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llvm-mirror/lib/CodeGen/StackProtector.cpp
Hongtao Yu 20454adfae [CSSPGO] Unblock optimizations with pseudo probe instrumentation. 
The IR/MIR pseudo probe intrinsics don't get materialized into real machine instructions and therefore they don't incur runtime cost directly. However, they come with indirect cost by blocking certain optimizations. Some of the blocking are intentional (such as blocking code merge) for better counts quality while the others are accidental. This change unblocks perf-critical optimizations that do not affect counts quality. They include:

1. IR InstCombine, sinking load operation to shorten lifetimes.
2. MIR LiveRangeShrink, similar to #1
3. MIR TwoAddressInstructionPass, i.e, opeq transform
4. MIR function argument copy elision
5. IR stack protection. (though not perf-critical but nice to have).

Reviewed By: wmi

Differential Revision: https://reviews.llvm.org/D95982
2021-02-10 12:43:17 -08:00

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//===- StackProtector.cpp - Stack Protector Insertion ---------------------===//
//
// 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 pass inserts stack protectors into functions which need them. A variable
// with a random value in it is stored onto the stack before the local variables
// are allocated. Upon exiting the block, the stored value is checked. If it's
// changed, then there was some sort of violation and the program aborts.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/StackProtector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/EHPersonalities.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "stack-protector"
STATISTIC(NumFunProtected, "Number of functions protected");
STATISTIC(NumAddrTaken, "Number of local variables that have their address"
" taken.");
static cl::opt<bool> EnableSelectionDAGSP("enable-selectiondag-sp",
cl::init(true), cl::Hidden);
char StackProtector::ID = 0;
StackProtector::StackProtector() : FunctionPass(ID), SSPBufferSize(8) {
initializeStackProtectorPass(*PassRegistry::getPassRegistry());
}
INITIALIZE_PASS_BEGIN(StackProtector, DEBUG_TYPE,
"Insert stack protectors", false, true)
INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(StackProtector, DEBUG_TYPE,
"Insert stack protectors", false, true)
FunctionPass *llvm::createStackProtectorPass() { return new StackProtector(); }
void StackProtector::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<TargetPassConfig>();
AU.addPreserved<DominatorTreeWrapperPass>();
}
bool StackProtector::runOnFunction(Function &Fn) {
F = &Fn;
M = F->getParent();
DominatorTreeWrapperPass *DTWP =
getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DT = DTWP ? &DTWP->getDomTree() : nullptr;
TM = &getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
Trip = TM->getTargetTriple();
TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
HasPrologue = false;
HasIRCheck = false;
Attribute Attr = Fn.getFnAttribute("stack-protector-buffer-size");
if (Attr.isStringAttribute() &&
Attr.getValueAsString().getAsInteger(10, SSPBufferSize))
return false; // Invalid integer string
if (!RequiresStackProtector())
return false;
// TODO(etienneb): Functions with funclets are not correctly supported now.
// Do nothing if this is funclet-based personality.
if (Fn.hasPersonalityFn()) {
EHPersonality Personality = classifyEHPersonality(Fn.getPersonalityFn());
if (isFuncletEHPersonality(Personality))
return false;
}
++NumFunProtected;
return InsertStackProtectors();
}
/// \param [out] IsLarge is set to true if a protectable array is found and
/// it is "large" ( >= ssp-buffer-size). In the case of a structure with
/// multiple arrays, this gets set if any of them is large.
bool StackProtector::ContainsProtectableArray(Type *Ty, bool &IsLarge,
bool Strong,
bool InStruct) const {
if (!Ty)
return false;
if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
if (!AT->getElementType()->isIntegerTy(8)) {
// If we're on a non-Darwin platform or we're inside of a structure, don't
// add stack protectors unless the array is a character array.
// However, in strong mode any array, regardless of type and size,
// triggers a protector.
if (!Strong && (InStruct || !Trip.isOSDarwin()))
return false;
}
// If an array has more than SSPBufferSize bytes of allocated space, then we
// emit stack protectors.
if (SSPBufferSize <= M->getDataLayout().getTypeAllocSize(AT)) {
IsLarge = true;
return true;
}
if (Strong)
// Require a protector for all arrays in strong mode
return true;
}
const StructType *ST = dyn_cast<StructType>(Ty);
if (!ST)
return false;
bool NeedsProtector = false;
for (StructType::element_iterator I = ST->element_begin(),
E = ST->element_end();
I != E; ++I)
if (ContainsProtectableArray(*I, IsLarge, Strong, true)) {
// If the element is a protectable array and is large (>= SSPBufferSize)
// then we are done. If the protectable array is not large, then
// keep looking in case a subsequent element is a large array.
if (IsLarge)
return true;
NeedsProtector = true;
}
return NeedsProtector;
}
bool StackProtector::HasAddressTaken(const Instruction *AI,
uint64_t AllocSize) {
const DataLayout &DL = M->getDataLayout();
for (const User *U : AI->users()) {
const auto *I = cast<Instruction>(U);
// If this instruction accesses memory make sure it doesn't access beyond
// the bounds of the allocated object.
Optional<MemoryLocation> MemLoc = MemoryLocation::getOrNone(I);
if (MemLoc.hasValue() && MemLoc->Size.hasValue() &&
MemLoc->Size.getValue() > AllocSize)
return true;
switch (I->getOpcode()) {
case Instruction::Store:
if (AI == cast<StoreInst>(I)->getValueOperand())
return true;
break;
case Instruction::AtomicCmpXchg:
// cmpxchg conceptually includes both a load and store from the same
// location. So, like store, the value being stored is what matters.
if (AI == cast<AtomicCmpXchgInst>(I)->getNewValOperand())
return true;
break;
case Instruction::PtrToInt:
if (AI == cast<PtrToIntInst>(I)->getOperand(0))
return true;
break;
case Instruction::Call: {
// Ignore intrinsics that do not become real instructions.
// TODO: Narrow this to intrinsics that have store-like effects.
const auto *CI = cast<CallInst>(I);
if (!CI->isDebugOrPseudoInst() && !CI->isLifetimeStartOrEnd())
return true;
break;
}
case Instruction::Invoke:
return true;
case Instruction::GetElementPtr: {
// If the GEP offset is out-of-bounds, or is non-constant and so has to be
// assumed to be potentially out-of-bounds, then any memory access that
// would use it could also be out-of-bounds meaning stack protection is
// required.
const GetElementPtrInst *GEP = cast<GetElementPtrInst>(I);
unsigned TypeSize = DL.getIndexTypeSizeInBits(I->getType());
APInt Offset(TypeSize, 0);
APInt MaxOffset(TypeSize, AllocSize);
if (!GEP->accumulateConstantOffset(DL, Offset) || Offset.ugt(MaxOffset))
return true;
// Adjust AllocSize to be the space remaining after this offset.
if (HasAddressTaken(I, AllocSize - Offset.getLimitedValue()))
return true;
break;
}
case Instruction::BitCast:
case Instruction::Select:
case Instruction::AddrSpaceCast:
if (HasAddressTaken(I, AllocSize))
return true;
break;
case Instruction::PHI: {
// Keep track of what PHI nodes we have already visited to ensure
// they are only visited once.
const auto *PN = cast<PHINode>(I);
if (VisitedPHIs.insert(PN).second)
if (HasAddressTaken(PN, AllocSize))
return true;
break;
}
case Instruction::Load:
case Instruction::AtomicRMW:
case Instruction::Ret:
// These instructions take an address operand, but have load-like or
// other innocuous behavior that should not trigger a stack protector.
// atomicrmw conceptually has both load and store semantics, but the
// value being stored must be integer; so if a pointer is being stored,
// we'll catch it in the PtrToInt case above.
break;
default:
// Conservatively return true for any instruction that takes an address
// operand, but is not handled above.
return true;
}
}
return false;
}
/// Search for the first call to the llvm.stackprotector intrinsic and return it
/// if present.
static const CallInst *findStackProtectorIntrinsic(Function &F) {
for (const BasicBlock &BB : F)
for (const Instruction &I : BB)
if (const auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::stackprotector)
return II;
return nullptr;
}
/// Check whether or not this function needs a stack protector based
/// upon the stack protector level.
///
/// We use two heuristics: a standard (ssp) and strong (sspstrong).
/// The standard heuristic which will add a guard variable to functions that
/// call alloca with a either a variable size or a size >= SSPBufferSize,
/// functions with character buffers larger than SSPBufferSize, and functions
/// with aggregates containing character buffers larger than SSPBufferSize. The
/// strong heuristic will add a guard variables to functions that call alloca
/// regardless of size, functions with any buffer regardless of type and size,
/// functions with aggregates that contain any buffer regardless of type and
/// size, and functions that contain stack-based variables that have had their
/// address taken.
bool StackProtector::RequiresStackProtector() {
bool Strong = false;
bool NeedsProtector = false;
if (F->hasFnAttribute(Attribute::SafeStack))
return false;
// We are constructing the OptimizationRemarkEmitter on the fly rather than
// using the analysis pass to avoid building DominatorTree and LoopInfo which
// are not available this late in the IR pipeline.
OptimizationRemarkEmitter ORE(F);
if (F->hasFnAttribute(Attribute::StackProtectReq)) {
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "StackProtectorRequested", F)
<< "Stack protection applied to function "
<< ore::NV("Function", F)
<< " due to a function attribute or command-line switch";
});
NeedsProtector = true;
Strong = true; // Use the same heuristic as strong to determine SSPLayout
} else if (F->hasFnAttribute(Attribute::StackProtectStrong))
Strong = true;
else if (!F->hasFnAttribute(Attribute::StackProtect))
return false;
for (const BasicBlock &BB : *F) {
for (const Instruction &I : BB) {
if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
if (AI->isArrayAllocation()) {
auto RemarkBuilder = [&]() {
return OptimizationRemark(DEBUG_TYPE, "StackProtectorAllocaOrArray",
&I)
<< "Stack protection applied to function "
<< ore::NV("Function", F)
<< " due to a call to alloca or use of a variable length "
"array";
};
if (const auto *CI = dyn_cast<ConstantInt>(AI->getArraySize())) {
if (CI->getLimitedValue(SSPBufferSize) >= SSPBufferSize) {
// A call to alloca with size >= SSPBufferSize requires
// stack protectors.
Layout.insert(std::make_pair(AI,
MachineFrameInfo::SSPLK_LargeArray));
ORE.emit(RemarkBuilder);
NeedsProtector = true;
} else if (Strong) {
// Require protectors for all alloca calls in strong mode.
Layout.insert(std::make_pair(AI,
MachineFrameInfo::SSPLK_SmallArray));
ORE.emit(RemarkBuilder);
NeedsProtector = true;
}
} else {
// A call to alloca with a variable size requires protectors.
Layout.insert(std::make_pair(AI,
MachineFrameInfo::SSPLK_LargeArray));
ORE.emit(RemarkBuilder);
NeedsProtector = true;
}
continue;
}
bool IsLarge = false;
if (ContainsProtectableArray(AI->getAllocatedType(), IsLarge, Strong)) {
Layout.insert(std::make_pair(AI, IsLarge
? MachineFrameInfo::SSPLK_LargeArray
: MachineFrameInfo::SSPLK_SmallArray));
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "StackProtectorBuffer", &I)
<< "Stack protection applied to function "
<< ore::NV("Function", F)
<< " due to a stack allocated buffer or struct containing a "
"buffer";
});
NeedsProtector = true;
continue;
}
if (Strong && HasAddressTaken(AI, M->getDataLayout().getTypeAllocSize(
AI->getAllocatedType()))) {
++NumAddrTaken;
Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf));
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "StackProtectorAddressTaken",
&I)
<< "Stack protection applied to function "
<< ore::NV("Function", F)
<< " due to the address of a local variable being taken";
});
NeedsProtector = true;
}
// Clear any PHIs that we visited, to make sure we examine all uses of
// any subsequent allocas that we look at.
VisitedPHIs.clear();
}
}
}
return NeedsProtector;
}
/// Create a stack guard loading and populate whether SelectionDAG SSP is
/// supported.
static Value *getStackGuard(const TargetLoweringBase *TLI, Module *M,
IRBuilder<> &B,
bool *SupportsSelectionDAGSP = nullptr) {
Value *Guard = TLI->getIRStackGuard(B);
auto GuardMode = TLI->getTargetMachine().Options.StackProtectorGuard;
if ((GuardMode == llvm::StackProtectorGuards::TLS ||
GuardMode == llvm::StackProtectorGuards::None) && Guard)
return B.CreateLoad(B.getInt8PtrTy(), Guard, true, "StackGuard");
// Use SelectionDAG SSP handling, since there isn't an IR guard.
//
// This is more or less weird, since we optionally output whether we
// should perform a SelectionDAG SP here. The reason is that it's strictly
// defined as !TLI->getIRStackGuard(B), where getIRStackGuard is also
// mutating. There is no way to get this bit without mutating the IR, so
// getting this bit has to happen in this right time.
//
// We could have define a new function TLI::supportsSelectionDAGSP(), but that
// will put more burden on the backends' overriding work, especially when it
// actually conveys the same information getIRStackGuard() already gives.
if (SupportsSelectionDAGSP)
*SupportsSelectionDAGSP = true;
TLI->insertSSPDeclarations(*M);
return B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackguard));
}
/// Insert code into the entry block that stores the stack guard
/// variable onto the stack:
///
/// entry:
/// StackGuardSlot = alloca i8*
/// StackGuard = <stack guard>
/// call void @llvm.stackprotector(StackGuard, StackGuardSlot)
///
/// Returns true if the platform/triple supports the stackprotectorcreate pseudo
/// node.
static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
const TargetLoweringBase *TLI, AllocaInst *&AI) {
bool SupportsSelectionDAGSP = false;
IRBuilder<> B(&F->getEntryBlock().front());
PointerType *PtrTy = Type::getInt8PtrTy(RI->getContext());
AI = B.CreateAlloca(PtrTy, nullptr, "StackGuardSlot");
Value *GuardSlot = getStackGuard(TLI, M, B, &SupportsSelectionDAGSP);
B.CreateCall(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
{GuardSlot, AI});
return SupportsSelectionDAGSP;
}
/// InsertStackProtectors - Insert code into the prologue and epilogue of the
/// function.
///
/// - The prologue code loads and stores the stack guard onto the stack.
/// - The epilogue checks the value stored in the prologue against the original
/// value. It calls __stack_chk_fail if they differ.
bool StackProtector::InsertStackProtectors() {
// If the target wants to XOR the frame pointer into the guard value, it's
// impossible to emit the check in IR, so the target *must* support stack
// protection in SDAG.
bool SupportsSelectionDAGSP =
TLI->useStackGuardXorFP() ||
(EnableSelectionDAGSP && !TM->Options.EnableFastISel &&
!TM->Options.EnableGlobalISel);
AllocaInst *AI = nullptr; // Place on stack that stores the stack guard.
for (Function::iterator I = F->begin(), E = F->end(); I != E;) {
BasicBlock *BB = &*I++;
ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
if (!RI)
continue;
// Generate prologue instrumentation if not already generated.
if (!HasPrologue) {
HasPrologue = true;
SupportsSelectionDAGSP &= CreatePrologue(F, M, RI, TLI, AI);
}
// SelectionDAG based code generation. Nothing else needs to be done here.
// The epilogue instrumentation is postponed to SelectionDAG.
if (SupportsSelectionDAGSP)
break;
// Find the stack guard slot if the prologue was not created by this pass
// itself via a previous call to CreatePrologue().
if (!AI) {
const CallInst *SPCall = findStackProtectorIntrinsic(*F);
assert(SPCall && "Call to llvm.stackprotector is missing");
AI = cast<AllocaInst>(SPCall->getArgOperand(1));
}
// Set HasIRCheck to true, so that SelectionDAG will not generate its own
// version. SelectionDAG called 'shouldEmitSDCheck' to check whether
// instrumentation has already been generated.
HasIRCheck = true;
// Generate epilogue instrumentation. The epilogue intrumentation can be
// function-based or inlined depending on which mechanism the target is
// providing.
if (Function *GuardCheck = TLI->getSSPStackGuardCheck(*M)) {
// Generate the function-based epilogue instrumentation.
// The target provides a guard check function, generate a call to it.
IRBuilder<> B(RI);
LoadInst *Guard = B.CreateLoad(B.getInt8PtrTy(), AI, true, "Guard");
CallInst *Call = B.CreateCall(GuardCheck, {Guard});
Call->setAttributes(GuardCheck->getAttributes());
Call->setCallingConv(GuardCheck->getCallingConv());
} else {
// Generate the epilogue with inline instrumentation.
// If we do not support SelectionDAG based tail calls, generate IR level
// tail calls.
//
// For each block with a return instruction, convert this:
//
// return:
// ...
// ret ...
//
// into this:
//
// return:
// ...
// %1 = <stack guard>
// %2 = load StackGuardSlot
// %3 = cmp i1 %1, %2
// br i1 %3, label %SP_return, label %CallStackCheckFailBlk
//
// SP_return:
// ret ...
//
// CallStackCheckFailBlk:
// call void @__stack_chk_fail()
// unreachable
// Create the FailBB. We duplicate the BB every time since the MI tail
// merge pass will merge together all of the various BB into one including
// fail BB generated by the stack protector pseudo instruction.
BasicBlock *FailBB = CreateFailBB();
// Split the basic block before the return instruction.
BasicBlock *NewBB = BB->splitBasicBlock(RI->getIterator(), "SP_return");
// Update the dominator tree if we need to.
if (DT && DT->isReachableFromEntry(BB)) {
DT->addNewBlock(NewBB, BB);
DT->addNewBlock(FailBB, BB);
}
// Remove default branch instruction to the new BB.
BB->getTerminator()->eraseFromParent();
// Move the newly created basic block to the point right after the old
// basic block so that it's in the "fall through" position.
NewBB->moveAfter(BB);
// Generate the stack protector instructions in the old basic block.
IRBuilder<> B(BB);
Value *Guard = getStackGuard(TLI, M, B);
LoadInst *LI2 = B.CreateLoad(B.getInt8PtrTy(), AI, true);
Value *Cmp = B.CreateICmpEQ(Guard, LI2);
auto SuccessProb =
BranchProbabilityInfo::getBranchProbStackProtector(true);
auto FailureProb =
BranchProbabilityInfo::getBranchProbStackProtector(false);
MDNode *Weights = MDBuilder(F->getContext())
.createBranchWeights(SuccessProb.getNumerator(),
FailureProb.getNumerator());
B.CreateCondBr(Cmp, NewBB, FailBB, Weights);
}
}
// Return if we didn't modify any basic blocks. i.e., there are no return
// statements in the function.
return HasPrologue;
}
/// CreateFailBB - Create a basic block to jump to when the stack protector
/// check fails.
BasicBlock *StackProtector::CreateFailBB() {
LLVMContext &Context = F->getContext();
BasicBlock *FailBB = BasicBlock::Create(Context, "CallStackCheckFailBlk", F);
IRBuilder<> B(FailBB);
if (F->getSubprogram())
B.SetCurrentDebugLocation(
DILocation::get(Context, 0, 0, F->getSubprogram()));
if (Trip.isOSOpenBSD()) {
FunctionCallee StackChkFail = M->getOrInsertFunction(
"__stack_smash_handler", Type::getVoidTy(Context),
Type::getInt8PtrTy(Context));
B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
} else {
FunctionCallee StackChkFail =
M->getOrInsertFunction("__stack_chk_fail", Type::getVoidTy(Context));
B.CreateCall(StackChkFail, {});
}
B.CreateUnreachable();
return FailBB;
}
bool StackProtector::shouldEmitSDCheck(const BasicBlock &BB) const {
return HasPrologue && !HasIRCheck && isa<ReturnInst>(BB.getTerminator());
}
void StackProtector::copyToMachineFrameInfo(MachineFrameInfo &MFI) const {
if (Layout.empty())
return;
for (int I = 0, E = MFI.getObjectIndexEnd(); I != E; ++I) {
if (MFI.isDeadObjectIndex(I))
continue;
const AllocaInst *AI = MFI.getObjectAllocation(I);
if (!AI)
continue;
SSPLayoutMap::const_iterator LI = Layout.find(AI);
if (LI == Layout.end())
continue;
MFI.setObjectSSPLayout(I, LI->second);
}
}
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