // Generate the stack protector instructions in the old basic block.
457 LoadInst *LI1 = B.CreateLoad(StackGuardVar);
458 LoadInst *LI2 = B.CreateLoad(AI);
459 Value *Cmp = B.CreateICmpEQ(LI1, LI2);
460 unsigned SuccessWeight =
461 BranchProbabilityInfo::getBranchWeightStackProtector(true);
462 unsigned FailureWeight =
463 BranchProbabilityInfo::getBranchWeightStackProtector(false);
464 MDNode *Weights = MDBuilder(F->getContext())
465 .createBranchWeights(SuccessWeight, FailureWeight);
466 B.CreateCondBr(Cmp, NewBB, FailBB, Weights);
470 // Return if we didn't modify any basic blocks. i.e., there are no return
471 // statements in the function.
478 /// CreateFailBB - Create a basic block to jump to when the stack protector
480 BasicBlock *StackProtector::CreateFailBB() {
481 LLVMContext &Context = F->getContext();
482 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
483 IRBuilder<> B(FailBB);
484 if (Trip.isOSOpenBSD()) {
485 Constant *StackChkFail =
486 M->getOrInsertFunction("__stack_smash_handler",
487 Type::getVoidTy(Context),
488 Type::getInt8PtrTy(Context), nullptr);
490 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
492 Constant *StackChkFail =
493 M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context),
495 B.CreateCall(StackChkFail);
497 B.CreateUnreachable();