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//===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains the custom lowering code required by the shadow-stack GC
// strategy.
//
// This pass implements the code transformation described in this paper:
// "Accurate Garbage Collection in an Uncooperative Environment"
// Fergus Henderson, ISMM, 2002
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/ShadowStackGCLowering.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/CodeGen/GCMetadata.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Transforms/Utils/EscapeEnumerator.h"
#include <cassert>
#include <optional>
#include <string>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "shadow-stack-gc-lowering"
namespace {
class ShadowStackGCLoweringImpl {
/// RootChain - This is the global linked-list that contains the chain of GC
/// roots.
GlobalVariable *Head = nullptr;
/// StackEntryTy - Abstract type of a link in the shadow stack.
StructType *StackEntryTy = nullptr;
StructType *FrameMapTy = nullptr;
/// 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:
ShadowStackGCLoweringImpl() = default;
bool doInitialization(Module &M);
bool runOnFunction(Function &F, DomTreeUpdater *DTU);
private:
bool IsNullValue(Value *V);
Constant *GetFrameMap(Function &F);
Type *GetConcreteStackEntryType(Function &F);
void CollectRoots(Function &F);
static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
Type *Ty, Value *BasePtr, int Idx1,
const char *Name);
static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
Type *Ty, Value *BasePtr, int Idx1, int Idx2,
const char *Name);
};
class ShadowStackGCLowering : public FunctionPass {
ShadowStackGCLoweringImpl Impl;
public:
static char ID;
ShadowStackGCLowering();
bool doInitialization(Module &M) override { return Impl.doInitialization(M); }
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addPreserved<DominatorTreeWrapperPass>();
}
bool runOnFunction(Function &F) override {
std::optional<DomTreeUpdater> DTU;
if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
DTU.emplace(DTWP->getDomTree(), DomTreeUpdater::UpdateStrategy::Lazy);
return Impl.runOnFunction(F, DTU ? &*DTU : nullptr);
}
};
} // end anonymous namespace
PreservedAnalyses ShadowStackGCLoweringPass::run(Module &M,
ModuleAnalysisManager &MAM) {
auto &Map = MAM.getResult<CollectorMetadataAnalysis>(M);
if (Map.StrategyMap.contains("shadow-stack"))
return PreservedAnalyses::all();
ShadowStackGCLoweringImpl Impl;
bool Changed = Impl.doInitialization(M);
for (auto &F : M) {
auto &FAM =
MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F);
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
Changed |= Impl.runOnFunction(F, DT ? &DTU : nullptr);
}
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserve<DominatorTreeAnalysis>();
return PA;
}
char ShadowStackGCLowering::ID = 0;
char &llvm::ShadowStackGCLoweringID = ShadowStackGCLowering::ID;
INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
"Shadow Stack GC Lowering", false, false)
INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
"Shadow Stack GC Lowering", false, false)
FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
}
Constant *ShadowStackGCLoweringImpl::GetFrameMap(Function &F) {
// doInitialization creates the abstract type of this value.
Type *VoidPtr = PointerType::getUnqual(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(C);
}
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(FrameMap->getType(), GV, GEPIndices);
}
Type *ShadowStackGCLoweringImpl::GetConcreteStackEntryType(Function &F) {
// doInitialization creates the generic version of this type.
std::vector<Type *> EltTys;
EltTys.push_back(StackEntryTy);
for (const std::pair<CallInst *, AllocaInst *> &Root : Roots)
EltTys.push_back(Root.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 ShadowStackGCLoweringImpl::doInitialization(Module &M) {
bool Active = false;
for (Function &F : M) {
if (F.hasGC() && F.getGC() == "shadow-stack") {
Active = true;
break;
}
}
if (!Active)
return false;
// 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).
// };
PointerType *StackEntryPtrTy = PointerType::getUnqual(M.getContext());
EltTys.clear();
EltTys.push_back(StackEntryPtrTy);
EltTys.push_back(FrameMapPtrTy);
StackEntryTy = StructType::create(EltTys, "gc_stackentry");
// 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 ShadowStackGCLoweringImpl::IsNullValue(Value *V) {
if (Constant *C = dyn_cast<Constant>(V))
return C->isNullValue();
return false;
}
void ShadowStackGCLoweringImpl::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 (BasicBlock &BB : F)
for (Instruction &I : BB)
if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(&I))
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 *
ShadowStackGCLoweringImpl::CreateGEP(LLVMContext &Context, IRBuilder<> &B,
Type *Ty, 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(Ty, BasePtr, Indices, Name);
assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
return dyn_cast<GetElementPtrInst>(Val);
}
GetElementPtrInst *ShadowStackGCLoweringImpl::CreateGEP(LLVMContext &Context,
IRBuilder<> &B,
Type *Ty,
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(Ty, BasePtr, Indices, Name);
assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
return dyn_cast<GetElementPtrInst>(Val);
}
/// runOnFunction - Insert code to maintain the shadow stack.
bool ShadowStackGCLoweringImpl::runOnFunction(Function &F,
DomTreeUpdater *DTU) {
// Quick exit for functions that do not use the shadow stack GC.
if (!F.hasGC() || F.getGC() != "shadow-stack")
return false;
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");
AtEntry.SetInsertPointPastAllocas(&F);
IP = AtEntry.GetInsertPoint();
// Initialize the map pointer and load the current head of the shadow stack.
Instruction *CurrentHead =
AtEntry.CreateLoad(AtEntry.getPtrTy(), Head, "gc_currhead");
Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
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, ConcreteStackEntryTy,
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, ConcreteStackEntryTy,
StackEntry, 0, 0, "gc_frame.next");
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", /*HandleExceptions=*/true, DTU);
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(AtExit->getPtrTy(), 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 (std::pair<CallInst *, AllocaInst *> &Root : Roots) {
Root.first->eraseFromParent();
Root.second->eraseFromParent();
}
Roots.clear();
return true;
}