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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/ADT/SmallVector.h"
+#include "llvm/ADT/StringExtras.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/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/Pass.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Transforms/Utils/EscapeEnumerator.h"
+#include <cassert>
+#include <cstddef>
+#include <string>
+#include <utility>
+#include <vector>
+
+using namespace llvm;
+
+#define DEBUG_TYPE "shadow-stack-gc-lowering"
+
+namespace {
+
+class ShadowStackGCLowering : public FunctionPass {
+ /// 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:
+ static char ID;
+
+ ShadowStackGCLowering();
+
+ bool doInitialization(Module &M) override;
+ bool runOnFunction(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,
+ 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);
+};
+
+} // end anonymous namespace
+
+char ShadowStackGCLowering::ID = 0;
+
+INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
+ "Shadow Stack GC Lowering", false, false)
+INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
+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 *ShadowStackGCLowering::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(FrameMap->getType(), GV, GEPIndices);
+}
+
+Type *ShadowStackGCLowering::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 ShadowStackGCLowering::doInitialization(Module &M) {
+ bool Active = false;
+ for (Function &F : M) {
+ if (F.hasGC() && F.getGC() == std::string("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).
+ // };
+
+ 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 ShadowStackGCLowering::IsNullValue(Value *V) {
+ if (Constant *C = dyn_cast<Constant>(V))
+ return C->isNullValue();
+ return false;
+}
+
+void ShadowStackGCLowering::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 *ShadowStackGCLowering::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 *ShadowStackGCLowering::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 ShadowStackGCLowering::runOnFunction(Function &F) {
+ // Quick exit for functions that do not use the shadow stack GC.
+ if (!F.hasGC() ||
+ F.getGC() != std::string("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");
+
+ 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(StackEntryTy->getPointerTo(), 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");
+ 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;
+}
generated by cgit v1.2.3 (git 2.25.1) at 2025-10-10 23:16:55 +0000