#include "llvm/CodeGen/GCs.h"
#include "llvm/ADT/StringExtras.h"
+#include "llvm/CodeGen/GCStrategy.h"
#include "llvm/IR/CallSite.h"
-#include "llvm/IR/GCStrategy.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#define DEBUG_TYPE "shadowstackgc"
namespace {
-
class ShadowStackGC : public GCStrategy {
- /// RootChain - This is the global linked-list that contains the chain of GC
- /// roots.
- GlobalVariable *Head;
-
- /// StackEntryTy - Abstract type of a link in the shadow stack.
- ///
- StructType *StackEntryTy;
- StructType *FrameMapTy;
-
- /// Roots - GC roots in the current function. Each is a pair of the
- /// intrinsic call and its corresponding alloca.
- std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
-
public:
ShadowStackGC();
-
- bool initializeCustomLowering(Module &M) override;
- bool performCustomLowering(Function &F) override;
-
-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 {
-/// EscapeEnumerator - This is a little algorithm to find all escape points
-/// from a function so that "finally"-style code can be inserted. In addition
-/// to finding the existing return and unwind instructions, it also (if
-/// necessary) transforms any call instructions into invokes and sends them to
-/// a landing pad.
-///
-/// It's wrapped up in a state machine using the same transform C# uses for
-/// 'yield return' enumerators, This transform allows it to be non-allocating.
-class EscapeEnumerator {
- Function &F;
- const char *CleanupBBName;
-
- // State.
- 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 nullptr;
-
- case 0:
- StateBB = F.begin();
- StateE = F.end();
- State = 1;
-
- case 1:
- // Find all 'return', 'resume', and 'unwind' instructions.
- while (StateBB != StateE) {
- BasicBlock *CurBB = StateBB++;
-
- // Branches and invokes do not escape, only unwind, resume, and return
- // do.
- TerminatorInst *TI = CurBB->getTerminator();
- if (!isa<ReturnInst>(TI) && !isa<ResumeInst>(TI))
- continue;
-
- Builder.SetInsertPoint(TI->getParent(), TI);
- return &Builder;
- }
-
- State = 2;
-
- // Find all 'call' instructions.
- 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 nullptr;
-
- // Create a cleanup block.
- LLVMContext &C = F.getContext();
- BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
- Type *ExnTy =
- StructType::get(Type::getInt8PtrTy(C), Type::getInt32Ty(C), nullptr);
- 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);
-
- // Transform the 'call' instructions into 'invoke's branching to the
- // cleanup block. Go in reverse order to make prettier BB names.
- SmallVector<Value *, 16> Args;
- for (unsigned I = Calls.size(); I != 0;) {
- CallInst *CI = cast<CallInst>(Calls[--I]);
-
- // Split the basic block containing the function call.
- BasicBlock *CallBB = CI->getParent();
- BasicBlock *NewBB =
- CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
-
- // Remove the unconditional branch inserted at the end of CallBB.
- CallBB->getInstList().pop_back();
- NewBB->getInstList().remove(CI);
-
- // Create a new invoke instruction.
- 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(nullptr), StackEntryTy(nullptr) {
+ShadowStackGC::ShadowStackGC() {
InitRoots = true;
CustomRoots = true;
}
-
-Constant *ShadowStackGC::GetFrameMap(Function &F) {
- // doInitialization creates the abstract type of this value.
- Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
-
- // Truncate the ShadowStackDescriptor if some metadata is null.
- 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);
-
- // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
- // that, short of multithreaded LLVM, it should be safe; all that is
- // necessary is that a simple Module::iterator loop not be invalidated.
- // Appending to the GlobalVariable list is safe in that sense.
- //
- // All of the output passes emit globals last. The ExecutionEngine
- // explicitly supports adding globals to the module after
- // initialization.
- //
- // Still, if it isn't deemed acceptable, then this transformation needs
- // to be a ModulePass (which means it cannot be in the 'llc' pipeline
- // (which uses a FunctionPassManager (which segfaults (not asserts) if
- // provided a ModulePass))).
- 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) {
- // doInitialization creates the generic version of this type.
- 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());
-}
-
-/// doInitialization - If this module uses the GC intrinsics, find them now. If
-/// not, exit fast.
-bool ShadowStackGC::initializeCustomLowering(Module &M) {
- // struct FrameMap {
- // int32_t NumRoots; // Number of roots in stack frame.
- // int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.
- // void *Meta[]; // May be absent for roots without metadata.
- // };
- std::vector<Type *> EltTys;
- // 32 bits is ok up to a 32GB stack frame. :)
- EltTys.push_back(Type::getInt32Ty(M.getContext()));
- // Specifies length of variable length array.
- EltTys.push_back(Type::getInt32Ty(M.getContext()));
- FrameMapTy = StructType::create(EltTys, "gc_map");
- PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
-
- // struct StackEntry {
- // ShadowStackEntry *Next; // Caller's stack entry.
- // FrameMap *Map; // Pointer to constant FrameMap.
- // void *Roots[]; // Stack roots (in-place array, so we pretend).
- // };
-
- 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);
-
- // Get the root chain if it already exists.
- Head = M.getGlobalVariable("llvm_gc_root_chain");
- if (!Head) {
- // If the root chain does not exist, insert a new one with linkonce
- // linkage!
- 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) {
- // FIXME: Account for original alignment. Could fragment the root array.
- // Approach 1: Null initialize empty slots at runtime. Yuck.
- // Approach 2: Emit a map of the array instead of just a count.
-
- 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);
- }
-
- // Number roots with metadata (usually empty) at the beginning, so that the
- // FrameMap::Meta array can be elided.
- 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);
-}
-
-/// runOnFunction - Insert code to maintain the shadow stack.
-bool ShadowStackGC::performCustomLowering(Function &F) {
- LLVMContext &Context = F.getContext();
-
- // Find calls to llvm.gcroot.
- CollectRoots(F);
-
- // If there are no roots in this function, then there is no need to add a
- // stack map entry for it.
- if (Roots.empty())
- return false;
-
- // Build the constant map and figure the type of the shadow stack entry.
- Value *FrameMap = GetFrameMap(F);
- Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
-
- // Build the shadow stack entry at the very start of the function.
- BasicBlock::iterator IP = F.getEntryBlock().begin();
- IRBuilder<> AtEntry(IP->getParent(), IP);
-
- Instruction *StackEntry =
- AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
-
- while (isa<AllocaInst>(IP))
- ++IP;
- AtEntry.SetInsertPoint(IP->getParent(), IP);
-
- // Initialize the map pointer and load the current head of the shadow stack.
- Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
- Instruction *EntryMapPtr =
- CreateGEP(Context, AtEntry, StackEntry, 0, 1, "gc_frame.map");
- AtEntry.CreateStore(FrameMap, EntryMapPtr);
-
- // After all the allocas...
- for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
- // For each root, find the corresponding slot in the aggregate...
- Value *SlotPtr = CreateGEP(Context, AtEntry, StackEntry, 1 + I, "gc_root");
-
- // And use it in lieu of the alloca.
- AllocaInst *OriginalAlloca = Roots[I].second;
- SlotPtr->takeName(OriginalAlloca);
- OriginalAlloca->replaceAllUsesWith(SlotPtr);
- }
-
- // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
- // really necessary (the collector would never see the intermediate state at
- // runtime), but it's nicer not to push the half-initialized entry onto the
- // shadow stack.
- while (isa<StoreInst>(IP))
- ++IP;
- AtEntry.SetInsertPoint(IP->getParent(), IP);
-
- // Push the entry onto the shadow stack.
- 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);
-
- // For each instruction that escapes...
- EscapeEnumerator EE(F, "gc_cleanup");
- while (IRBuilder<> *AtExit = EE.Next()) {
- // Pop the entry from the shadow stack. Don't reuse CurrentHead from
- // AtEntry, since that would make the value live for the entire function.
- Instruction *EntryNextPtr2 =
- CreateGEP(Context, *AtExit, StackEntry, 0, 0, "gc_frame.next");
- Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
- AtExit->CreateStore(SavedHead, Head);
- }
-
- // Delete the original allocas (which are no longer used) and the intrinsic
- // calls (which are no longer valid). Doing this last avoids invalidating
- // iterators.
- for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
- Roots[I].first->eraseFromParent();
- Roots[I].second->eraseFromParent();
- }
-
- Roots.clear();
- return true;
-}
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