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//===---- X86IndirectBranchTracking.cpp - Enables CET IBT mechanism -------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file defines a pass that enables Indirect Branch Tracking (IBT) as part
// of Control-Flow Enforcement Technology (CET).
// The pass adds ENDBR (End Branch) machine instructions at the beginning of
// each basic block or function that is referenced by an indrect jump/call
// instruction.
// The ENDBR instructions have a NOP encoding and as such are ignored in
// targets that do not support CET IBT mechanism.
#include "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
using namespace llvm;
#define DEBUG_TYPE "x86-indirect-branch-tracking"
static cl::opt<bool> IndirectBranchTracking(
"x86-indirect-branch-tracking", cl::init(false), cl::Hidden,
cl::desc("Enable X86 indirect branch tracking pass."));
STATISTIC(NumEndBranchAdded, "Number of ENDBR instructions added");
namespace {
class X86IndirectBranchTrackingPass : public MachineFunctionPass {
X86IndirectBranchTrackingPass() : MachineFunctionPass(ID) {}
StringRef getPassName() const override {
return "X86 Indirect Branch Tracking";
bool runOnMachineFunction(MachineFunction &MF) override;
static char ID;
/// Machine instruction info used throughout the class.
const X86InstrInfo *TII;
/// Endbr opcode for the current machine function.
unsigned int EndbrOpcode;
/// Adds a new ENDBR instruction to the begining of the MBB.
/// The function will not add it if already exists.
/// It will add ENDBR32 or ENDBR64 opcode, depending on the target.
/// \returns true if the ENDBR was added and false otherwise.
bool addENDBR(MachineBasicBlock &MBB) const;
} // end anonymous namespace
char X86IndirectBranchTrackingPass::ID = 0;
FunctionPass *llvm::createX86IndirectBranchTrackingPass() {
return new X86IndirectBranchTrackingPass();
bool X86IndirectBranchTrackingPass::addENDBR(MachineBasicBlock &MBB) const {
assert(TII && "Target instruction info was not initialized");
assert((X86::ENDBR64 == EndbrOpcode || X86::ENDBR32 == EndbrOpcode) &&
"Unexpected Endbr opcode");
auto MI = MBB.begin();
// If the MBB is empty or the first instruction is not ENDBR,
// add the ENDBR instruction to the beginning of the MBB.
if (MI == MBB.end() || EndbrOpcode != MI->getOpcode()) {
BuildMI(MBB, MI, MBB.findDebugLoc(MI), TII->get(EndbrOpcode));
return true;
return false;
bool X86IndirectBranchTrackingPass::runOnMachineFunction(MachineFunction &MF) {
const X86Subtarget &SubTarget = MF.getSubtarget<X86Subtarget>();
// Check that the cf-protection-branch is enabled.
Metadata *isCFProtectionSupported =
if (!isCFProtectionSupported && !IndirectBranchTracking)
return false;
// True if the current MF was changed and false otherwise.
bool Changed = false;
TII = SubTarget.getInstrInfo();
EndbrOpcode = SubTarget.is64Bit() ? X86::ENDBR64 : X86::ENDBR32;
// Non-internal function or function whose address was taken, can be
// accessed through indirect calls. Mark the first BB with ENDBR instruction
// unless nocf_check attribute is used.
if ((MF.getFunction().hasAddressTaken() ||
!MF.getFunction().hasLocalLinkage()) &&
!MF.getFunction().doesNoCfCheck()) {
auto MBB = MF.begin();
Changed |= addENDBR(*MBB);
for (auto &MBB : MF)
// Find all basic blocks that their address was taken (for example
// in the case of indirect jump) and add ENDBR instruction.
if (MBB.hasAddressTaken())
Changed |= addENDBR(MBB);
return Changed;