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//===- StackProtector.cpp - Stack Protector Insertion ---------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass inserts stack protectors into functions which need them. A variable
// with a random value in it is stored onto the stack before the local variables
// are allocated. Upon exiting the block, the stored value is checked. If it's
// changed, then there was some sort of violation and the program aborts.
#include "llvm/CodeGen/StackProtector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/EHPersonalities.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "stack-protector"
STATISTIC(NumFunProtected, "Number of functions protected");
STATISTIC(NumAddrTaken, "Number of local variables that have their address"
" taken.");
static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp",
cl::init(true), cl::Hidden);
char StackProtector::ID = 0;
StackProtector::StackProtector() : FunctionPass(ID), SSPBufferSize(8) {
"Insert stack protectors", false, true)
"Insert stack protectors", false, true)
FunctionPass *llvm::createStackProtectorPass() { return new StackProtector(); }
void StackProtector::getAnalysisUsage(AnalysisUsage &AU) const {
bool StackProtector::runOnFunction(Function &Fn) {
F = &Fn;
M = F->getParent();
DominatorTreeWrapperPass *DTWP =
DT = DTWP ? &DTWP->getDomTree() : nullptr;
TM = &getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
Trip = TM->getTargetTriple();
TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
HasPrologue = false;
HasIRCheck = false;
Attribute Attr = Fn.getFnAttribute("stack-protector-buffer-size");
if (Attr.isStringAttribute() &&
Attr.getValueAsString().getAsInteger(10, SSPBufferSize))
return false; // Invalid integer string
if (!RequiresStackProtector())
return false;
// TODO(etienneb): Functions with funclets are not correctly supported now.
// Do nothing if this is funclet-based personality.
if (Fn.hasPersonalityFn()) {
EHPersonality Personality = classifyEHPersonality(Fn.getPersonalityFn());
if (isFuncletEHPersonality(Personality))
return false;
return InsertStackProtectors();
/// \param [out] IsLarge is set to true if a protectable array is found and
/// it is "large" ( >= ssp-buffer-size). In the case of a structure with
/// multiple arrays, this gets set if any of them is large.
bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
bool Strong,
bool InStruct) const {
if (!Ty)
return false;
if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
if (!AT->getElementType()->isIntegerTy(8)) {
// If we're on a non-Darwin platform or we're inside of a structure, don't
// add stack protectors unless the array is a character array.
// However, in strong mode any array, regardless of type and size,
// triggers a protector.
if (!Strong && (InStruct || !Trip.isOSDarwin()))
return false;
// If an array has more than SSPBufferSize bytes of allocated space, then we
// emit stack protectors.
if (SSPBufferSize <= M->getDataLayout().getTypeAllocSize(AT)) {
IsLarge = true;
return true;
if (Strong)
// Require a protector for all arrays in strong mode
return true;
const StructType *ST = dyn_cast<StructType>(Ty);
if (!ST)
return false;
bool NeedsProtector = false;
for (StructType::element_iterator I = ST->element_begin(),
E = ST->element_end();
I != E; ++I)
if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
// If the element is a protectable array and is large (>= SSPBufferSize)
// then we are done. If the protectable array is not large, then
// keep looking in case a subsequent element is a large array.
if (IsLarge)
return true;
NeedsProtector = true;
return NeedsProtector;
bool StackProtector::HasAddressTaken(const Instruction *AI,
uint64_t AllocSize) {
const DataLayout &DL = M->getDataLayout();
for (const User *U : AI->users()) {
const auto *I = cast<Instruction>(U);
// If this instruction accesses memory make sure it doesn't access beyond
// the bounds of the allocated object.
Optional<MemoryLocation> MemLoc = MemoryLocation::getOrNone(I);
if (MemLoc.hasValue() && MemLoc->Size.hasValue() &&
MemLoc->Size.getValue() > AllocSize)
return true;
switch (I->getOpcode()) {
case Instruction::Store:
if (AI == cast<StoreInst>(I)->getValueOperand())
return true;
case Instruction::AtomicCmpXchg:
// cmpxchg conceptually includes both a load and store from the same
// location. So, like store, the value being stored is what matters.
if (AI == cast<AtomicCmpXchgInst>(I)->getNewValOperand())
return true;
case Instruction::PtrToInt:
if (AI == cast<PtrToIntInst>(I)->getOperand(0))
return true;
case Instruction::Call: {
// Ignore intrinsics that do not become real instructions.
// TODO: Narrow this to intrinsics that have store-like effects.
const auto *CI = cast<CallInst>(I);
if (!CI->isDebugOrPseudoInst() && !CI->isLifetimeStartOrEnd())
return true;
case Instruction::Invoke:
return true;
case Instruction::GetElementPtr: {
// If the GEP offset is out-of-bounds, or is non-constant and so has to be
// assumed to be potentially out-of-bounds, then any memory access that
// would use it could also be out-of-bounds meaning stack protection is
// required.
const GetElementPtrInst *GEP = cast<GetElementPtrInst>(I);
unsigned TypeSize = DL.getIndexTypeSizeInBits(I->getType());
APInt Offset(TypeSize, 0);
APInt MaxOffset(TypeSize, AllocSize);
if (!GEP->accumulateConstantOffset(DL, Offset) || Offset.ugt(MaxOffset))
return true;
// Adjust AllocSize to be the space remaining after this offset.
if (HasAddressTaken(I, AllocSize - Offset.getLimitedValue()))
return true;
case Instruction::BitCast:
case Instruction::Select:
case Instruction::AddrSpaceCast:
if (HasAddressTaken(I, AllocSize))
return true;
case Instruction::PHI: {
// Keep track of what PHI nodes we have already visited to ensure
// they are only visited once.
const auto *PN = cast<PHINode>(I);
if (VisitedPHIs.insert(PN).second)
if (HasAddressTaken(PN, AllocSize))
return true;
case Instruction::Load:
case Instruction::AtomicRMW:
case Instruction::Ret:
// These instructions take an address operand, but have load-like or
// other innocuous behavior that should not trigger a stack protector.
// atomicrmw conceptually has both load and store semantics, but the
// value being stored must be integer; so if a pointer is being stored,
// we'll catch it in the PtrToInt case above.
// Conservatively return true for any instruction that takes an address
// operand, but is not handled above.
return true;
return false;
/// Search for the first call to the llvm.stackprotector intrinsic and return it
/// if present.
static const CallInst *findStackProtectorIntrinsic(Function &F) {
for (const BasicBlock &BB : F)
for (const Instruction &I : BB)
if (const auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::stackprotector)
return II;
return nullptr;
/// Check whether or not this function needs a stack protector based
/// upon the stack protector level.
/// We use two heuristics: a standard (ssp) and strong (sspstrong).
/// The standard heuristic which will add a guard variable to functions that
/// call alloca with a either a variable size or a size >= SSPBufferSize,
/// functions with character buffers larger than SSPBufferSize, and functions
/// with aggregates containing character buffers larger than SSPBufferSize. The
/// strong heuristic will add a guard variables to functions that call alloca
/// regardless of size, functions with any buffer regardless of type and size,
/// functions with aggregates that contain any buffer regardless of type and
/// size, and functions that contain stack-based variables that have had their
/// address taken.
bool StackProtector::RequiresStackProtector() {
bool Strong = false;
bool NeedsProtector = false;
if (F->hasFnAttribute(Attribute::SafeStack))
return false;
// We are constructing the OptimizationRemarkEmitter on the fly rather than
// using the analysis pass to avoid building DominatorTree and LoopInfo which
// are not available this late in the IR pipeline.
OptimizationRemarkEmitter ORE(F);
if (F->hasFnAttribute(Attribute::StackProtectReq)) {
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "StackProtectorRequested", F)
<< "Stack protection applied to function "
<< ore::NV("Function", F)
<< " due to a function attribute or command-line switch";
NeedsProtector = true;
Strong = true; // Use the same heuristic as strong to determine SSPLayout
} else if (F->hasFnAttribute(Attribute::StackProtectStrong))
Strong = true;
else if (!F->hasFnAttribute(Attribute::StackProtect))
return false;
for (const BasicBlock &BB : *F) {
for (const Instruction &I : BB) {
if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
if (AI->isArrayAllocation()) {
auto RemarkBuilder = [&]() {
return OptimizationRemark(DEBUG_TYPE, "StackProtectorAllocaOrArray",
<< "Stack protection applied to function "
<< ore::NV("Function", F)
<< " due to a call to alloca or use of a variable length "
if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) {
if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
// A call to alloca with size >= SSPBufferSize requires
// stack protectors.
NeedsProtector = true;
} else if (Strong) {
// Require protectors for all alloca calls in strong mode.
NeedsProtector = true;
} else {
// A call to alloca with a variable size requires protectors.
NeedsProtector = true;
bool IsLarge = false;
if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
Layout.insert(std::make_pair(AI, IsLarge
? MachineFrameInfo::SSPLK_LargeArray
: MachineFrameInfo::SSPLK_SmallArray));
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "StackProtectorBuffer", &I)
<< "Stack protection applied to function "
<< ore::NV("Function", F)
<< " due to a stack allocated buffer or struct containing a "
NeedsProtector = true;
if (Strong && HasAddressTaken(AI, M->getDataLayout().getTypeAllocSize(
AI->getAllocatedType()))) {
Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf));
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "StackProtectorAddressTaken",
<< "Stack protection applied to function "
<< ore::NV("Function", F)
<< " due to the address of a local variable being taken";
NeedsProtector = true;
// Clear any PHIs that we visited, to make sure we examine all uses of
// any subsequent allocas that we look at.
return NeedsProtector;
/// Create a stack guard loading and populate whether SelectionDAG SSP is
/// supported.
static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M,
IRBuilder<> &B,
bool *SupportsSelectionDAGSP = nullptr) {
Value *Guard = TLI->getIRStackGuard(B);
auto GuardMode = TLI->getTargetMachine().Options.StackProtectorGuard;
if ((GuardMode == llvm::StackProtectorGuards::TLS ||
GuardMode == llvm::StackProtectorGuards::None) && Guard)
return B.CreateLoad(B.getInt8PtrTy(), Guard, true, "StackGuard");
// Use SelectionDAG SSP handling, since there isn't an IR guard.
// This is more or less weird, since we optionally output whether we
// should perform a SelectionDAG SP here. The reason is that it's strictly
// defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also
// mutating. There is no way to get this bit without mutating the IR, so
// getting this bit has to happen in this right time.
// We could have define a new function TLI::supportsSelectionDAGSP(), but that
// will put more burden on the backends' overriding work, especially when it
// actually conveys the same information getIRStackGuard() already gives.
if (SupportsSelectionDAGSP)
*SupportsSelectionDAGSP = true;
return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard));
/// Insert code into the entry block that stores the stack guard
/// variable onto the stack:
/// entry:
/// StackGuardSlot = alloca i8*
/// StackGuard = <stack guard>
/// call void @llvm.stackprotector(StackGuard, StackGuardSlot)
/// Returns true if the platform/triple supports the stackprotectorcreate pseudo
/// node.
static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
const TargetLoweringBase *TLI, AllocaInst *&AI) {
bool SupportsSelectionDAGSP = false;
IRBuilder<> B(&F->getEntryBlock().front());
PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot");
Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP);
B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
{GuardSlot, AI});
return SupportsSelectionDAGSP;
/// InsertStackProtectors - Insert code into the prologue and epilogue of the
/// function.
/// - The prologue code loads and stores the stack guard onto the stack.
/// - The epilogue checks the value stored in the prologue against the original
/// value. It calls __stack_chk_fail if they differ.
bool StackProtector::InsertStackProtectors() {
// If the target wants to XOR the frame pointer into the guard value, it's
// impossible to emit the check in IR, so the target *must* support stack
// protection in SDAG.
bool SupportsSelectionDAGSP =
TLI->useStackGuardXorFP() ||
(EnableSelectionDAGSP && !TM->Options.EnableFastISel &&
AllocaInst *AI = nullptr; // Place on stack that stores the stack guard.
for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
BasicBlock *BB = &*I++;
ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
if (!RI)
// Generate prologue instrumentation if not already generated.
if (!HasPrologue) {
HasPrologue = true;
SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI);
// SelectionDAG based code generation. Nothing else needs to be done here.
// The epilogue instrumentation is postponed to SelectionDAG.
if (SupportsSelectionDAGSP)
// Find the stack guard slot if the prologue was not created by this pass
// itself via a previous call to CreatePrologue().
if (!AI) {
const CallInst *SPCall = findStackProtectorIntrinsic(*F);
assert(SPCall && "Call to llvm.stackprotector is missing");
AI = cast<AllocaInst>(SPCall->getArgOperand(1));
// Set HasIRCheck to true, so that SelectionDAG will not generate its own
// version. SelectionDAG called 'shouldEmitSDCheck' to check whether
// instrumentation has already been generated.
HasIRCheck = true;
// If we're instrumenting a block with a musttail call, the check has to be
// inserted before the call rather than between it and the return. The
// verifier guarantees that a musttail call is either directly before the
// return or with a single correct bitcast of the return value in between so
// we don't need to worry about many situations here.
Instruction *CheckLoc = RI;
Instruction *Prev = RI->getPrevNonDebugInstruction();
if (Prev && isa<CallInst>(Prev) && cast<CallInst>(Prev)->isMustTailCall())
CheckLoc = Prev;
else if (Prev) {
Prev = Prev->getPrevNonDebugInstruction();
if (Prev && isa<CallInst>(Prev) && cast<CallInst>(Prev)->isMustTailCall())
CheckLoc = Prev;
// Generate epilogue instrumentation. The epilogue intrumentation can be
// function-based or inlined depending on which mechanism the target is
// providing.
if (Function *GuardCheck = TLI->getSSPStackGuardCheck(*M)) {
// Generate the function-based epilogue instrumentation.
// The target provides a guard check function, generate a call to it.
IRBuilder<> B(CheckLoc);
LoadInst *Guard = B.CreateLoad(B.getInt8PtrTy(), AI, true, "Guard");
CallInst *Call = B.CreateCall(GuardCheck, {Guard});
} else {
// Generate the epilogue with inline instrumentation.
// If we do not support SelectionDAG based calls, generate IR level
// calls.
// For each block with a return instruction, convert this:
// return:
// ...
// ret ...
// into this:
// return:
// ...
// %1 = <stack guard>
// %2 = load StackGuardSlot
// %3 = cmp i1 %1, %2
// br i1 %3, label %SP_return, label %CallStackCheckFailBlk
// SP_return:
// ret ...
// CallStackCheckFailBlk:
// call void @__stack_chk_fail()
// unreachable
// Create the FailBB. We duplicate the BB every time since the MI tail
// merge pass will merge together all of the various BB into one including
// fail BB generated by the stack protector pseudo instruction.
BasicBlock *FailBB = CreateFailBB();
// Split the basic block before the return instruction.
BasicBlock *NewBB =
BB->splitBasicBlock(CheckLoc->getIterator(), "SP_return");
// Update the dominator tree if we need to.
if (DT && DT->isReachableFromEntry(BB)) {
DT->addNewBlock(NewBB, BB);
DT->addNewBlock(FailBB, BB);
// Remove default branch instruction to the new BB.
// Move the newly created basic block to the point right after the old
// basic block so that it's in the "fall through" position.
// Generate the stack protector instructions in the old basic block.
IRBuilder<> B(BB);
Value *Guard = getStackGuard(TLI, M, B);
LoadInst *LI2 = B.CreateLoad(B.getInt8PtrTy(), AI, true);
Value *Cmp = B.CreateICmpEQ(Guard, LI2);
auto SuccessProb =
auto FailureProb =
MDNode *Weights = MDBuilder(F->getContext())
B.CreateCondBr(Cmp, NewBB, FailBB, Weights);
// Return if we didn't modify any basic blocks. i.e., there are no return
// statements in the function.
return HasPrologue;
/// CreateFailBB - Create a basic block to jump to when the stack protector
/// check fails.
BasicBlock *StackProtector::CreateFailBB() {
LLVMContext &Context = F->getContext();
BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
IRBuilder<> B(FailBB);
if (F->getSubprogram())
DILocation::get(Context, 0, 0, F->getSubprogram()));
if (Trip.isOSOpenBSD()) {
FunctionCallee StackChkFail = M->getOrInsertFunction(
"__stack_smash_handler", Type::getVoidTy(Context),
B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
} else {
FunctionCallee StackChkFail =
M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context));
B.CreateCall(StackChkFail, {});
return FailBB;
bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const {
return HasPrologue && !HasIRCheck && isa<ReturnInst>(BB.getTerminator());
void StackProtector::copyToMachineFrameInfo(MachineFrameInfo &MFI) const {
if (Layout.empty())
for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) {
if (MFI.isDeadObjectIndex(I))
const AllocaInst *AI = MFI.getObjectAllocation(I);
if (!AI)
SSPLayoutMap::const_iterator LI = Layout.find(AI);
if (LI == Layout.end())
MFI.setObjectSSPLayout(I, LI->second);