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//===- StackProtector.cpp - Stack Protector Insertion ---------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// 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/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/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;
"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;
static bool isLifetimeInst(const Instruction *I) {
if (const auto Intrinsic = dyn_cast<IntrinsicInst>(I)) {
const auto Id = Intrinsic->getIntrinsicID();
return Id == Intrinsic::lifetime_start || Id == Intrinsic::lifetime_end;
return false;
bool StackProtector::HasAddressTaken(const Instruction *AI) {
for (const User *U : AI->users()) {
if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
if (AI == SI->getValueOperand())
return true;
} else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
if (AI == SI->getOperand(0))
return true;
} else if (const CallInst *CI = dyn_cast<CallInst>(U)) {
// Ignore intrinsics that are not calls. TODO: Use isLoweredToCall().
if (!isa<DbgInfoIntrinsic>(CI) && !isLifetimeInst(CI))
return true;
} else if (isa<InvokeInst>(U)) {
return true;
} else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
if (HasAddressTaken(SI))
return true;
} else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
// Keep track of what PHI nodes we have already visited to ensure
// they are only visited once.
if (VisitedPHIs.insert(PN).second)
if (HasAddressTaken(PN))
return true;
} else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
if (HasAddressTaken(GEP))
return true;
} else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
if (HasAddressTaken(BI))
return true;
return false;
/// 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;
for (const BasicBlock &BB : *F)
for (const Instruction &I : BB)
if (const CallInst *CI = dyn_cast<CallInst>(&I))
if (CI->getCalledFunction() ==
HasPrologue = true;
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 (HasPrologue)
NeedsProtector = 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)) {
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;
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) {
if (Value *Guard = TLI->getIRStackGuard(B))
return B.CreateLoad(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)
// 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;
// Generate epilogue instrumentation. The epilogue intrumentation can be
// function-based or inlined depending on which mechanism the target is
// providing.
if (Value* GuardCheck = TLI->getSSPStackGuardCheck(*M)) {
// Generate the function-based epilogue instrumentation.
// The target provides a guard check function, generate a call to it.
IRBuilder<> B(RI);
LoadInst *Guard = B.CreateLoad(AI, true, "Guard");
CallInst *Call = B.CreateCall(GuardCheck, {Guard});
llvm::Function *Function = cast<llvm::Function>(GuardCheck);
} else {
// Generate the epilogue with inline instrumentation.
// If we do not support SelectionDAG based tail calls, generate IR level
// tail 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(RI->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(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);
B.SetCurrentDebugLocation(DebugLoc::get(0, 0, F->getSubprogram()));
if (Trip.isOSOpenBSD()) {
Constant *StackChkFail =
B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
} else {
Constant *StackChkFail =
M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context));
B.CreateCall(StackChkFail, {});
return FailBB;
bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const {
return HasPrologue && !HasIRCheck && dyn_cast<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);