453 lines
17 KiB
C++
453 lines
17 KiB
C++
//===-- ShadowStackGC.cpp - GC support for uncooperative targets ----------===//
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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 implements lowering for the llvm.gc* intrinsics for targets that do
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// not natively support them (which includes the C backend). Note that the code
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// generated is not quite as efficient as algorithms which generate stack maps
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// to identify roots.
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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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// In runtime/GC/SemiSpace.cpp is a prototype runtime which is compatible with
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// ShadowStackGC.
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//
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// In order to support this particular transformation, all stack roots are
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// coallocated in the stack. This allows a fully target-independent stack map
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// while introducing only minor runtime overhead.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "shadowstackgc"
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#include "llvm/IRBuilder.h"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/Module.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/CodeGen/GCs.h"
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#include "llvm/Support/CallSite.h"
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using namespace llvm;
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namespace {
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class ShadowStackGC : public GCStrategy {
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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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ShadowStackGC();
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bool initializeCustomLowering(Module &M);
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bool performCustomLowering(Function &F);
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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,
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IRBuilder<> &B, Value *BasePtr,
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int Idx1, const char *Name);
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static GetElementPtrInst *CreateGEP(LLVMContext &Context,
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IRBuilder<> &B, Value *BasePtr,
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int Idx1, int Idx2, const char *Name);
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};
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}
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static GCRegistry::Add<ShadowStackGC>
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X("shadow-stack", "Very portable GC for uncooperative code generators");
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namespace {
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/// EscapeEnumerator - This is a little algorithm to find all escape points
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/// from a function so that "finally"-style code can be inserted. In addition
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/// to finding the existing return and unwind instructions, it also (if
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/// necessary) transforms any call instructions into invokes and sends them to
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/// a landing pad.
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///
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/// It's wrapped up in a state machine using the same transform C# uses for
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/// 'yield return' enumerators, This transform allows it to be non-allocating.
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class EscapeEnumerator {
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Function &F;
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const char *CleanupBBName;
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// State.
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int State;
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Function::iterator StateBB, StateE;
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IRBuilder<> Builder;
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public:
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EscapeEnumerator(Function &F, const char *N = "cleanup")
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: F(F), CleanupBBName(N), State(0), Builder(F.getContext()) {}
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IRBuilder<> *Next() {
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switch (State) {
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default:
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return 0;
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case 0:
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StateBB = F.begin();
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StateE = F.end();
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State = 1;
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case 1:
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// Find all 'return', 'resume', and 'unwind' instructions.
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while (StateBB != StateE) {
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BasicBlock *CurBB = StateBB++;
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// Branches and invokes do not escape, only unwind, resume, and return
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// do.
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TerminatorInst *TI = CurBB->getTerminator();
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if (!isa<ReturnInst>(TI) && !isa<ResumeInst>(TI))
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continue;
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Builder.SetInsertPoint(TI->getParent(), TI);
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return &Builder;
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}
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State = 2;
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// Find all 'call' instructions.
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SmallVector<Instruction*,16> Calls;
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for (Function::iterator BB = F.begin(),
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E = F.end(); BB != E; ++BB)
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for (BasicBlock::iterator II = BB->begin(),
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EE = BB->end(); II != EE; ++II)
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if (CallInst *CI = dyn_cast<CallInst>(II))
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if (!CI->getCalledFunction() ||
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!CI->getCalledFunction()->getIntrinsicID())
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Calls.push_back(CI);
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if (Calls.empty())
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return 0;
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// Create a cleanup block.
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LLVMContext &C = F.getContext();
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BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
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Type *ExnTy = StructType::get(Type::getInt8PtrTy(C),
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Type::getInt32Ty(C), NULL);
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Constant *PersFn =
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F.getParent()->
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getOrInsertFunction("__gcc_personality_v0",
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FunctionType::get(Type::getInt32Ty(C), true));
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LandingPadInst *LPad = LandingPadInst::Create(ExnTy, PersFn, 1,
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"cleanup.lpad",
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CleanupBB);
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LPad->setCleanup(true);
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ResumeInst *RI = ResumeInst::Create(LPad, CleanupBB);
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// Transform the 'call' instructions into 'invoke's branching to the
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// cleanup block. Go in reverse order to make prettier BB names.
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SmallVector<Value*,16> Args;
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for (unsigned I = Calls.size(); I != 0; ) {
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CallInst *CI = cast<CallInst>(Calls[--I]);
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// Split the basic block containing the function call.
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BasicBlock *CallBB = CI->getParent();
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BasicBlock *NewBB =
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CallBB->splitBasicBlock(CI, CallBB->getName() + ".cont");
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// Remove the unconditional branch inserted at the end of CallBB.
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CallBB->getInstList().pop_back();
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NewBB->getInstList().remove(CI);
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// Create a new invoke instruction.
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Args.clear();
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CallSite CS(CI);
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Args.append(CS.arg_begin(), CS.arg_end());
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InvokeInst *II = InvokeInst::Create(CI->getCalledValue(),
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NewBB, CleanupBB,
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Args, CI->getName(), CallBB);
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II->setCallingConv(CI->getCallingConv());
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II->setAttributes(CI->getAttributes());
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CI->replaceAllUsesWith(II);
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delete CI;
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}
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Builder.SetInsertPoint(RI->getParent(), RI);
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return &Builder;
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}
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}
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};
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}
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// -----------------------------------------------------------------------------
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void llvm::linkShadowStackGC() { }
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ShadowStackGC::ShadowStackGC() : Head(0), StackEntryTy(0) {
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InitRoots = true;
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CustomRoots = true;
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}
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Constant *ShadowStackGC::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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};
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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,
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FrameMap, "__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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};
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return ConstantExpr::getGetElementPtr(GV, GEPIndices);
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}
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Type* ShadowStackGC::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 ShadowStackGC::initializeCustomLowering(Module &M) {
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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(M, StackEntryPtrTy, false,
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GlobalValue::LinkOnceAnyLinkage,
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Constant::getNullValue(StackEntryPtrTy),
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"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 ShadowStackGC::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 ShadowStackGC::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, 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 *
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ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, Value *BasePtr,
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int Idx, int Idx2, 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(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 *
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ShadowStackGC::CreateGEP(LLVMContext &Context, IRBuilder<> &B, 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(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 ShadowStackGC::performCustomLowering(Function &F) {
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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 = AtEntry.CreateAlloca(ConcreteStackEntryTy, 0,
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"gc_frame");
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while (isa<AllocaInst>(IP)) ++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, StackEntry,
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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, 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)) ++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,
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StackEntry,0,0,"gc_frame.next");
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Instruction *NewHeadVal = CreateGEP(Context, AtEntry,
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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 = CreateGEP(Context, *AtExit, 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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