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llvm-6502/lib/Transforms/Instrumentation/SafeStack.cpp
Peter Collingbourne 47e71da4b3 SafeStack: Create the unsafe stack pointer on demand. 
This avoids creating an unnecessary undefined reference on targets such as
NVPTX that require such references to be declared in asm output.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240321 91177308-0d34-0410-b5e6-96231b3b80d8
2015-06-22 20:26:54 +00:00

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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).
//
// http://clang.llvm.org/docs/SafeStack.html
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Instrumentation.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Triple.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Format.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_os_ostream.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
using namespace llvm;
#define DEBUG_TYPE "safestack"
namespace llvm {
STATISTIC(NumFunctions, "Total number of functions");
STATISTIC(NumUnsafeStackFunctions, "Number of functions with unsafe stack");
STATISTIC(NumUnsafeStackRestorePointsFunctions,
"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(NumUnsafeStackRestorePoints, "Number of setjmps and landingpads");
} // namespace llvm
namespace {
/// Check whether a given alloca instruction (AI) should 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 IsSafeStackAlloca(const AllocaInst *AI) {
// 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 Instruction *, 8> WorkList;
WorkList.push_back(AI);
// A DFS search through all uses of the alloca in bitcasts/PHI/GEPs/etc.
while (!WorkList.empty()) {
const Instruction *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:
// Loading from a pointer is safe.
break;
case Instruction::VAArg:
// "va-arg" from a pointer is safe.
break;
case Instruction::Store:
if (V == I->getOperand(0))
// Stored the pointer - conservatively assume it may be unsafe.
return false;
// Storing to the pointee is safe.
break;
case Instruction::GetElementPtr:
if (!cast<const GetElementPtrInst>(I)->hasAllConstantIndices())
// GEP with non-constant indices can lead to memory errors.
// This also applies to inbounds GEPs, as the inbounds attribute
// represents an assumption that the address is in bounds, rather than
// an assertion that it is.
return false;
// We assume that GEP on static alloca with constant indices is safe,
// otherwise a compiler would detect it and warn during compilation.
if (!isa<const ConstantInt>(AI->getArraySize()))
// However, if the array size itself is not constant, the access
// might still be unsafe at runtime.
return false;
/* fallthrough */
case Instruction::BitCast:
case Instruction::IntToPtr:
case Instruction::PHI:
case Instruction::PtrToInt:
case Instruction::Select:
// The object can be safe or not, depending on how the result of the
// instruction is used.
if (Visited.insert(I).second)
WorkList.push_back(cast<const Instruction>(I));
break;
case Instruction::Call:
case Instruction::Invoke: {
// FIXME: add support for memset and memcpy intrinsics.
ImmutableCallSite CS(I);
// 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'
// 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 && !CS.doesNotCapture(A - B))
// The parameter is not marked 'nocapture' - unsafe.
return false;
continue;
}
default:
// The object is unsafe if it is used in any other way.
return false;
}
}
}
// All uses of the alloca are safe, we can place it on the safe stack.
return true;
}
/// 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 unsafe ways.
class SafeStack : public FunctionPass {
const DataLayout *DL;
Type *StackPtrTy;
Type *IntPtrTy;
Type *Int32Ty;
Type *Int8Ty;
Constant *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 };
/// \brief Build a constant representing a pointer to the unsafe stack
/// pointer.
Constant *getOrCreateUnsafeStackPtr(Module &M);
/// \brief 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<ReturnInst *> &Returns,
SmallVectorImpl<Instruction *> &StackRestorePoints);
/// \brief 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(Function &F,
ArrayRef<AllocaInst *> StaticAllocas,
ArrayRef<ReturnInst *> Returns);
/// \brief 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(Function &F,
ArrayRef<Instruction *> StackRestorePoints,
Value *StaticTop, bool NeedDynamicTop);
/// \brief 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);
public:
static char ID; // Pass identification, replacement for typeid.
SafeStack() : FunctionPass(ID), DL(nullptr) {
initializeSafeStackPass(*PassRegistry::getPassRegistry());
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<AliasAnalysis>();
}
virtual bool doInitialization(Module &M) {
DL = &M.getDataLayout();
StackPtrTy = Type::getInt8PtrTy(M.getContext());
IntPtrTy = DL->getIntPtrType(M.getContext());
Int32Ty = Type::getInt32Ty(M.getContext());
Int8Ty = Type::getInt8Ty(M.getContext());
return false;
}
bool runOnFunction(Function &F);
}; // class SafeStack
Constant *SafeStack::getOrCreateUnsafeStackPtr(Module &M) {
// The unsafe stack pointer is stored in a global variable with a magic name.
const char *kUnsafeStackPtrVar = "__safestack_unsafe_stack_ptr";
auto UnsafeStackPtr =
dyn_cast_or_null<GlobalVariable>(M.getNamedValue(kUnsafeStackPtrVar));
if (!UnsafeStackPtr) {
// The global variable is not defined yet, define it ourselves.
// We use the initial-exec TLS model because we do not support the variable
// living anywhere other than in the main executable.
UnsafeStackPtr = new GlobalVariable(
/*Module=*/M, /*Type=*/StackPtrTy,
/*isConstant=*/false, /*Linkage=*/GlobalValue::ExternalLinkage,
/*Initializer=*/0, /*Name=*/kUnsafeStackPtrVar,
/*InsertBefore=*/nullptr,
/*ThreadLocalMode=*/GlobalValue::InitialExecTLSModel);
} else {
// The variable exists, check its type and attributes.
if (UnsafeStackPtr->getValueType() != StackPtrTy) {
report_fatal_error(Twine(kUnsafeStackPtrVar) + " must have void* type");
}
if (!UnsafeStackPtr->isThreadLocal()) {
report_fatal_error(Twine(kUnsafeStackPtrVar) + " must be thread-local");
}
}
return UnsafeStackPtr;
}
void SafeStack::findInsts(Function &F,
SmallVectorImpl<AllocaInst *> &StaticAllocas,
SmallVectorImpl<AllocaInst *> &DynamicAllocas,
SmallVectorImpl<ReturnInst *> &Returns,
SmallVectorImpl<Instruction *> &StackRestorePoints) {
for (Instruction &I : inst_range(&F)) {
if (auto AI = dyn_cast<AllocaInst>(&I)) {
++NumAllocas;
if (IsSafeStackAlloca(AI))
continue;
if (AI->isStaticAlloca()) {
++NumUnsafeStaticAllocas;
StaticAllocas.push_back(AI);
} else {
++NumUnsafeDynamicAllocas;
DynamicAllocas.push_back(AI);
}
} else if (auto RI = dyn_cast<ReturnInst>(&I)) {
Returns.push_back(RI);
} else if (auto CI = dyn_cast<CallInst>(&I)) {
// setjmps require stack restore.
if (CI->getCalledFunction() && CI->canReturnTwice())
StackRestorePoints.push_back(CI);
} else if (auto LP = dyn_cast<LandingPadInst>(&I)) {
// Exception landing pads require stack restore.
StackRestorePoints.push_back(LP);
} else if (auto II = dyn_cast<IntrinsicInst>(&I)) {
if (II->getIntrinsicID() == Intrinsic::gcroot)
llvm::report_fatal_error(
"gcroot intrinsic not compatible with safestack attribute");
}
}
}
AllocaInst *
SafeStack::createStackRestorePoints(Function &F,
ArrayRef<Instruction *> StackRestorePoints,
Value *StaticTop, bool NeedDynamicTop) {
if (StackRestorePoints.empty())
return nullptr;
IRBuilder<> IRB(StaticTop
? cast<Instruction>(StaticTop)->getNextNode()
: (Instruction *)F.getEntryBlock().getFirstInsertionPt());
// 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,
"unsafe_stack_dynamic_ptr");
if (!StaticTop)
// We need the original unsafe stack pointer value, even if there are
// no unsafe static allocas.
StaticTop = IRB.CreateLoad(UnsafeStackPtr, false, "unsafe_stack_ptr");
if (NeedDynamicTop)
IRB.CreateStore(StaticTop, DynamicTop);
// Restore current stack pointer after longjmp/exception catch.
for (Instruction *I : StackRestorePoints) {
++NumUnsafeStackRestorePoints;
IRB.SetInsertPoint(cast<Instruction>(I->getNextNode()));
Value *CurrentTop = DynamicTop ? IRB.CreateLoad(DynamicTop) : StaticTop;
IRB.CreateStore(CurrentTop, UnsafeStackPtr);
}
return DynamicTop;
}
Value *
SafeStack::moveStaticAllocasToUnsafeStack(Function &F,
ArrayRef<AllocaInst *> StaticAllocas,
ArrayRef<ReturnInst *> Returns) {
if (StaticAllocas.empty())
return nullptr;
IRBuilder<> IRB(F.getEntryBlock().getFirstInsertionPt());
DIBuilder DIB(*F.getParent());
// 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.
// 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);
for (ReturnInst *RI : Returns) {
IRB.SetInsertPoint(RI);
IRB.CreateStore(BasePointer, UnsafeStackPtr);
}
// Compute maximum alignment among static objects on the unsafe stack.
unsigned MaxAlignment = 0;
for (AllocaInst *AI : StaticAllocas) {
Type *Ty = AI->getAllocatedType();
unsigned Align =
std::max((unsigned)DL->getPrefTypeAlignment(Ty), AI->getAlignment());
if (Align > MaxAlignment)
MaxAlignment = Align;
}
if (MaxAlignment > StackAlignment) {
// Re-align the base pointer according to the max requested alignment.
assert(isPowerOf2_32(MaxAlignment));
IRB.SetInsertPoint(cast<Instruction>(BasePointer->getNextNode()));
BasePointer = cast<Instruction>(IRB.CreateIntToPtr(
IRB.CreateAnd(IRB.CreatePtrToInt(BasePointer, IntPtrTy),
ConstantInt::get(IntPtrTy, ~uint64_t(MaxAlignment - 1))),
StackPtrTy));
}
// Allocate space for every unsafe static AllocaInst on the unsafe stack.
int64_t StaticOffset = 0; // Current stack top.
for (AllocaInst *AI : StaticAllocas) {
IRB.SetInsertPoint(AI);
auto CArraySize = cast<ConstantInt>(AI->getArraySize());
Type *Ty = AI->getAllocatedType();
uint64_t Size = DL->getTypeAllocSize(Ty) * CArraySize->getZExtValue();
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());
// Add alignment.
// NOTE: we ensure that BasePointer itself is aligned to >= Align.
StaticOffset += Size;
StaticOffset = RoundUpToAlignment(StaticOffset, Align);
Value *Off = IRB.CreateGEP(BasePointer, // BasePointer is i8*
ConstantInt::get(Int32Ty, -StaticOffset));
Value *NewAI = IRB.CreateBitCast(Off, AI->getType(), AI->getName());
if (AI->hasName() && isa<Instruction>(NewAI))
cast<Instruction>(NewAI)->takeName(AI);
// Replace alloc with the new location.
replaceDbgDeclareForAlloca(AI, NewAI, DIB, /*Deref=*/true);
AI->replaceAllUsesWith(NewAI);
AI->eraseFromParent();
}
// Re-align BasePointer so that our callees would see it aligned as
// expected.
// FIXME: no need to update BasePointer in leaf functions.
StaticOffset = RoundUpToAlignment(StaticOffset, StackAlignment);
// Update shadow stack pointer in the function epilogue.
IRB.SetInsertPoint(cast<Instruction>(BasePointer->getNextNode()));
Value *StaticTop =
IRB.CreateGEP(BasePointer, ConstantInt::get(Int32Ty, -StaticOffset),
"unsafe_stack_static_top");
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()),
(unsigned)StackAlignment);
assert(isPowerOf2_32(Align));
Value *NewTop = IRB.CreateIntToPtr(
IRB.CreateAnd(SP, ConstantInt::get(IntPtrTy, ~uint64_t(Align - 1))),
StackPtrTy);
// Save the stack pointer.
IRB.CreateStore(NewTop, UnsafeStackPtr);
if (DynamicTop)
IRB.CreateStore(NewTop, DynamicTop);
Value *NewAI = IRB.CreateIntToPtr(SP, AI->getType());
if (AI->hasName() && isa<Instruction>(NewAI))
NewAI->takeName(AI);
replaceDbgDeclareForAlloca(AI, NewAI, DIB, /*Deref=*/true);
AI->replaceAllUsesWith(NewAI);
AI->eraseFromParent();
}
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)
continue;
if (II->getIntrinsicID() == Intrinsic::stacksave) {
IRBuilder<> IRB(II);
Instruction *LI = IRB.CreateLoad(UnsafeStackPtr);
LI->takeName(II);
II->replaceAllUsesWith(LI);
II->eraseFromParent();
} else if (II->getIntrinsicID() == Intrinsic::stackrestore) {
IRBuilder<> IRB(II);
Instruction *SI = IRB.CreateStore(II->getArgOperand(0), UnsafeStackPtr);
SI->takeName(II);
assert(II->use_empty());
II->eraseFromParent();
}
}
}
}
bool SafeStack::runOnFunction(Function &F) {
auto AA = &getAnalysis<AliasAnalysis>();
DEBUG(dbgs() << "[SafeStack] Function: " << F.getName() << "\n");
if (!F.hasFnAttribute(Attribute::SafeStack)) {
DEBUG(dbgs() << "[SafeStack] safestack is not requested"
" for this function\n");
return false;
}
if (F.isDeclaration()) {
DEBUG(dbgs() << "[SafeStack] function definition"
" is not available\n");
return false;
}
{
// Make sure the regular stack protector won't run on this function
// (safestack attribute takes precedence).
AttrBuilder B;
B.addAttribute(Attribute::StackProtect)
.addAttribute(Attribute::StackProtectReq)
.addAttribute(Attribute::StackProtectStrong);
F.removeAttributes(
AttributeSet::FunctionIndex,
AttributeSet::get(F.getContext(), AttributeSet::FunctionIndex, B));
}
if (AA->onlyReadsMemory(&F)) {
// XXX: we don't protect against information leak attacks for now.
DEBUG(dbgs() << "[SafeStack] function only reads memory\n");
return false;
}
++NumFunctions;
SmallVector<AllocaInst *, 16> StaticAllocas;
SmallVector<AllocaInst *, 4> DynamicAllocas;
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 prorerly.
// 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, Returns, StackRestorePoints);
if (StaticAllocas.empty() && DynamicAllocas.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())
++NumUnsafeStackRestorePointsFunctions;
if (!UnsafeStackPtr)
UnsafeStackPtr = getOrCreateUnsafeStackPtr(*F.getParent());
// The top of the unsafe stack after all unsafe static allocas are allocated.
Value *StaticTop = moveStaticAllocasToUnsafeStack(F, StaticAllocas, Returns);
// 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(
F, StackRestorePoints, StaticTop, !DynamicAllocas.empty());
// Handle dynamic allocas.
moveDynamicAllocasToUnsafeStack(F, UnsafeStackPtr, DynamicTop,
DynamicAllocas);
DEBUG(dbgs() << "[SafeStack] safestack applied\n");
return true;
}
} // end anonymous namespace
char SafeStack::ID = 0;
INITIALIZE_PASS_BEGIN(SafeStack, "safe-stack",
"Safe Stack instrumentation pass", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(SafeStack, "safe-stack", "Safe Stack instrumentation pass",
false, false)
FunctionPass *llvm::createSafeStackPass() { return new SafeStack(); }
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