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 DominatorTreeWrapperPass *DTWP =
64 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
65 DT = DTWP ? &DTWP->getDomTree() : 0;
66 TLI = TM->getTargetLowering();
68 if (!RequiresStackProtector())
71 Attribute Attr = Fn.getAttributes().getAttribute(
72 AttributeSet::FunctionIndex, "stack-protector-buffer-size");
73 if (Attr.isStringAttribute())
74 Attr.getValueAsString().getAsInteger(10, SSPBufferSize);
77 return InsertStackProtectors();
80 /// \param [out] IsLarge is set to true if a protectable array is found and
81 /// it is "large" ( >= ssp-buffer-size). In the case of a structure with
82 /// multiple arrays, this gets set if any of them is large.
83 bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
85 bool InStruct) const {
88 if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
89 if (!AT->getElementType()->isIntegerTy(8)) {
90 // If we're on a non-Darwin platform or we're inside of a structure, don't
91 // add stack protectors unless the array is a character array.
92 // However, in strong mode any array, regardless of type and size,
93 // triggers a protector.
94 if (!Strong && (InStruct || !Trip.isOSDarwin()))
98 // If an array has more than SSPBufferSize bytes of allocated space, then we
99 // emit stack protectors.
100 if (SSPBufferSize <= TLI->getDataLayout()->getTypeAllocSize(AT)) {
106 // Require a protector for all arrays in strong mode
110 const StructType *ST = dyn_cast<StructType>(Ty);
114 bool NeedsProtector = false;
115 for (StructType::element_iterator I = ST->element_begin(),
116 E = ST->element_end();
118 if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
119 // If the element is a protectable array and is large (>= SSPBufferSize)
120 // then we are done. If the protectable array is not large, then
121 // keep looking in case a subsequent element is a large array.
124 NeedsProtector = true;
127 return NeedsProtector;
130 bool StackProtector::HasAddressTaken(const Instruction *AI) {
131 for (Value::const_use_iterator UI = AI->use_begin(), UE = AI->use_end();
134 if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
135 if (AI == SI->getValueOperand())
137 } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
138 if (AI == SI->getOperand(0))
140 } else if (isa<CallInst>(U)) {
142 } else if (isa<InvokeInst>(U)) {
144 } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
145 if (HasAddressTaken(SI))
147 } else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
148 // Keep track of what PHI nodes we have already visited to ensure
149 // they are only visited once.
150 if (VisitedPHIs.insert(PN))
151 if (HasAddressTaken(PN))
153 } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
154 if (HasAddressTaken(GEP))
156 } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
157 if (HasAddressTaken(BI))
164 /// \brief Check whether or not this function needs a stack protector based
165 /// upon the stack protector level.
167 /// We use two heuristics: a standard (ssp) and strong (sspstrong).
168 /// The standard heuristic which will add a guard variable to functions that
169 /// call alloca with a either a variable size or a size >= SSPBufferSize,
170 /// functions with character buffers larger than SSPBufferSize, and functions
171 /// with aggregates containing character buffers larger than SSPBufferSize. The
172 /// strong heuristic will add a guard variables to functions that call alloca
173 /// regardless of size, functions with any buffer regardless of type and size,
174 /// functions with aggregates that contain any buffer regardless of type and
175 /// size, and functions that contain stack-based variables that have had their
177 bool StackProtector::RequiresStackProtector() {
179 bool NeedsProtector = false;
180 if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
181 Attribute::StackProtectReq)) {
182 NeedsProtector = true;
183 Strong = true; // Use the same heuristic as strong to determine SSPLayout
184 } else if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
185 Attribute::StackProtectStrong))
187 else if (!F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
188 Attribute::StackProtect))
191 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
194 for (BasicBlock::iterator II = BB->begin(), IE = BB->end(); II != IE;
196 if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
197 if (AI->isArrayAllocation()) {
198 // SSP-Strong: Enable protectors for any call to alloca, regardless
203 if (const ConstantInt *CI =
204 dyn_cast<ConstantInt>(AI->getArraySize())) {
205 if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
206 // A call to alloca with size >= SSPBufferSize requires
208 Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
209 NeedsProtector = true;
211 // Require protectors for all alloca calls in strong mode.
212 Layout.insert(std::make_pair(AI, SSPLK_SmallArray));
213 NeedsProtector = true;
216 // A call to alloca with a variable size requires protectors.
217 Layout.insert(std::make_pair(AI, SSPLK_LargeArray));
218 NeedsProtector = true;
223 bool IsLarge = false;
224 if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
225 Layout.insert(std::make_pair(AI, IsLarge ? SSPLK_LargeArray
226 : SSPLK_SmallArray));
227 NeedsProtector = true;
231 if (Strong && HasAddressTaken(AI)) {
233 Layout.insert(std::make_pair(AI, SSPLK_AddrOf));
234 NeedsProtector = true;
240 return NeedsProtector;
243 static bool InstructionWillNotHaveChain(const Instruction *I) {
244 return !I->mayHaveSideEffects() && !I->mayReadFromMemory() &&
245 isSafeToSpeculativelyExecute(I);
248 /// Identify if RI has a previous instruction in the "Tail Position" and return
249 /// it. Otherwise return 0.
251 /// This is based off of the code in llvm::isInTailCallPosition. The difference
252 /// is that it inverts the first part of llvm::isInTailCallPosition since
253 /// isInTailCallPosition is checking if a call is in a tail call position, and
254 /// we are searching for an unknown tail call that might be in the tail call
255 /// position. Once we find the call though, the code uses the same refactored
256 /// code, returnTypeIsEligibleForTailCall.
257 static CallInst *FindPotentialTailCall(BasicBlock *BB, ReturnInst *RI,
258 const TargetLoweringBase *TLI) {
259 // Establish a reasonable upper bound on the maximum amount of instructions we
260 // will look through to find a tail call.
261 unsigned SearchCounter = 0;
262 const unsigned MaxSearch = 4;
263 bool NoInterposingChain = true;
265 for (BasicBlock::reverse_iterator I = llvm::next(BB->rbegin()),
267 I != E && SearchCounter < MaxSearch; ++I) {
268 Instruction *Inst = &*I;
270 // Skip over debug intrinsics and do not allow them to affect our MaxSearch
272 if (isa<DbgInfoIntrinsic>(Inst))
275 // If we find a call and the following conditions are satisifed, then we
276 // have found a tail call that satisfies at least the target independent
277 // requirements of a tail call:
279 // 1. The call site has the tail marker.
281 // 2. The call site either will not cause the creation of a chain or if a
282 // chain is necessary there are no instructions in between the callsite and
283 // the call which would create an interposing chain.
285 // 3. The return type of the function does not impede tail call
287 if (CallInst *CI = dyn_cast<CallInst>(Inst)) {
288 if (CI->isTailCall() &&
289 (InstructionWillNotHaveChain(CI) || NoInterposingChain) &&
290 returnTypeIsEligibleForTailCall(BB->getParent(), CI, RI, *TLI))
294 // If we did not find a call see if we have an instruction that may create
295 // an interposing chain.
297 NoInterposingChain && InstructionWillNotHaveChain(Inst);
299 // Increment max search.
306 /// Insert code into the entry block that stores the __stack_chk_guard
307 /// variable onto the stack:
310 /// StackGuardSlot = alloca i8*
311 /// StackGuard = load __stack_chk_guard
312 /// call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
314 /// Returns true if the platform/triple supports the stackprotectorcreate pseudo
316 static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
317 const TargetLoweringBase *TLI, const Triple &Trip,
318 AllocaInst *&AI, Value *&StackGuardVar) {
319 bool SupportsSelectionDAGSP = false;
320 PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
321 unsigned AddressSpace, Offset;
322 if (TLI->getStackCookieLocation(AddressSpace, Offset)) {
323 Constant *OffsetVal =
324 ConstantInt::get(Type::getInt32Ty(RI->getContext()), Offset);
326 StackGuardVar = ConstantExpr::getIntToPtr(
327 OffsetVal, PointerType::get(PtrTy, AddressSpace));
328 } else if (Trip.getOS() == llvm::Triple::OpenBSD) {
329 StackGuardVar = M->getOrInsertGlobal("__guard_local", PtrTy);
330 cast<GlobalValue>(StackGuardVar)
331 ->setVisibility(GlobalValue::HiddenVisibility);
333 SupportsSelectionDAGSP = true;
334 StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);
337 IRBuilder<> B(&F->getEntryBlock().front());
338 AI = B.CreateAlloca(PtrTy, 0, "StackGuardSlot");
339 LoadInst *LI = B.CreateLoad(StackGuardVar, "StackGuard");
340 B.CreateCall2(Intrinsic::getDeclaration(M, Intrinsic::stackprotector), LI,
343 return SupportsSelectionDAGSP;
346 /// InsertStackProtectors - Insert code into the prologue and epilogue of the
349 /// - The prologue code loads and stores the stack guard onto the stack.
350 /// - The epilogue checks the value stored in the prologue against the original
351 /// value. It calls __stack_chk_fail if they differ.
352 bool StackProtector::InsertStackProtectors() {
353 bool HasPrologue = false;
354 bool SupportsSelectionDAGSP =
355 EnableSelectionDAGSP && !TM->Options.EnableFastISel;
356 AllocaInst *AI = 0; // Place on stack that stores the stack guard.
357 Value *StackGuardVar = 0; // The stack guard variable.
359 for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
360 BasicBlock *BB = I++;
361 ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
367 SupportsSelectionDAGSP &=
368 CreatePrologue(F, M, RI, TLI, Trip, AI, StackGuardVar);
371 if (SupportsSelectionDAGSP) {
372 // Since we have a potential tail call, insert the special stack check
374 Instruction *InsertionPt = 0;
375 if (CallInst *CI = FindPotentialTailCall(BB, RI, TLI)) {
379 // At this point we know that BB has a return statement so it *DOES*
380 // have a terminator.
381 assert(InsertionPt != 0 && "BB must have a terminator instruction at "
385 Function *Intrinsic =
386 Intrinsic::getDeclaration(M, Intrinsic::stackprotectorcheck);
387 CallInst::Create(Intrinsic, StackGuardVar, "", InsertionPt);
390 // If we do not support SelectionDAG based tail calls, generate IR level
393 // For each block with a return instruction, convert this:
403 // %1 = load __stack_chk_guard
404 // %2 = load StackGuardSlot
405 // %3 = cmp i1 %1, %2
406 // br i1 %3, label %SP_return, label %CallStackCheckFailBlk
411 // CallStackCheckFailBlk:
412 // call void @__stack_chk_fail()
415 // Create the FailBB. We duplicate the BB every time since the MI tail
416 // merge pass will merge together all of the various BB into one including
417 // fail BB generated by the stack protector pseudo instruction.
418 BasicBlock *FailBB = CreateFailBB();
420 // Split the basic block before the return instruction.
421 BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
423 // Update the dominator tree if we need to.
424 if (DT && DT->isReachableFromEntry(BB)) {
425 DT->addNewBlock(NewBB, BB);
426 DT->addNewBlock(FailBB, BB);
429 // Remove default branch instruction to the new BB.
430 BB->getTerminator()->eraseFromParent();
432 // Move the newly created basic block to the point right after the old
433 // basic block so that it's in the "fall through" position.
434 NewBB->moveAfter(BB);
436 // Generate the stack protector instructions in the old basic block.
438 LoadInst *LI1 = B.CreateLoad(StackGuardVar);
439 LoadInst *LI2 = B.CreateLoad(AI);
440 Value *Cmp = B.CreateICmpEQ(LI1, LI2);
441 B.CreateCondBr(Cmp, NewBB, FailBB);
445 // Return if we didn't modify any basic blocks. I.e., there are no return
446 // statements in the function.
453 /// CreateFailBB - Create a basic block to jump to when the stack protector
455 BasicBlock *StackProtector::CreateFailBB() {
456 LLVMContext &Context = F->getContext();
457 BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
458 IRBuilder<> B(FailBB);
459 if (Trip.getOS() == llvm::Triple::OpenBSD) {
460 Constant *StackChkFail = M->getOrInsertFunction(
461 "__stack_smash_handler", Type::getVoidTy(Context),
462 Type::getInt8PtrTy(Context), NULL);
464 B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
466 Constant *StackChkFail = M->getOrInsertFunction(
467 "__stack_chk_fail", Type::getVoidTy(Context), NULL);
468 B.CreateCall(StackChkFail);
470 B.CreateUnreachable();