[AArch64][PAC] Move emission of LR checks in tail calls to AsmPrinter

Move the emission of the checks performed on the authenticated LR value
during tail calls to AArch64AsmPrinter class, so that different checker
sequences can be reused by pseudo instructions expanded there.
This adds one more option to AuthCheckMethod enumeration, the generic
XPAC variant which is not restricted to checking the LR register.

Author: atrosinenko

llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp

@@ -153,6 +153,7 @@ class AArch64AsmPrinter : public AsmPrinter {
   void emitPtrauthCheckAuthenticatedValue(Register TestedReg,
                                           Register ScratchReg,
                                           AArch64PACKey::ID Key,
+                                          AArch64PAuth::AuthCheckMethod Method,
                                           bool ShouldTrap,
                                           const MCSymbol *OnFailure);
 
@@ -1731,7 +1732,8 @@ unsigned AArch64AsmPrinter::emitPtrauthDiscriminator(uint16_t Disc,
 /// of proceeding to the next instruction (only if ShouldTrap is false).
 void AArch64AsmPrinter::emitPtrauthCheckAuthenticatedValue(
     Register TestedReg, Register ScratchReg, AArch64PACKey::ID Key,
-    bool ShouldTrap, const MCSymbol *OnFailure) {
+    AArch64PAuth::AuthCheckMethod Method, bool ShouldTrap,
+    const MCSymbol *OnFailure) {
   // Insert a sequence to check if authentication of TestedReg succeeded,
   // such as:
   //
@@ -1757,38 +1759,70 @@ void AArch64AsmPrinter::emitPtrauthCheckAuthenticatedValue(
   //    Lsuccess:
   //      ...
   //
-  // This sequence is expensive, but we need more information to be able to
-  // do better.
-  //
-  // We can't TBZ the poison bit because EnhancedPAC2 XORs the PAC bits
-  // on failure.
-  // We can't TST the PAC bits because we don't always know how the address
-  // space is setup for the target environment (and the bottom PAC bit is
-  // based on that).
-  // Either way, we also don't always know whether TBI is enabled or not for
-  // the specific target environment.
+  // See the documentation on AuthCheckMethod enumeration constants for
+  // the specific code sequences that can be used to perform the check.
+  using AArch64PAuth::AuthCheckMethod;
 
-  unsigned XPACOpc = getXPACOpcodeForKey(Key);
+  if (Method == AuthCheckMethod::None)
+    return;
+  if (Method == AuthCheckMethod::DummyLoad) {
+    EmitToStreamer(MCInstBuilder(AArch64::LDRWui)
+                       .addReg(getWRegFromXReg(ScratchReg))
+                       .addReg(TestedReg)
+                       .addImm(0));
+    assert(ShouldTrap && !OnFailure && "DummyLoad always traps on error");
+    return;
+  }
 
   MCSymbol *SuccessSym = createTempSymbol("auth_success_");
+  if (Method == AuthCheckMethod::XPAC || Method == AuthCheckMethod::XPACHint) {
+    //  mov Xscratch, Xtested
+    emitMovXReg(ScratchReg, TestedReg);
 
-  //  mov Xscratch, Xtested
-  emitMovXReg(ScratchReg, TestedReg);
-
-  //  xpac(i|d) Xscratch
-  EmitToStreamer(MCInstBuilder(XPACOpc).addReg(ScratchReg).addReg(ScratchReg));
+    if (Method == AuthCheckMethod::XPAC) {
+      //  xpac(i|d) Xscratch
+      unsigned XPACOpc = getXPACOpcodeForKey(Key);
+      EmitToStreamer(
+          MCInstBuilder(XPACOpc).addReg(ScratchReg).addReg(ScratchReg));
+    } else {
+      //  xpaclri
+
+      // Note that this method applies XPAC to TestedReg instead of ScratchReg.
+      assert(TestedReg == AArch64::LR &&
+             "XPACHint mode is only compatible with checking the LR register");
+      assert((Key == AArch64PACKey::IA || Key == AArch64PACKey::IB) &&
+             "XPACHint mode is only compatible with I-keys");
+      EmitToStreamer(MCInstBuilder(AArch64::XPACLRI));
+    }
 
-  //  cmp Xtested, Xscratch
-  EmitToStreamer(MCInstBuilder(AArch64::SUBSXrs)
-                     .addReg(AArch64::XZR)
-                     .addReg(TestedReg)
-                     .addReg(ScratchReg)
-                     .addImm(0));
+    //  cmp Xtested, Xscratch
+    EmitToStreamer(MCInstBuilder(AArch64::SUBSXrs)
+                       .addReg(AArch64::XZR)
+                       .addReg(TestedReg)
+                       .addReg(ScratchReg)
+                       .addImm(0));
 
-  //  b.eq Lsuccess
-  EmitToStreamer(MCInstBuilder(AArch64::Bcc)
-                     .addImm(AArch64CC::EQ)
-                     .addExpr(MCSymbolRefExpr::create(SuccessSym, OutContext)));
+    //  b.eq Lsuccess
+    EmitToStreamer(
+        MCInstBuilder(AArch64::Bcc)
+            .addImm(AArch64CC::EQ)
+            .addExpr(MCSymbolRefExpr::create(SuccessSym, OutContext)));
+  } else if (Method == AuthCheckMethod::HighBitsNoTBI) {
+    //  eor Xscratch, Xtested, Xtested, lsl #1
+    EmitToStreamer(MCInstBuilder(AArch64::EORXrs)
+                       .addReg(ScratchReg)
+                       .addReg(TestedReg)
+                       .addReg(TestedReg)
+                       .addImm(1));
+    //  tbz Xscratch, #62, Lsuccess
+    EmitToStreamer(
+        MCInstBuilder(AArch64::TBZX)
+            .addReg(ScratchReg)
+            .addImm(62)
+            .addExpr(MCSymbolRefExpr::create(SuccessSym, OutContext)));
+  } else {
+    llvm_unreachable("Unsupported check method");
+  }
 
   if (ShouldTrap) {
     assert(!OnFailure && "Cannot specify OnFailure with ShouldTrap");
@@ -1802,9 +1836,26 @@ void AArch64AsmPrinter::emitPtrauthCheckAuthenticatedValue(
     // Note that this can introduce an authentication oracle (such as based on
     // the high bits of the re-signed value).
 
-    // FIXME: Can we simply return the AUT result, already in TestedReg?
-    //  mov Xtested, Xscratch
-    emitMovXReg(TestedReg, ScratchReg);
+    // FIXME: The XPAC method can be optimized by applying XPAC to TestedReg
+    //        instead of ScratchReg, thus eliminating one `mov` instruction.
+    //        Both XPAC and XPACHint can be further optimized by not using a
+    //        conditional branch jumping over an unconditional one.
+
+    switch (Method) {
+    case AuthCheckMethod::XPACHint:
+      // LR is already XPAC-ed at this point.
+      break;
+    case AuthCheckMethod::XPAC:
+      //  mov Xtested, Xscratch
+      emitMovXReg(TestedReg, ScratchReg);
+      break;
+    default:
+      // If Xtested was not XPAC-ed so far, emit XPAC here.
+      //  xpac(i|d) Xtested
+      unsigned XPACOpc = getXPACOpcodeForKey(Key);
+      EmitToStreamer(
+          MCInstBuilder(XPACOpc).addReg(TestedReg).addReg(TestedReg));
+    }
 
     if (OnFailure) {
       //  b Lend
@@ -1830,7 +1881,7 @@ void AArch64AsmPrinter::emitPtrauthAuthResign(const MachineInstr *MI) {
   //      ; sign x16 (if AUTPAC)
   //    Lend:   ; if not trapping on failure
   //
-  // with the checking sequence chosen depending on whether we should check
+  // with the checking sequence chosen depending on whether/how we should check
   // the pointer and whether we should trap on failure.
 
   // By default, auth/resign sequences check for auth failures.
@@ -1890,6 +1941,7 @@ void AArch64AsmPrinter::emitPtrauthAuthResign(const MachineInstr *MI) {
       EndSym = createTempSymbol("resign_end_");
 
     emitPtrauthCheckAuthenticatedValue(AArch64::X16, AArch64::X17, AUTKey,
+                                       AArch64PAuth::AuthCheckMethod::XPAC,
                                        ShouldTrap, EndSym);
   }
 
@@ -2260,11 +2312,34 @@ void AArch64AsmPrinter::emitInstruction(const MachineInstr *MI) {
     OutStreamer->emitLabel(LOHLabel);
   }
 
+  // With Pointer Authentication, it may be needed to explicitly check the
+  // authenticated value in LR when performing a tail call.
+  // Otherwise, the callee may re-sign the invalid return address,
+  // introducing a signing oracle.
+  auto CheckLRInTailCall = [this](Register CallDestinationReg) {
+    if (!AArch64FI->shouldSignReturnAddress(*MF))
+      return;
+
+    auto LRCheckMethod = STI->getAuthenticatedLRCheckMethod(*MF);
+    if (LRCheckMethod == AArch64PAuth::AuthCheckMethod::None)
+      return;
+
+    Register ScratchReg =
+        CallDestinationReg == AArch64::X16 ? AArch64::X17 : AArch64::X16;
+    AArch64PACKey::ID Key =
+        AArch64FI->shouldSignWithBKey() ? AArch64PACKey::IB : AArch64PACKey::IA;
+    emitPtrauthCheckAuthenticatedValue(
+        AArch64::LR, ScratchReg, Key, LRCheckMethod,
+        /*ShouldTrap=*/true, /*OnFailure=*/nullptr);
+  };
+
   AArch64TargetStreamer *TS =
     static_cast<AArch64TargetStreamer *>(OutStreamer->getTargetStreamer());
   // Do any manual lowerings.
   switch (MI->getOpcode()) {
   default:
+    assert(!AArch64InstrInfo::isTailCallReturnInst(*MI) &&
+           "Unhandled tail call instruction");
     break;
   case AArch64::HINT: {
     // CurrentPatchableFunctionEntrySym can be CurrentFnBegin only for
@@ -2404,6 +2479,8 @@ void AArch64AsmPrinter::emitInstruction(const MachineInstr *MI) {
                               ? AArch64::X17
                               : AArch64::X16;
 
+    CheckLRInTailCall(MI->getOperand(0).getReg());
+
     unsigned DiscReg = AddrDisc;
     if (Disc) {
       if (AddrDisc != AArch64::NoRegister) {
@@ -2434,13 +2511,17 @@ void AArch64AsmPrinter::emitInstruction(const MachineInstr *MI) {
   case AArch64::TCRETURNrix17:
   case AArch64::TCRETURNrinotx16:
   case AArch64::TCRETURNriALL: {
+    CheckLRInTailCall(MI->getOperand(0).getReg());
+
     MCInst TmpInst;
     TmpInst.setOpcode(AArch64::BR);
     TmpInst.addOperand(MCOperand::createReg(MI->getOperand(0).getReg()));
     EmitToStreamer(*OutStreamer, TmpInst);
     return;
   }
   case AArch64::TCRETURNdi: {
+    CheckLRInTailCall(AArch64::NoRegister);
+
     MCOperand Dest;
     MCInstLowering.lowerOperand(MI->getOperand(0), Dest);
     MCInst TmpInst;

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -107,6 +107,19 @@ unsigned AArch64InstrInfo::getInstSizeInBytes(const MachineInstr &MI) const {
   unsigned NumBytes = 0;
   const MCInstrDesc &Desc = MI.getDesc();
 
+  if (!MI.isBundle() && isTailCallReturnInst(MI)) {
+    NumBytes = Desc.getSize() ? Desc.getSize() : 4;
+
+    const auto *MFI = MF->getInfo<AArch64FunctionInfo>();
+    if (!MFI->shouldSignReturnAddress(MF))
+      return NumBytes;
+
+    auto &STI = MF->getSubtarget<AArch64Subtarget>();
+    auto Method = STI.getAuthenticatedLRCheckMethod(*MF);
+    NumBytes += AArch64PAuth::getCheckerSizeInBytes(Method);
+    return NumBytes;
+  }
+
   // Size should be preferably set in
   // llvm/lib/Target/AArch64/AArch64InstrInfo.td (default case).
   // Specific cases handle instructions of variable sizes

llvm/lib/Target/AArch64/AArch64InstrInfo.td

@@ -1952,6 +1952,8 @@ let Predicates = [HasPAuth] in {
   }
 
   // Size 16: 4 fixed + 8 variable, to compute discriminator.
+  // The size returned by getInstSizeInBytes() is incremented according
+  // to the variant of LR check.
   let isCall = 1, isTerminator = 1, isReturn = 1, isBarrier = 1, Size = 16,
       Uses = [SP] in {
     def AUTH_TCRETURN