#define DEBUG_TYPE "shadowstackgc"
#include "llvm/CodeGen/GCs.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/CodeGen/GCStrategy.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Module.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/IRBuilder.h"
using namespace llvm;
namespace {
class ShadowStackGC : public GCStrategy {
GlobalVariable *Head;
StructType *StackEntryTy;
StructType *FrameMapTy;
std::vector<std::pair<CallInst*,AllocaInst*> > Roots;
public:
ShadowStackGC();
bool initializeCustomLowering(Module &M);
bool performCustomLowering(Function &F);
private:
bool IsNullValue(Value *V);
Constant *GetFrameMap(Function &F);
Type* GetConcreteStackEntryType(Function &F);
void CollectRoots(Function &F);
static GetElementPtrInst *CreateGEP(LLVMContext &Context,
IRBuilder<> &B, Value *BasePtr,
int Idx1, const char *Name);
static GetElementPtrInst *CreateGEP(LLVMContext &Context,
IRBuilder<> &B, Value *BasePtr,
int Idx1, int Idx2, const char *Name);
};
}
static GCRegistry::Add<ShadowStackGC>
X("shadow-stack", "Very portable GC for uncooperative code generators");
namespace {
class EscapeEnumerator {
Function &F;
const char *CleanupBBName;
int State;
Function::iterator StateBB, StateE;
IRBuilder<> Builder;
public:
EscapeEnumerator(Function &F, const char *N = "cleanup")
: F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
IRBuilder<> *Next() {
switch (State) {
default:
return 0;
case 0:
StateBB = F.begin();
StateE = F.end();
State = 1;
case 1:
while (StateBB != StateE) {
BasicBlock *CurBB = StateBB++;
TerminatorInst *TI = CurBB->getTerminator();
if (!isa<ReturnInst>(TI) && !isa<ResumeInst>(TI))
continue;
Builder.SetInsertPoint(TI->getParent(), TI);
return &Builder;
}
State = 2;
SmallVector<Instruction*,16> Calls;
for (Function::iterator BB = F.begin(),
E = F.end(); BB != E; ++BB)
for (BasicBlock::iterator II = BB->begin(),
EE = BB->end(); II != EE; ++II)
if (CallInst *CI = dyn_cast<CallInst>(II))
if (!CI->getCalledFunction() ||
!CI->getCalledFunction()->getIntrinsicID())
Calls.push_back(CI);
if (Calls.empty())
return 0;
LLVMContext &C = F.getContext();
BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
Type *ExnTy = StructType::get(Type::getInt8PtrTy(C),
Type::getInt32Ty(C), NULL);
Constant *PersFn =
F.getParent()->
getOrInsertFunction("__gcc_personality_v0",
FunctionType::get(Type::getInt32Ty(C), true));
LandingPadInst *LPad = LandingPadInst::Create(ExnTy, PersFn, 1,
"cleanup.lpad",
CleanupBB);
LPad->setCleanup(true);
ResumeInst *RI = ResumeInst::Create(LPad, CleanupBB);
SmallVector<Value*,16> Args;
for (unsigned I = Calls.size(); I != 0; ) {
CallInst *CI = cast<CallInst>(Calls[--I]);
BasicBlock *CallBB = CI->getParent();
BasicBlock *NewBB =
CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
CallBB->getInstList().pop_back();
NewBB->getInstList().remove(CI);
Args.clear();
CallSite CS(CI);
Args.append(CS.arg_begin(), CS.arg_end());
InvokeInst *II = InvokeInst::Create(CI->getCalledValue(),
NewBB, CleanupBB,
Args, CI->getName(), CallBB);
II->setCallingConv(CI->getCallingConv());
II->setAttributes(CI->getAttributes());
CI->replaceAllUsesWith(II);
delete CI;
}
Builder.SetInsertPoint(RI->getParent(), RI);
return &Builder;
}
}
};
}
void llvm::linkShadowStackGC() { }
ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) {
InitRoots = true;
CustomRoots = true;
}
Constant *ShadowStackGC::GetFrameMap(Function &F) {
Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
unsigned NumMeta = 0;
SmallVector<Constant*, 16> Metadata;
for (unsigned I = 0; I != Roots.size(); ++I) {
Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
if (!C->isNullValue())
NumMeta = I + 1;
Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
}
Metadata.resize(NumMeta);
Type *Int32Ty = Type::getInt32Ty(F.getContext());
Constant *BaseElts[] = {
ConstantInt::get(Int32Ty, Roots.size(), false),
ConstantInt::get(Int32Ty, NumMeta, false),
};
Constant *DescriptorElts[] = {
ConstantStruct::get(FrameMapTy, BaseElts),
ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)
};
Type *EltTys[] = { DescriptorElts[0]->getType(),DescriptorElts[1]->getType()};
StructType *STy = StructType::create(EltTys, "gc_map."+utostr(NumMeta));
Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
GlobalVariable::InternalLinkage,
FrameMap, "__gc_" + F.getName());
Constant *GEPIndices[2] = {
ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)
};
return ConstantExpr::getGetElementPtr(GV, GEPIndices);
}
Type* ShadowStackGC::GetConcreteStackEntryType(Function &F) {
std::vector<Type*> EltTys;
EltTys.push_back(StackEntryTy);
for (size_t I = 0; I != Roots.size(); I++)
EltTys.push_back(Roots[I].second->getAllocatedType());
return StructType::create(EltTys, "gc_stackentry."+F.getName().str());
}
bool ShadowStackGC::initializeCustomLowering(Module &M) {
std::vector<Type*> EltTys;
EltTys.push_back(Type::getInt32Ty(M.getContext()));
EltTys.push_back(Type::getInt32Ty(M.getContext()));
FrameMapTy = StructType::create(EltTys, "gc_map");
PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
EltTys.clear();
EltTys.push_back(PointerType::getUnqual(StackEntryTy));
EltTys.push_back(FrameMapPtrTy);
StackEntryTy->setBody(EltTys);
PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
Head = M.getGlobalVariable("llvm_gc_root_chain");
if (!Head) {
Head = new GlobalVariable(M, StackEntryPtrTy, false,
GlobalValue::LinkOnceAnyLinkage,
Constant::getNullValue(StackEntryPtrTy),
"llvm_gc_root_chain");
} else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
}
return true;
}
bool ShadowStackGC::IsNullValue(Value *V) {
if (Constant *C = dyn_cast<Constant>(V))
return C->isNullValue();
return false;
}
void ShadowStackGC::CollectRoots(Function &F) {
assert(Roots.empty() && "Not cleaned up?");
SmallVector<std::pair<CallInst*, AllocaInst*>, 16> MetaRoots;
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
if (Function *F = CI->getCalledFunction())
if (F->getIntrinsicID() == Intrinsic::gcroot) {
std::pair<CallInst*, AllocaInst*> Pair = std::make_pair(
CI, cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
if (IsNullValue(CI->getArgOperand(1)))
Roots.push_back(Pair);
else
MetaRoots.push_back(Pair);
}
Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
}
GetElementPtrInst *
ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
int Idx, int Idx2, const char *Name) {
Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
ConstantInt::get(Type::getInt32Ty(Context), Idx),
ConstantInt::get(Type::getInt32Ty(Context), Idx2) };
Value* Val = B.CreateGEP(BasePtr, Indices, Name);
assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
return dyn_cast<GetElementPtrInst>(Val);
}
GetElementPtrInst *
ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
int Idx, const char *Name) {
Value *Indices[] = { ConstantInt::get(Type::getInt32Ty(Context), 0),
ConstantInt::get(Type::getInt32Ty(Context), Idx) };
Value *Val = B.CreateGEP(BasePtr, Indices, Name);
assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
return dyn_cast<GetElementPtrInst>(Val);
}
bool ShadowStackGC::performCustomLowering(Function &F) {
LLVMContext &Context = F.getContext();
CollectRoots(F);
if (Roots.empty())
return false;
Value *FrameMap = GetFrameMap(F);
Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
BasicBlock::iterator IP = F.getEntryBlock().begin();
IRBuilder<> AtEntry(IP->getParent(), IP);
Instruction *StackEntry = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0,
"gc_frame");
while (isa<AllocaInst>(IP)) ++IP;
AtEntry.SetInsertPoint(IP->getParent(), IP);
Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, StackEntry,
0,1,"gc_frame.map");
AtEntry.CreateStore(FrameMap, EntryMapPtr);
for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root");
AllocaInst *OriginalAlloca = Roots[I].second;
SlotPtr->takeName(OriginalAlloca);
OriginalAlloca->replaceAllUsesWith(SlotPtr);
}
while (isa<StoreInst>(IP)) ++IP;
AtEntry.SetInsertPoint(IP->getParent(), IP);
Instruction *EntryNextPtr = CreateGEP(Context, AtEntry,
StackEntry,0,0,"gc_frame.next");
Instruction *NewHeadVal = CreateGEP(Context, AtEntry,
StackEntry, 0, "gc_newhead");
AtEntry.CreateStore(CurrentHead, EntryNextPtr);
AtEntry.CreateStore(NewHeadVal, Head);
EscapeEnumerator EE(F, "gc_cleanup");
while (IRBuilder<> *AtExit = EE.Next()) {
Instruction *EntryNextPtr2 = CreateGEP(Context, *AtExit, StackEntry, 0, 0,
"gc_frame.next");
Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
AtExit->CreateStore(SavedHead, Head);
}
for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
Roots[I].first->eraseFromParent();
Roots[I].second->eraseFromParent();
}
Roots.clear();
return true;
}