MC Helpers for llvm-cfi-verify.

[hctim]

Summary

Add instruction analysis and machinecode traversal helpers in preparation for control flow graph generation implementation.

[hctim]

Added getControlFlowXRefs() to FileVerifier, including the associated unit tests.

[hctim]

Implemented FileVerifier::getIndirectInstructions() and made it return a std::set.

[vlad.tsyrklevich]

Could this change and the follow-ons be uploaded as diffs from the previous change?

[hctim]

Updated diff to only contain changes from stack.

[hctim]

Fix gtest unsigned/signed comparison warnings.

[vlad.tsyrklevich]

tools/llvm-cfi-verify/FileVerifier.cpp

132 ↗

(On Diff #117449)

getOpcode() == Instruction::Undefined? Also, what if InstrMeta.Bad?

171 ↗

(On Diff #117449)

Here you don't check NextKV->second.bad but below you do, which should it be?

tools/llvm-cfi-verify/FileVerifier.h

90 ↗

(On Diff #117449)

s/flow/fall/

95 ↗

(On Diff #117449)

s/qua/que/

96 ↗

(On Diff #117449)

s/instruction/method/

[hctim]

Merged changes from previous in stack, updated for vlad.tsyrklevich's comments.

tools/llvm-cfi-verify/FileVerifier.cpp

132 ↗

(On Diff #117449)

Made it more clear, this should only return true when the provided instruction is a trap used by CFI.

171 ↗

(On Diff #117449)

Should be both. Updated the unit test for this.

[hctim]

Merge prev in stack.

[hctim]

clang format

[vlad.tsyrklevich]

tools/llvm-cfi-verify/FileAnalysis.cpp

149

Make this an if{}else{} that calculates a uint64_t Target so that the identical tails can be merged?

tools/llvm-cfi-verify/FileAnalysis.h

103

getDirectControlFlowXRefs()?

[hctim]

Updated from Vlad's comments

[hctim]

Backported usesRegisterOperand.

[hctim]

Merged updates from stack.

[hctim]

Merged unit test modifications up stack.

tools/llvm-cfi-verify/FileAnalysis.h

public:
   const Instr &getInstructionOrDie(uint64_t Address) const;
 
   // Returns a pointer to the previous/next instruction in sequence,
   // respectively. Returns nullptr if the next/prev instruction doesn't exist,
   // or if the provided instruction doesn't exist.
   const Instr *getPrevInstructionSequential(const Instr &InstrMeta) const;
   const Instr *getNextInstructionSequential(const Instr &InstrMeta) const;
 
-  // Returns the list of indirect instructions.
+  // Returns whether this instruction is used by CFI to trap the program.
+  bool isCFITrap(const Instr &InstrMeta) const;
+
+  // Returns whether this function can fall through to the next instruction.
+  // Undefined (and bad) instructions cannot fall through, and instruction that
+  // modify the control flow can only fall through if they are conditional
+  // branches or calls.
+  bool canFallThrough(const Instr &InstrMeta) const;
+
+  // Returns the definitive next instruction. This is different from the next
+  // instruction sequentially as it will follow unconditional branches (assuming
+  // they can be resolved at compile time, i.e. not indirect). This method
+  // returns nullptr if the provided instruction does not transfer control flow
+  // to exactly one instruction that is known deterministically at compile time.
+  // Also returns nullptr if the deterministic target does not exist in this
+  // file.
+  const Instr *getDefiniteNextInstruction(const Instr &InstrMeta) const;
+
+  // Get a list of deterministic control flows that lead to the provided
+  // instruction. This list includes all static control flow cross-references as
+  // well as the previous instruction if it can fall through.
+  std::set<const Instr *> getControlFlowXRefs(const Instr &InstrMeta) const;
+
   const std::set<uint64_t> &getIndirectInstructions() const;
 
   const MCRegisterInfo *getRegisterInfo() const;
   const MCInstrInfo *getMCInstrInfo() const;
   const MCInstrAnalysis *getMCInstrAnalysis() const;
 
 protected:
   // Construct a blank object with the provided triple and features. Used in

tools/llvm-cfi-verify/FileAnalysis.cpp

 }
 
 const Instr &FileAnalysis::getInstructionOrDie(uint64_t Address) const {
   const auto &InstrKV = Instructions.find(Address);
   assert(InstrKV != Instructions.end() && "Address doesn't exist.");
   return InstrKV->second;
 }
 
+bool FileAnalysis::isCFITrap(const Instr &InstrMeta) const {
+  return MII->getName(InstrMeta.Instruction.getOpcode()) == "TRAP";
+}
+
+bool FileAnalysis::canFallThrough(const Instr &InstrMeta) const {
+  if (!InstrMeta.Valid)
+    return false;
+
+  if (isCFITrap(InstrMeta))
+    return false;
+
+  const auto &InstrDesc = MII->get(InstrMeta.Instruction.getOpcode());
+  if (InstrDesc.mayAffectControlFlow(InstrMeta.Instruction, *RegisterInfo))
+    return InstrDesc.isConditionalBranch();
+
+  return true;
+}
+
+const Instr *
+FileAnalysis::getDefiniteNextInstruction(const Instr &InstrMeta) const {
+  if (!InstrMeta.Valid)
+    return nullptr;
+
+  if (isCFITrap(InstrMeta))
+    return nullptr;
+
+  const auto &InstrDesc = MII->get(InstrMeta.Instruction.getOpcode());
+  if (InstrDesc.mayAffectControlFlow(InstrMeta.Instruction, *RegisterInfo)) {
+    if (InstrDesc.isConditionalBranch())
+      return nullptr;
+
+    uint64_t Target;
+    if (!MIA->evaluateBranch(InstrMeta.Instruction, InstrMeta.VMAddress,
+                             InstrMeta.InstructionSize, Target))
+      return nullptr;
+
+    const auto &NextKV = Instructions.find(Target);
+    if (NextKV == Instructions.end())
+      return nullptr;
+
+    if (!NextKV->second.Valid)
+      return nullptr;
+
+    return &NextKV->second;
+  }
+
+  const auto &NextKV =
+      Instructions.find(InstrMeta.VMAddress + InstrMeta.InstructionSize);
+  if (NextKV == Instructions.end())
+    return nullptr;
+
+  if (!NextKV->second.Valid)
+    return nullptr;
+
+  return &NextKV->second;
+}
+
+std::set<const Instr *>
+FileAnalysis::getControlFlowXRefs(const Instr &InstrMeta) const {
+  std::set<const Instr *> CFCrossReferences;
+  const Instr *PrevInstruction = getPrevInstructionSequential(InstrMeta);
+
+  if (PrevInstruction && canFallThrough(*PrevInstruction))
+    CFCrossReferences.insert(PrevInstruction);
+
+  const auto &TargetRefsKV = StaticBranchTargetings.find(InstrMeta.VMAddress);
+  if (TargetRefsKV == StaticBranchTargetings.end())
+    return CFCrossReferences;
+
+  for (uint64_t SourceInstrAddress : TargetRefsKV->second) {
+    const auto &SourceInstrKV = Instructions.find(SourceInstrAddress);
+    if (SourceInstrKV == Instructions.end()) {
+      errs() << "Failed to find source instruction at address "
+             << format_hex(SourceInstrAddress, 2)
+             << " for the cross-reference to instruction at address "
+             << format_hex(InstrMeta.VMAddress, 2) << ".\n";
+      continue;
+    }
+
+    CFCrossReferences.insert(&SourceInstrKV->second);
+  }
+
+  return CFCrossReferences;
+}
+
 const std::set<uint64_t> &FileAnalysis::getIndirectInstructions() const {
   return IndirectInstructions;
 }
 
 const MCRegisterInfo *FileAnalysis::getRegisterInfo() const {
   return RegisterInfo.get();
 }
 

tools/llvm-cfi-verify/unittests/FileAnalysis.cpp

 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/raw_ostream.h"
 
 #include <cstdlib>
 
 using Instr = ::llvm::cfi_verify::FileAnalysis::Instr;
 using ::testing::Eq;
+using ::testing::Field;
 
 namespace llvm {
 namespace cfi_verify {
 namespace {
 class ELFx86TestFileAnalysis : public FileAnalysis {
 public:
   ELFx86TestFileAnalysis()
       : FileAnalysis(Triple("x86_64--"), SubtargetFeatures()) {}
TEST_F(BasicFileAnalysisTest, PrevAndNextFromBadInst) {
   EXPECT_EQ(1u, GoodInstrMeta->InstructionSize);
 
   GoodInstrMeta = Verifier.getNextInstructionSequential(BadInstrMeta);
   EXPECT_NE(nullptr, GoodInstrMeta);
   EXPECT_EQ(0xDEADBEEF + 2, GoodInstrMeta->VMAddress);
   EXPECT_EQ(1u, GoodInstrMeta->InstructionSize);
 }
 
+TEST_F(BasicFileAnalysisTest, CFITrapTest) {
+  Verifier.parseSectionContents(
+      {
+          0x90,                   // 0: nop
+          0xb0, 0x00,             // 1: mov $0x0, %al
+          0x48, 0x89, 0xe5,       // 3: mov %rsp, %rbp
+          0x48, 0x83, 0xec, 0x18, // 6: sub $0x18, %rsp
+          0x48, 0xbe, 0xc4, 0x07, 0x40,
+          0x00, 0x00, 0x00, 0x00, 0x00, // 10: movabs $0x4007c4, %rsi
+          0x2f,                         // 20: (bad)
+          0x41, 0x0e,                   // 21: rex.B (bad)
+          0x62, 0x72, 0x65, 0x61, 0x6b, // 23: (bad) {%k1}
+          0x0f, 0x0b                    // 28: ud2
+      },
+      0xDEADBEEF);
+
+  EXPECT_FALSE(Verifier.isCFITrap(
+      Verifier.getInstructionOrDie(0xDEADBEEF)));
+  EXPECT_FALSE(Verifier.isCFITrap(
+      Verifier.getInstructionOrDie(0xDEADBEEF + 3)));
+  EXPECT_FALSE(Verifier.isCFITrap(
+      Verifier.getInstructionOrDie(0xDEADBEEF + 6)));
+  EXPECT_FALSE(Verifier.isCFITrap(
+      Verifier.getInstructionOrDie(0xDEADBEEF + 10)));
+  EXPECT_FALSE(Verifier.isCFITrap(
+      Verifier.getInstructionOrDie(0xDEADBEEF + 20)));
+  EXPECT_FALSE(Verifier.isCFITrap(
+      Verifier.getInstructionOrDie(0xDEADBEEF + 21)));
+  EXPECT_FALSE(Verifier.isCFITrap(
+      Verifier.getInstructionOrDie(0xDEADBEEF + 23)));
+  EXPECT_TRUE(Verifier.isCFITrap(
+      Verifier.getInstructionOrDie(0xDEADBEEF + 28)));
+}
+
+TEST_F(BasicFileAnalysisTest, FallThroughTest) {
+  Verifier.parseSectionContents(
+      {
+          0x90,                         // 0: nop
+          0xb0, 0x00,                   // 1: mov $0x0, %al
+          0x2f,                         // 3: (bad)
+          0x0f, 0x0b,                   // 4: ud2
+          0xff, 0x20,                   // 6: jmpq *(%rax)
+          0xeb, 0x00,                   // 8: jmp +0
+          0xe8, 0x45, 0xfe, 0xff, 0xff, // 10: callq [some loc]
+          0xff, 0x10,                   // 15: callq *(rax)
+          0x75, 0x00,                   // 17: jne +0
+          0xc3,                         // 19: retq
+      },
+      0xDEADBEEF);
+
+  EXPECT_TRUE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF)));
+  EXPECT_TRUE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 1)));
+  EXPECT_FALSE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 3)));
+  EXPECT_FALSE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 4)));
+  EXPECT_FALSE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 6)));
+  EXPECT_FALSE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 8)));
+  EXPECT_FALSE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 10)));
+  EXPECT_FALSE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 15)));
+  EXPECT_TRUE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 17)));
+  EXPECT_FALSE(
+      Verifier.canFallThrough(Verifier.getInstructionOrDie(0xDEADBEEF + 19)));
+}
+
+TEST_F(BasicFileAnalysisTest, DefiniteNextInstructionTest) {
+  Verifier.parseSectionContents(
+      {
+          0x90,                         // 0: nop
+          0xb0, 0x00,                   // 1: mov $0x0, %al
+          0x2f,                         // 3: (bad)
+          0x0f, 0x0b,                   // 4: ud2
+          0xff, 0x20,                   // 6: jmpq *(%rax)
+          0xeb, 0x00,                   // 8: jmp 10 [+0]
+          0xeb, 0x05,                   // 10: jmp 17 [+5]
+          0xe8, 0x00, 0x00, 0x00, 0x00, // 12: callq 17 [+0]
+          0xe8, 0x78, 0x56, 0x34, 0x12, // 17: callq 0x1234569f [+0x12345678]
+          0xe8, 0x04, 0x00, 0x00, 0x00, // 22: callq 31 [+4]
+          0xff, 0x10,                   // 27: callq *(rax)
+          0x75, 0x00,                   // 29: jne 31 [+0]
+          0x75, 0xe0,                   // 31: jne 1 [-32]
+          0xc3,                         // 33: retq
+          0xeb, 0xdd,                   // 34: jmp 1 [-35]
+          0xeb, 0xdd,                   // 36: jmp 3 [-35]
+          0xeb, 0xdc,                   // 38: jmp 4 [-36]
+      },
+      0xDEADBEEF);
+
+  const auto *Current = Verifier.getInstruction(0xDEADBEEF);
+  const auto *Next = Verifier.getDefiniteNextInstruction(*Current);
+  EXPECT_NE(nullptr, Next);
+  EXPECT_EQ(0xDEADBEEF + 1, Next->VMAddress);
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 1);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 3);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 4);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 6);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 8);
+  Next = Verifier.getDefiniteNextInstruction(*Current);
+  EXPECT_NE(nullptr, Next);
+  EXPECT_EQ(0xDEADBEEF + 10, Next->VMAddress);
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 10);
+  Next = Verifier.getDefiniteNextInstruction(*Current);
+  EXPECT_NE(nullptr, Next);
+  EXPECT_EQ(0xDEADBEEF + 17, Next->VMAddress);
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 12);
+  Next = Verifier.getDefiniteNextInstruction(*Current);
+  EXPECT_NE(nullptr, Next);
+  EXPECT_EQ(0xDEADBEEF + 17, Next->VMAddress);
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 17);
+  // Note, definite next instruction address is out of range and should fail.
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+  Next = Verifier.getDefiniteNextInstruction(*Current);
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 22);
+  Next = Verifier.getDefiniteNextInstruction(*Current);
+  EXPECT_NE(nullptr, Next);
+  EXPECT_EQ(0xDEADBEEF + 31, Next->VMAddress);
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 27);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+  Current = Verifier.getInstruction(0xDEADBEEF + 29);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+  Current = Verifier.getInstruction(0xDEADBEEF + 31);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+  Current = Verifier.getInstruction(0xDEADBEEF + 33);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 34);
+  Next = Verifier.getDefiniteNextInstruction(*Current);
+  EXPECT_NE(nullptr, Next);
+  EXPECT_EQ(0xDEADBEEF + 1, Next->VMAddress);
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 36);
+  EXPECT_EQ(nullptr, Verifier.getDefiniteNextInstruction(*Current));
+
+  Current = Verifier.getInstruction(0xDEADBEEF + 38);
+  Next = Verifier.getDefiniteNextInstruction(*Current);
+  EXPECT_NE(nullptr, Next);
+  EXPECT_EQ(0xDEADBEEF + 4, Next->VMAddress);
+}
+
+TEST_F(BasicFileAnalysisTest, ControlFlowXRefsTest) {
+  Verifier.parseSectionContents(
+      {
+          0x90,                         // 0: nop
+          0xb0, 0x00,                   // 1: mov $0x0, %al
+          0x2f,                         // 3: (bad)
+          0x0f, 0x0b,                   // 4: ud2
+          0xff, 0x20,                   // 6: jmpq *(%rax)
+          0xeb, 0x00,                   // 8: jmp 10 [+0]
+          0xeb, 0x05,                   // 10: jmp 17 [+5]
+          0xe8, 0x00, 0x00, 0x00, 0x00, // 12: callq 17 [+0]
+          0xe8, 0x78, 0x56, 0x34, 0x12, // 17: callq 0x1234569f [+0x12345678]
+          0xe8, 0x04, 0x00, 0x00, 0x00, // 22: callq 31 [+4]
+          0xff, 0x10,                   // 27: callq *(rax)
+          0x75, 0x00,                   // 29: jne 31 [+0]
+          0x75, 0xe0,                   // 31: jne 1 [-32]
+          0xc3,                         // 33: retq
+          0xeb, 0xdd,                   // 34: jmp 1 [-35]
+          0xeb, 0xdd,                   // 36: jmp 3 [-35]
+          0xeb, 0xdc,                   // 38: jmp 4 [-36]
+      },
+      0xDEADBEEF);
+  const auto *InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF);
+  std::set<const Instr *> XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(XRefs.empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 1);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_THAT(XRefs, UnorderedElementsAre(
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF)),
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 31)),
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 34))));
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 3);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_THAT(XRefs, UnorderedElementsAre(
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 1)),
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 36))));
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 4);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_THAT(XRefs, UnorderedElementsAre(
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 38))));
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 6);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 8);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 10);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_THAT(XRefs, UnorderedElementsAre(
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 8))));
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 12);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 17);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_THAT(XRefs, UnorderedElementsAre(
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 10)),
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 12))));
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 22);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 27);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 29);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 31);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_THAT(XRefs, UnorderedElementsAre(
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 22)),
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 29))));
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 33);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_THAT(XRefs, UnorderedElementsAre(
+                         Field(&Instr::VMAddress, Eq(0xDEADBEEF + 31))));
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 34);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 36);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+
+  InstrMetaPtr = &Verifier.getInstructionOrDie(0xDEADBEEF + 38);
+  XRefs = Verifier.getControlFlowXRefs(*InstrMetaPtr);
+  EXPECT_TRUE(Verifier.getControlFlowXRefs(*InstrMetaPtr).empty());
+}
+
+
 } // anonymous namespace
 } // end namespace cfi_verify
 } // end namespace llvm
 
 int main(int argc, char **argv) {
   ::testing::InitGoogleTest(&argc, argv);
   llvm::cl::ParseCommandLineOptions(argc, argv);
 
   llvm::InitializeAllTargetInfos();
   llvm::InitializeAllTargetMCs();
   llvm::InitializeAllAsmParsers();
   llvm::InitializeAllDisassemblers();
 
   return RUN_ALL_TESTS();
 }