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06fe9815da
This adds support for TSan C++ exception handling, where we need to add extra calls to __tsan_func_exit when a function is exitted via exception mechanisms. Otherwise the shadow stack gets corrupted (leaked). This patch moves and enhances the existing implementation of EscapeEnumerator that finds all possible function exit points, and adds extra EH cleanup blocks where needed. Differential Revision: https://reviews.llvm.org/D26177 llvm-svn: 286893
355 lines
14 KiB
C++
355 lines
14 KiB
C++
//===-- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ---===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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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/CodeGen/Passes.h"
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#include "llvm/ADT/StringExtras.h"
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#include "llvm/CodeGen/GCStrategy.h"
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#include "llvm/IR/CallSite.h"
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#include "llvm/IR/IRBuilder.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Transforms/Utils/EscapeEnumerator.h"
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using namespace llvm;
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#define DEBUG_TYPE "shadowstackgclowering"
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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;
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/// StackEntryTy - Abstract type of a link in the shadow stack.
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///
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StructType *StackEntryTy;
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StructType *FrameMapTy;
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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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}
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INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, "shadow-stack-gc-lowering",
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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, "shadow-stack-gc-lowering",
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"Shadow Stack GC Lowering", false, false)
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FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
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char ShadowStackGCLowering::ID = 0;
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ShadowStackGCLowering::ShadowStackGCLowering()
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: FunctionPass(ID), Head(nullptr), StackEntryTy(nullptr),
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FrameMapTy(nullptr) {
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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 = AtEntry.CreateLoad(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,
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StackEntry, 0, "gc_newhead");
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AtEntry.CreateStore(CurrentHead, EntryNextPtr);
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AtEntry.CreateStore(NewHeadVal, Head);
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// For each instruction that escapes...
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EscapeEnumerator EE(F, "gc_cleanup");
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while (IRBuilder<> *AtExit = EE.Next()) {
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// Pop the entry from the shadow stack. Don't reuse CurrentHead from
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// AtEntry, since that would make the value live for the entire function.
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Instruction *EntryNextPtr2 =
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CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
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"gc_frame.next");
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Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
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AtExit->CreateStore(SavedHead, Head);
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}
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// Delete the original allocas (which are no longer used) and the intrinsic
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// calls (which are no longer valid). Doing this last avoids invalidating
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// iterators.
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for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
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Roots[I].first->eraseFromParent();
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Roots[I].second->eraseFromParent();
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}
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Roots.clear();
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return true;
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}
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