374 lines
14 KiB
C++
374 lines
14 KiB
C++
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//===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file contains the custom lowering code required by the shadow-stack GC
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// strategy.
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//
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// This pass implements the code transformation described in this paper:
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// "Accurate Garbage Collection in an Uncooperative Environment"
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// Fergus Henderson, ISMM, 2002
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/StringExtras.h"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/Constant.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DerivedTypes.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/GlobalValue.h"
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#include "llvm/IR/GlobalVariable.h"
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#include "llvm/IR/IRBuilder.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Intrinsics.h"
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#include "llvm/IR/Module.h"
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#include "llvm/IR/Type.h"
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#include "llvm/IR/Value.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Casting.h"
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#include "llvm/Transforms/Utils/EscapeEnumerator.h"
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#include <cassert>
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#include <cstddef>
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#include <string>
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#include <utility>
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#include <vector>
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using namespace llvm;
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#define DEBUG_TYPE "shadow-stack-gc-lowering"
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namespace {
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class ShadowStackGCLowering : public FunctionPass {
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/// RootChain - This is the global linked-list that contains the chain of GC
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/// roots.
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GlobalVariable *Head = nullptr;
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/// StackEntryTy - Abstract type of a link in the shadow stack.
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StructType *StackEntryTy = nullptr;
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StructType *FrameMapTy = nullptr;
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/// Roots - GC roots in the current function. Each is a pair of the
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/// intrinsic call and its corresponding alloca.
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std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
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public:
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static char ID;
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ShadowStackGCLowering();
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bool doInitialization(Module &M) override;
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bool runOnFunction(Function &F) override;
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private:
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bool IsNullValue(Value *V);
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Constant *GetFrameMap(Function &F);
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Type *GetConcreteStackEntryType(Function &F);
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void CollectRoots(Function &F);
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static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
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Type *Ty, Value *BasePtr, int Idx1,
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const char *Name);
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static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
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Type *Ty, Value *BasePtr, int Idx1, int Idx2,
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const char *Name);
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};
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} // end anonymous namespace
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char ShadowStackGCLowering::ID = 0;
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INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
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"Shadow Stack GC Lowering", false, false)
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INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
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INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
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"Shadow Stack GC Lowering", false, false)
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FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
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ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
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initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
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}
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Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
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// doInitialization creates the abstract type of this value.
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Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
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// Truncate the ShadowStackDescriptor if some metadata is null.
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unsigned NumMeta = 0;
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SmallVector<Constant *, 16> Metadata;
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for (unsigned I = 0; I != Roots.size(); ++I) {
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Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
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if (!C->isNullValue())
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NumMeta = I + 1;
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Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
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}
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Metadata.resize(NumMeta);
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Type *Int32Ty = Type::getInt32Ty(F.getContext());
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Constant *BaseElts[] = {
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ConstantInt::get(Int32Ty, Roots.size(), false),
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ConstantInt::get(Int32Ty, NumMeta, false),
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};
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Constant *DescriptorElts[] = {
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ConstantStruct::get(FrameMapTy, BaseElts),
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ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
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Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
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StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
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Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
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// FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
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// that, short of multithreaded LLVM, it should be safe; all that is
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// necessary is that a simple Module::iterator loop not be invalidated.
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// Appending to the GlobalVariable list is safe in that sense.
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//
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// All of the output passes emit globals last. The ExecutionEngine
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// explicitly supports adding globals to the module after
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// initialization.
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//
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// Still, if it isn't deemed acceptable, then this transformation needs
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// to be a ModulePass (which means it cannot be in the 'llc' pipeline
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// (which uses a FunctionPassManager (which segfaults (not asserts) if
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// provided a ModulePass))).
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Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
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GlobalVariable::InternalLinkage, FrameMap,
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"__gc_" + F.getName());
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Constant *GEPIndices[2] = {
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ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
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ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
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return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
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}
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Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
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// doInitialization creates the generic version of this type.
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std::vector<Type *> EltTys;
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EltTys.push_back(StackEntryTy);
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for (size_t I = 0; I != Roots.size(); I++)
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EltTys.push_back(Roots[I].second->getAllocatedType());
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return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
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}
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/// doInitialization - If this module uses the GC intrinsics, find them now. If
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/// not, exit fast.
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bool ShadowStackGCLowering::doInitialization(Module &M) {
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bool Active = false;
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for (Function &F : M) {
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if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
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Active = true;
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break;
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}
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}
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if (!Active)
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return false;
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// struct FrameMap {
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// int32_t NumRoots; // Number of roots in stack frame.
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// int32_t NumMeta; // Number of metadata descriptors. May be < NumRoots.
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// void *Meta[]; // May be absent for roots without metadata.
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// };
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std::vector<Type *> EltTys;
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// 32 bits is ok up to a 32GB stack frame. :)
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EltTys.push_back(Type::getInt32Ty(M.getContext()));
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// Specifies length of variable length array.
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EltTys.push_back(Type::getInt32Ty(M.getContext()));
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FrameMapTy = StructType::create(EltTys, "gc_map");
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PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
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// struct StackEntry {
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// ShadowStackEntry *Next; // Caller's stack entry.
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// FrameMap *Map; // Pointer to constant FrameMap.
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// void *Roots[]; // Stack roots (in-place array, so we pretend).
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// };
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StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
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EltTys.clear();
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EltTys.push_back(PointerType::getUnqual(StackEntryTy));
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EltTys.push_back(FrameMapPtrTy);
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StackEntryTy->setBody(EltTys);
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PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
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// Get the root chain if it already exists.
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Head = M.getGlobalVariable("llvm_gc_root_chain");
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if (!Head) {
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// If the root chain does not exist, insert a new one with linkonce
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// linkage!
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Head = new GlobalVariable(
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M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
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Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
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} else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
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Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
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Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
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}
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return true;
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}
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bool ShadowStackGCLowering::IsNullValue(Value *V) {
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if (Constant *C = dyn_cast<Constant>(V))
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return C->isNullValue();
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return false;
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}
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void ShadowStackGCLowering::CollectRoots(Function &F) {
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// FIXME: Account for original alignment. Could fragment the root array.
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// Approach 1: Null initialize empty slots at runtime. Yuck.
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// Approach 2: Emit a map of the array instead of just a count.
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assert(Roots.empty() && "Not cleaned up?");
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SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
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if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
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if (Function *F = CI->getCalledFunction())
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if (F->getIntrinsicID() == Intrinsic::gcroot) {
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std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
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CI,
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cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
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if (IsNullValue(CI->getArgOperand(1)))
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Roots.push_back(Pair);
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else
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MetaRoots.push_back(Pair);
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}
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// Number roots with metadata (usually empty) at the beginning, so that the
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// FrameMap::Meta array can be elided.
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Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
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}
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GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
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IRBuilder<> &B, Type *Ty,
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Value *BasePtr, int Idx,
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int Idx2,
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const char *Name) {
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Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
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ConstantInt::get(Type::getInt32Ty(Context), Idx),
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ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
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Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
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assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
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return dyn_cast<GetElementPtrInst>(Val);
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}
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GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
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IRBuilder<> &B, Type *Ty, Value *BasePtr,
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int Idx, const char *Name) {
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Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
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ConstantInt::get(Type::getInt32Ty(Context), Idx)};
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Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
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assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
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return dyn_cast<GetElementPtrInst>(Val);
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}
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/// runOnFunction - Insert code to maintain the shadow stack.
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bool ShadowStackGCLowering::runOnFunction(Function &F) {
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// Quick exit for functions that do not use the shadow stack GC.
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if (!F.hasGC() ||
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F.getGC() != std::string("shadow-stack"))
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return false;
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LLVMContext &Context = F.getContext();
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// Find calls to llvm.gcroot.
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CollectRoots(F);
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// If there are no roots in this function, then there is no need to add a
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// stack map entry for it.
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if (Roots.empty())
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return false;
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// Build the constant map and figure the type of the shadow stack entry.
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Value *FrameMap = GetFrameMap(F);
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Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
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// Build the shadow stack entry at the very start of the function.
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BasicBlock::iterator IP = F.getEntryBlock().begin();
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IRBuilder<> AtEntry(IP->getParent(), IP);
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Instruction *StackEntry =
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AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
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while (isa<AllocaInst>(IP))
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++IP;
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AtEntry.SetInsertPoint(IP->getParent(), IP);
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// Initialize the map pointer and load the current head of the shadow stack.
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Instruction *CurrentHead =
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AtEntry.CreateLoad(StackEntryTy->getPointerTo(), Head, "gc_currhead");
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Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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StackEntry, 0, 1, "gc_frame.map");
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AtEntry.CreateStore(FrameMap, EntryMapPtr);
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// After all the allocas...
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for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
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// For each root, find the corresponding slot in the aggregate...
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Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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StackEntry, 1 + I, "gc_root");
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// And use it in lieu of the alloca.
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AllocaInst *OriginalAlloca = Roots[I].second;
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SlotPtr->takeName(OriginalAlloca);
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OriginalAlloca->replaceAllUsesWith(SlotPtr);
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}
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// Move past the original stores inserted by GCStrategy::InitRoots. This isn't
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// really necessary (the collector would never see the intermediate state at
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// runtime), but it's nicer not to push the half-initialized entry onto the
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// shadow stack.
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while (isa<StoreInst>(IP))
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++IP;
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AtEntry.SetInsertPoint(IP->getParent(), IP);
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// Push the entry onto the shadow stack.
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Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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StackEntry, 0, 0, "gc_frame.next");
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Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
|
||
|
|
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, ConcreteStackEntryTy, StackEntry, 0, 0,
|
||
|
|
"gc_frame.next");
|
||
|
|
Value *SavedHead = AtExit->CreateLoad(StackEntryTy->getPointerTo(),
|
||
|
|
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;
|
||
|
|
}
|