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//===- SafeStack.cpp - Safe Stack Insertion -------------------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This pass splits the stack into the safe stack (kept as-is for LLVM backend)
// and the unsafe stack (explicitly allocated and managed through the runtime
// support library).
#include "SafeStackColoring.h"
#include "SafeStackLayout.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/InlineCost.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.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/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <string>
#include <utility>
using namespace llvm;
using namespace llvm::safestack;
#define DEBUG_TYPE "safe-stack"
namespace llvm {
STATISTIC(NumFunctions, "Total number of functions");
STATISTIC(NumUnsafeStackFunctions, "Number of functions with unsafe stack");
"Number of functions that use setjmp or exceptions");
STATISTIC(NumAllocas, "Total number of allocas");
STATISTIC(NumUnsafeStaticAllocas, "Number of unsafe static allocas");
STATISTIC(NumUnsafeDynamicAllocas, "Number of unsafe dynamic allocas");
STATISTIC(NumUnsafeByValArguments, "Number of unsafe byval arguments");
STATISTIC(NumUnsafeStackRestorePoints, "Number of setjmps and landingpads");
} // namespace llvm
/// Use __safestack_pointer_address even if the platform has a faster way of
/// access safe stack pointer.
static cl::opt<bool>
cl::init(false), cl::Hidden);
namespace {
/// Rewrite an SCEV expression for a memory access address to an expression that
/// represents offset from the given alloca.
/// The implementation simply replaces all mentions of the alloca with zero.
class AllocaOffsetRewriter : public SCEVRewriteVisitor<AllocaOffsetRewriter> {
const Value *AllocaPtr;
AllocaOffsetRewriter(ScalarEvolution &SE, const Value *AllocaPtr)
: SCEVRewriteVisitor(SE), AllocaPtr(AllocaPtr) {}
const SCEV *visitUnknown(const SCEVUnknown *Expr) {
if (Expr->getValue() == AllocaPtr)
return SE.getZero(Expr->getType());
return Expr;
/// The SafeStack pass splits the stack of each function into the safe
/// stack, which is only accessed through memory safe dereferences (as
/// determined statically), and the unsafe stack, which contains all
/// local variables that are accessed in ways that we can't prove to
/// be safe.
class SafeStack {
Function &F;
const TargetLoweringBase &TL;
const DataLayout &DL;
ScalarEvolution &SE;
Type *StackPtrTy;
Type *IntPtrTy;
Type *Int32Ty;
Type *Int8Ty;
Value *UnsafeStackPtr = nullptr;
/// Unsafe stack alignment. Each stack frame must ensure that the stack is
/// aligned to this value. We need to re-align the unsafe stack if the
/// alignment of any object on the stack exceeds this value.
/// 16 seems like a reasonable upper bound on the alignment of objects that we
/// might expect to appear on the stack on most common targets.
enum { StackAlignment = 16 };
/// Return the value of the stack canary.
Value *getStackGuard(IRBuilder<> &IRB, Function &F);
/// Load stack guard from the frame and check if it has changed.
void checkStackGuard(IRBuilder<> &IRB, Function &F, ReturnInst &RI,
AllocaInst *StackGuardSlot, Value *StackGuard);
/// Find all static allocas, dynamic allocas, return instructions and
/// stack restore points (exception unwind blocks and setjmp calls) in the
/// given function and append them to the respective vectors.
void findInsts(Function &F, SmallVectorImpl<AllocaInst *> &StaticAllocas,
SmallVectorImpl<AllocaInst *> &DynamicAllocas,
SmallVectorImpl<Argument *> &ByValArguments,
SmallVectorImpl<ReturnInst *> &Returns,
SmallVectorImpl<Instruction *> &StackRestorePoints);
/// Calculate the allocation size of a given alloca. Returns 0 if the
/// size can not be statically determined.
uint64_t getStaticAllocaAllocationSize(const AllocaInst* AI);
/// Allocate space for all static allocas in \p StaticAllocas,
/// replace allocas with pointers into the unsafe stack and generate code to
/// restore the stack pointer before all return instructions in \p Returns.
/// \returns A pointer to the top of the unsafe stack after all unsafe static
/// allocas are allocated.
Value *moveStaticAllocasToUnsafeStack(IRBuilder<> &IRB, Function &F,
ArrayRef<AllocaInst *> StaticAllocas,
ArrayRef<Argument *> ByValArguments,
ArrayRef<ReturnInst *> Returns,
Instruction *BasePointer,
AllocaInst *StackGuardSlot);
/// Generate code to restore the stack after all stack restore points
/// in \p StackRestorePoints.
/// \returns A local variable in which to maintain the dynamic top of the
/// unsafe stack if needed.
AllocaInst *
createStackRestorePoints(IRBuilder<> &IRB, Function &F,
ArrayRef<Instruction *> StackRestorePoints,
Value *StaticTop, bool NeedDynamicTop);
/// Replace all allocas in \p DynamicAllocas with code to allocate
/// space dynamically on the unsafe stack and store the dynamic unsafe stack
/// top to \p DynamicTop if non-null.
void moveDynamicAllocasToUnsafeStack(Function &F, Value *UnsafeStackPtr,
AllocaInst *DynamicTop,
ArrayRef<AllocaInst *> DynamicAllocas);
bool IsSafeStackAlloca(const Value *AllocaPtr, uint64_t AllocaSize);
bool IsMemIntrinsicSafe(const MemIntrinsic *MI, const Use &U,
const Value *AllocaPtr, uint64_t AllocaSize);
bool IsAccessSafe(Value *Addr, uint64_t Size, const Value *AllocaPtr,
uint64_t AllocaSize);
bool ShouldInlinePointerAddress(CallSite &CS);
void TryInlinePointerAddress();
SafeStack(Function &F, const TargetLoweringBase &TL, const DataLayout &DL,
ScalarEvolution &SE)
: F(F), TL(TL), DL(DL), SE(SE),
Int8Ty(Type::getInt8Ty(F.getContext())) {}
// Run the transformation on the associated function.
// Returns whether the function was changed.
bool run();
uint64_t SafeStack::getStaticAllocaAllocationSize(const AllocaInst* AI) {
uint64_t Size = DL.getTypeAllocSize(AI->getAllocatedType());
if (AI->isArrayAllocation()) {
auto C = dyn_cast<ConstantInt>(AI->getArraySize());
if (!C)
return 0;
Size *= C->getZExtValue();
return Size;
bool SafeStack::IsAccessSafe(Value *Addr, uint64_t AccessSize,
const Value *AllocaPtr, uint64_t AllocaSize) {
AllocaOffsetRewriter Rewriter(SE, AllocaPtr);
const SCEV *Expr = Rewriter.visit(SE.getSCEV(Addr));
uint64_t BitWidth = SE.getTypeSizeInBits(Expr->getType());
ConstantRange AccessStartRange = SE.getUnsignedRange(Expr);
ConstantRange SizeRange =
ConstantRange(APInt(BitWidth, 0), APInt(BitWidth, AccessSize));
ConstantRange AccessRange = AccessStartRange.add(SizeRange);
ConstantRange AllocaRange =
ConstantRange(APInt(BitWidth, 0), APInt(BitWidth, AllocaSize));
bool Safe = AllocaRange.contains(AccessRange);
dbgs() << "[SafeStack] "
<< (isa<AllocaInst>(AllocaPtr) ? "Alloca " : "ByValArgument ")
<< *AllocaPtr << "\n"
<< " Access " << *Addr << "\n"
<< " SCEV " << *Expr
<< " U: " << SE.getUnsignedRange(Expr)
<< ", S: " << SE.getSignedRange(Expr) << "\n"
<< " Range " << AccessRange << "\n"
<< " AllocaRange " << AllocaRange << "\n"
<< " " << (Safe ? "safe" : "unsafe") << "\n");
return Safe;
bool SafeStack::IsMemIntrinsicSafe(const MemIntrinsic *MI, const Use &U,
const Value *AllocaPtr,
uint64_t AllocaSize) {
// All MemIntrinsics have destination address in Arg0 and size in Arg2.
if (MI->getRawDest() != U) return true;
const auto *Len = dyn_cast<ConstantInt>(MI->getLength());
// Non-constant size => unsafe. FIXME: try SCEV getRange.
if (!Len) return false;
return IsAccessSafe(U, Len->getZExtValue(), AllocaPtr, AllocaSize);
/// Check whether a given allocation must be put on the safe
/// stack or not. The function analyzes all uses of AI and checks whether it is
/// only accessed in a memory safe way (as decided statically).
bool SafeStack::IsSafeStackAlloca(const Value *AllocaPtr, uint64_t AllocaSize) {
// Go through all uses of this alloca and check whether all accesses to the
// allocated object are statically known to be memory safe and, hence, the
// object can be placed on the safe stack.
SmallPtrSet<const Value *, 16> Visited;
SmallVector<const Value *, 8> WorkList;
// A DFS search through all uses of the alloca in bitcasts/PHI/GEPs/etc.
while (!WorkList.empty()) {
const Value *V = WorkList.pop_back_val();
for (const Use &UI : V->uses()) {
auto I = cast<const Instruction>(UI.getUser());
assert(V == UI.get());
switch (I->getOpcode()) {
case Instruction::Load:
if (!IsAccessSafe(UI, DL.getTypeStoreSize(I->getType()), AllocaPtr,
return false;
case Instruction::VAArg:
// "va-arg" from a pointer is safe.
case Instruction::Store:
if (V == I->getOperand(0)) {
// Stored the pointer - conservatively assume it may be unsafe.
<< "[SafeStack] Unsafe alloca: " << *AllocaPtr
<< "\n store of address: " << *I << "\n");
return false;
if (!IsAccessSafe(UI, DL.getTypeStoreSize(I->getOperand(0)->getType()),
AllocaPtr, AllocaSize))
return false;
case Instruction::Ret:
// Information leak.
return false;
case Instruction::Call:
case Instruction::Invoke: {
ImmutableCallSite CS(I);
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
II->getIntrinsicID() == Intrinsic::lifetime_end)
if (const MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
if (!IsMemIntrinsicSafe(MI, UI, AllocaPtr, AllocaSize)) {
<< "[SafeStack] Unsafe alloca: " << *AllocaPtr
<< "\n unsafe memintrinsic: " << *I << "\n");
return false;
// LLVM 'nocapture' attribute is only set for arguments whose address
// is not stored, passed around, or used in any other non-trivial way.
// We assume that passing a pointer to an object as a 'nocapture
// readnone' argument is safe.
// FIXME: a more precise solution would require an interprocedural
// analysis here, which would look at all uses of an argument inside
// the function being called.
ImmutableCallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
for (ImmutableCallSite::arg_iterator A = B; A != E; ++A)
if (A->get() == V)
if (!(CS.doesNotCapture(A - B) && (CS.doesNotAccessMemory(A - B) ||
CS.doesNotAccessMemory()))) {
LLVM_DEBUG(dbgs() << "[SafeStack] Unsafe alloca: " << *AllocaPtr
<< "\n unsafe call: " << *I << "\n");
return false;
if (Visited.insert(I).second)
WorkList.push_back(cast<const Instruction>(I));
// All uses of the alloca are safe, we can place it on the safe stack.
return true;
Value *SafeStack::getStackGuard(IRBuilder<> &IRB, Function &F) {
Value *StackGuardVar = TL.getIRStackGuard(IRB);
if (!StackGuardVar)
StackGuardVar =
F.getParent()->getOrInsertGlobal("__stack_chk_guard", StackPtrTy);
return IRB.CreateLoad(StackGuardVar, "StackGuard");
void SafeStack::findInsts(Function &F,
SmallVectorImpl<AllocaInst *> &StaticAllocas,
SmallVectorImpl<AllocaInst *> &DynamicAllocas,
SmallVectorImpl<Argument *> &ByValArguments,
SmallVectorImpl<ReturnInst *> &Returns,
SmallVectorImpl<Instruction *> &StackRestorePoints) {
for (Instruction &I : instructions(&F)) {
if (auto AI = dyn_cast<AllocaInst>(&I)) {
uint64_t Size = getStaticAllocaAllocationSize(AI);
if (IsSafeStackAlloca(AI, Size))
if (AI->isStaticAlloca()) {
} else {
} else if (auto RI = dyn_cast<ReturnInst>(&I)) {
} else if (auto CI = dyn_cast<CallInst>(&I)) {
// setjmps require stack restore.
if (CI->getCalledFunction() && CI->canReturnTwice())
} else if (auto LP = dyn_cast<LandingPadInst>(&I)) {
// Exception landing pads require stack restore.
} else if (auto II = dyn_cast<IntrinsicInst>(&I)) {
if (II->getIntrinsicID() == Intrinsic::gcroot)
"gcroot intrinsic not compatible with safestack attribute");
for (Argument &Arg : F.args()) {
if (!Arg.hasByValAttr())
uint64_t Size =
if (IsSafeStackAlloca(&Arg, Size))
AllocaInst *
SafeStack::createStackRestorePoints(IRBuilder<> &IRB, Function &F,
ArrayRef<Instruction *> StackRestorePoints,
Value *StaticTop, bool NeedDynamicTop) {
assert(StaticTop && "The stack top isn't set.");
if (StackRestorePoints.empty())
return nullptr;
// We need the current value of the shadow stack pointer to restore
// after longjmp or exception catching.
// FIXME: On some platforms this could be handled by the longjmp/exception
// runtime itself.
AllocaInst *DynamicTop = nullptr;
if (NeedDynamicTop) {
// If we also have dynamic alloca's, the stack pointer value changes
// throughout the function. For now we store it in an alloca.
DynamicTop = IRB.CreateAlloca(StackPtrTy, /*ArraySize=*/nullptr,
IRB.CreateStore(StaticTop, DynamicTop);
// Restore current stack pointer after longjmp/exception catch.
for (Instruction *I : StackRestorePoints) {
Value *CurrentTop = DynamicTop ? IRB.CreateLoad(DynamicTop) : StaticTop;
IRB.CreateStore(CurrentTop, UnsafeStackPtr);
return DynamicTop;
void SafeStack::checkStackGuard(IRBuilder<> &IRB, Function &F, ReturnInst &RI,
AllocaInst *StackGuardSlot, Value *StackGuard) {
Value *V = IRB.CreateLoad(StackGuardSlot);
Value *Cmp = IRB.CreateICmpNE(StackGuard, V);
auto SuccessProb = BranchProbabilityInfo::getBranchProbStackProtector(true);
auto FailureProb = BranchProbabilityInfo::getBranchProbStackProtector(false);
MDNode *Weights = MDBuilder(F.getContext())
Instruction *CheckTerm =
SplitBlockAndInsertIfThen(Cmp, &RI,
/* Unreachable */ true, Weights);
IRBuilder<> IRBFail(CheckTerm);
// FIXME: respect -fsanitize-trap / -ftrap-function here?
Constant *StackChkFail = F.getParent()->getOrInsertFunction(
"__stack_chk_fail", IRB.getVoidTy());
IRBFail.CreateCall(StackChkFail, {});
/// We explicitly compute and set the unsafe stack layout for all unsafe
/// static alloca instructions. We save the unsafe "base pointer" in the
/// prologue into a local variable and restore it in the epilogue.
Value *SafeStack::moveStaticAllocasToUnsafeStack(
IRBuilder<> &IRB, Function &F, ArrayRef<AllocaInst *> StaticAllocas,
ArrayRef<Argument *> ByValArguments, ArrayRef<ReturnInst *> Returns,
Instruction *BasePointer, AllocaInst *StackGuardSlot) {
if (StaticAllocas.empty() && ByValArguments.empty())
return BasePointer;
DIBuilder DIB(*F.getParent());
StackColoring SSC(F, StaticAllocas);;
// Unsafe stack always grows down.
StackLayout SSL(StackAlignment);
if (StackGuardSlot) {
Type *Ty = StackGuardSlot->getAllocatedType();
unsigned Align =
std::max(DL.getPrefTypeAlignment(Ty), StackGuardSlot->getAlignment());
SSL.addObject(StackGuardSlot, getStaticAllocaAllocationSize(StackGuardSlot),
Align, SSC.getFullLiveRange());
for (Argument *Arg : ByValArguments) {
Type *Ty = Arg->getType()->getPointerElementType();
uint64_t Size = DL.getTypeStoreSize(Ty);
if (Size == 0)
Size = 1; // Don't create zero-sized stack objects.
// Ensure the object is properly aligned.
unsigned Align = std::max((unsigned)DL.getPrefTypeAlignment(Ty),
SSL.addObject(Arg, Size, Align, SSC.getFullLiveRange());
for (AllocaInst *AI : StaticAllocas) {
Type *Ty = AI->getAllocatedType();
uint64_t Size = getStaticAllocaAllocationSize(AI);
if (Size == 0)
Size = 1; // Don't create zero-sized stack objects.
// Ensure the object is properly aligned.
unsigned Align =
std::max((unsigned)DL.getPrefTypeAlignment(Ty), AI->getAlignment());
SSL.addObject(AI, Size, Align, SSC.getLiveRange(AI));
unsigned FrameAlignment = SSL.getFrameAlignment();
// FIXME: tell SSL that we start at a less-then-MaxAlignment aligned location
// (AlignmentSkew).
if (FrameAlignment > StackAlignment) {
// Re-align the base pointer according to the max requested alignment.
BasePointer = cast<Instruction>(IRB.CreateIntToPtr(
IRB.CreateAnd(IRB.CreatePtrToInt(BasePointer, IntPtrTy),
ConstantInt::get(IntPtrTy, ~uint64_t(FrameAlignment - 1))),
if (StackGuardSlot) {
unsigned Offset = SSL.getObjectOffset(StackGuardSlot);
Value *Off = IRB.CreateGEP(BasePointer, // BasePointer is i8*
ConstantInt::get(Int32Ty, -Offset));
Value *NewAI =
IRB.CreateBitCast(Off, StackGuardSlot->getType(), "StackGuardSlot");
// Replace alloc with the new location.
for (Argument *Arg : ByValArguments) {
unsigned Offset = SSL.getObjectOffset(Arg);
unsigned Align = SSL.getObjectAlignment(Arg);
Type *Ty = Arg->getType()->getPointerElementType();
uint64_t Size = DL.getTypeStoreSize(Ty);
if (Size == 0)
Size = 1; // Don't create zero-sized stack objects.
Value *Off = IRB.CreateGEP(BasePointer, // BasePointer is i8*
ConstantInt::get(Int32Ty, -Offset));
Value *NewArg = IRB.CreateBitCast(Off, Arg->getType(),
Arg->getName() + ".unsafe-byval");
// Replace alloc with the new location.
replaceDbgDeclare(Arg, BasePointer, BasePointer->getNextNode(), DIB,
DIExpression::NoDeref, -Offset, DIExpression::NoDeref);
IRB.CreateMemCpy(Off, Align, Arg, Arg->getParamAlignment(), Size);
// Allocate space for every unsafe static AllocaInst on the unsafe stack.
for (AllocaInst *AI : StaticAllocas) {
unsigned Offset = SSL.getObjectOffset(AI);
uint64_t Size = getStaticAllocaAllocationSize(AI);
if (Size == 0)
Size = 1; // Don't create zero-sized stack objects.
replaceDbgDeclareForAlloca(AI, BasePointer, DIB, DIExpression::NoDeref,
-Offset, DIExpression::NoDeref);
replaceDbgValueForAlloca(AI, BasePointer, DIB, -Offset);
// Replace uses of the alloca with the new location.
// Insert address calculation close to each use to work around PR27844.
std::string Name = std::string(AI->getName()) + ".unsafe";
while (!AI->use_empty()) {
Use &U = *AI->use_begin();
Instruction *User = cast<Instruction>(U.getUser());
Instruction *InsertBefore;
if (auto *PHI = dyn_cast<PHINode>(User))
InsertBefore = PHI->getIncomingBlock(U)->getTerminator();
InsertBefore = User;
IRBuilder<> IRBUser(InsertBefore);
Value *Off = IRBUser.CreateGEP(BasePointer, // BasePointer is i8*
ConstantInt::get(Int32Ty, -Offset));
Value *Replacement = IRBUser.CreateBitCast(Off, AI->getType(), Name);
if (auto *PHI = dyn_cast<PHINode>(User)) {
// PHI nodes may have multiple incoming edges from the same BB (why??),
// all must be updated at once with the same incoming value.
auto *BB = PHI->getIncomingBlock(U);
for (unsigned I = 0; I < PHI->getNumIncomingValues(); ++I)
if (PHI->getIncomingBlock(I) == BB)
PHI->setIncomingValue(I, Replacement);
} else {
// Re-align BasePointer so that our callees would see it aligned as
// expected.
// FIXME: no need to update BasePointer in leaf functions.
unsigned FrameSize = alignTo(SSL.getFrameSize(), StackAlignment);
// Update shadow stack pointer in the function epilogue.
Value *StaticTop =
IRB.CreateGEP(BasePointer, ConstantInt::get(Int32Ty, -FrameSize),
IRB.CreateStore(StaticTop, UnsafeStackPtr);
return StaticTop;
void SafeStack::moveDynamicAllocasToUnsafeStack(
Function &F, Value *UnsafeStackPtr, AllocaInst *DynamicTop,
ArrayRef<AllocaInst *> DynamicAllocas) {
DIBuilder DIB(*F.getParent());
for (AllocaInst *AI : DynamicAllocas) {
IRBuilder<> IRB(AI);
// Compute the new SP value (after AI).
Value *ArraySize = AI->getArraySize();
if (ArraySize->getType() != IntPtrTy)
ArraySize = IRB.CreateIntCast(ArraySize, IntPtrTy, false);
Type *Ty = AI->getAllocatedType();
uint64_t TySize = DL.getTypeAllocSize(Ty);
Value *Size = IRB.CreateMul(ArraySize, ConstantInt::get(IntPtrTy, TySize));
Value *SP = IRB.CreatePtrToInt(IRB.CreateLoad(UnsafeStackPtr), IntPtrTy);
SP = IRB.CreateSub(SP, Size);
// Align the SP value to satisfy the AllocaInst, type and stack alignments.
unsigned Align = std::max(
std::max((unsigned)DL.getPrefTypeAlignment(Ty), AI->getAlignment()),
Value *NewTop = IRB.CreateIntToPtr(
IRB.CreateAnd(SP, ConstantInt::get(IntPtrTy, ~uint64_t(Align - 1))),
// Save the stack pointer.
IRB.CreateStore(NewTop, UnsafeStackPtr);
if (DynamicTop)
IRB.CreateStore(NewTop, DynamicTop);
Value *NewAI = IRB.CreatePointerCast(NewTop, AI->getType());
if (AI->hasName() && isa<Instruction>(NewAI))
replaceDbgDeclareForAlloca(AI, NewAI, DIB, DIExpression::NoDeref, 0,
if (!DynamicAllocas.empty()) {
// Now go through the instructions again, replacing stacksave/stackrestore.
for (inst_iterator It = inst_begin(&F), Ie = inst_end(&F); It != Ie;) {
Instruction *I = &*(It++);
auto II = dyn_cast<IntrinsicInst>(I);
if (!II)
if (II->getIntrinsicID() == Intrinsic::stacksave) {
IRBuilder<> IRB(II);
Instruction *LI = IRB.CreateLoad(UnsafeStackPtr);
} else if (II->getIntrinsicID() == Intrinsic::stackrestore) {
IRBuilder<> IRB(II);
Instruction *SI = IRB.CreateStore(II->getArgOperand(0), UnsafeStackPtr);
bool SafeStack::ShouldInlinePointerAddress(CallSite &CS) {
Function *Callee = CS.getCalledFunction();
if (CS.hasFnAttr(Attribute::AlwaysInline) && isInlineViable(*Callee))
return true;
if (Callee->isInterposable() || Callee->hasFnAttribute(Attribute::NoInline) ||
return false;
return true;
void SafeStack::TryInlinePointerAddress() {
if (!isa<CallInst>(UnsafeStackPtr))
CallSite CS(UnsafeStackPtr);
Function *Callee = CS.getCalledFunction();
if (!Callee || Callee->isDeclaration())
if (!ShouldInlinePointerAddress(CS))
InlineFunctionInfo IFI;
InlineFunction(CS, IFI);
bool SafeStack::run() {
assert(F.hasFnAttribute(Attribute::SafeStack) &&
"Can't run SafeStack on a function without the attribute");
assert(!F.isDeclaration() && "Can't run SafeStack on a function declaration");
SmallVector<AllocaInst *, 16> StaticAllocas;
SmallVector<AllocaInst *, 4> DynamicAllocas;
SmallVector<Argument *, 4> ByValArguments;
SmallVector<ReturnInst *, 4> Returns;
// Collect all points where stack gets unwound and needs to be restored
// This is only necessary because the runtime (setjmp and unwind code) is
// not aware of the unsafe stack and won't unwind/restore it properly.
// To work around this problem without changing the runtime, we insert
// instrumentation to restore the unsafe stack pointer when necessary.
SmallVector<Instruction *, 4> StackRestorePoints;
// Find all static and dynamic alloca instructions that must be moved to the
// unsafe stack, all return instructions and stack restore points.
findInsts(F, StaticAllocas, DynamicAllocas, ByValArguments, Returns,
if (StaticAllocas.empty() && DynamicAllocas.empty() &&
ByValArguments.empty() && StackRestorePoints.empty())
return false; // Nothing to do in this function.
if (!StaticAllocas.empty() || !DynamicAllocas.empty() ||
++NumUnsafeStackFunctions; // This function has the unsafe stack.
if (!StackRestorePoints.empty())
IRBuilder<> IRB(&F.front(), F.begin()->getFirstInsertionPt());
if (SafeStackUsePointerAddress) {
Value *Fn = F.getParent()->getOrInsertFunction(
"__safestack_pointer_address", StackPtrTy->getPointerTo(0));
UnsafeStackPtr = IRB.CreateCall(Fn);
} else {
UnsafeStackPtr = TL.getSafeStackPointerLocation(IRB);
// Load the current stack pointer (we'll also use it as a base pointer).
// FIXME: use a dedicated register for it ?
Instruction *BasePointer =
IRB.CreateLoad(UnsafeStackPtr, false, "unsafe_stack_ptr");
assert(BasePointer->getType() == StackPtrTy);
AllocaInst *StackGuardSlot = nullptr;
// FIXME: implement weaker forms of stack protector.
if (F.hasFnAttribute(Attribute::StackProtect) ||
F.hasFnAttribute(Attribute::StackProtectStrong) ||
F.hasFnAttribute(Attribute::StackProtectReq)) {
Value *StackGuard = getStackGuard(IRB, F);
StackGuardSlot = IRB.CreateAlloca(StackPtrTy, nullptr);
IRB.CreateStore(StackGuard, StackGuardSlot);
for (ReturnInst *RI : Returns) {
IRBuilder<> IRBRet(RI);
checkStackGuard(IRBRet, F, *RI, StackGuardSlot, StackGuard);
// The top of the unsafe stack after all unsafe static allocas are
// allocated.
Value *StaticTop =
moveStaticAllocasToUnsafeStack(IRB, F, StaticAllocas, ByValArguments,
Returns, BasePointer, StackGuardSlot);
// Safe stack object that stores the current unsafe stack top. It is updated
// as unsafe dynamic (non-constant-sized) allocas are allocated and freed.
// This is only needed if we need to restore stack pointer after longjmp
// or exceptions, and we have dynamic allocations.
// FIXME: a better alternative might be to store the unsafe stack pointer
// before setjmp / invoke instructions.
AllocaInst *DynamicTop = createStackRestorePoints(
IRB, F, StackRestorePoints, StaticTop, !DynamicAllocas.empty());
// Handle dynamic allocas.
moveDynamicAllocasToUnsafeStack(F, UnsafeStackPtr, DynamicTop,
// Restore the unsafe stack pointer before each return.
for (ReturnInst *RI : Returns) {
IRB.CreateStore(BasePointer, UnsafeStackPtr);
LLVM_DEBUG(dbgs() << "[SafeStack] safestack applied\n");
return true;
class SafeStackLegacyPass : public FunctionPass {
const TargetMachine *TM = nullptr;
static char ID; // Pass identification, replacement for typeid..
SafeStackLegacyPass() : FunctionPass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnFunction(Function &F) override {
LLVM_DEBUG(dbgs() << "[SafeStack] Function: " << F.getName() << "\n");
if (!F.hasFnAttribute(Attribute::SafeStack)) {
LLVM_DEBUG(dbgs() << "[SafeStack] safestack is not requested"
" for this function\n");
return false;
if (F.isDeclaration()) {
LLVM_DEBUG(dbgs() << "[SafeStack] function definition"
" is not available\n");
return false;
TM = &getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
auto *TL = TM->getSubtargetImpl(F)->getTargetLowering();
if (!TL)
report_fatal_error("TargetLowering instance is required");
auto *DL = &F.getParent()->getDataLayout();
auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
auto &ACT = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
// Compute DT and LI only for functions that have the attribute.
// This is only useful because the legacy pass manager doesn't let us
// compute analyzes lazily.
// In the backend pipeline, nothing preserves DT before SafeStack, so we
// would otherwise always compute it wastefully, even if there is no
// function with the safestack attribute.
DominatorTree DT(F);
LoopInfo LI(DT);
ScalarEvolution SE(F, TLI, ACT, DT, LI);
return SafeStack(F, *TL, *DL, SE).run();
} // end anonymous namespace
char SafeStackLegacyPass::ID = 0;
"Safe Stack instrumentation pass", false, false)
"Safe Stack instrumentation pass", false, false)
FunctionPass *llvm::createSafeStackPass() { return new SafeStackLegacyPass(); }