[clang][CGObjC] Sign the v-table pointer in ObjC exception RTTI.

[ojhunt]

If clang is configured to apply pointer authentication to type_info's vtable pointer we
ensure that the selected schema is applied to the RTTI objects generated for objc++.

[llvmbot]

@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Oliver Hunt (ojhunt)

Changes

If clang is configured to apply pointer authentication to type_info's vtable pointer we
ensure that the selected schema is applied to the RTTI objects generated for objc++.

---

Full diff: https://github.com/llvm/llvm-project/pull/135562.diff

5 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/include/clang/CodeGen/ConstantInitBuilder.h (+3)
• (modified) clang/lib/CodeGen/CGObjCMac.cpp (+18-2)
• (modified) clang/lib/CodeGen/ConstantInitBuilder.cpp (+12)
• (added) clang/test/CodeGenObjC/ptrauth-attr-exception.m (+16)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 11f62bc881b03..67fef2fa0c37f 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -191,6 +191,7 @@ Non-comprehensive list of changes in this release
 - Support parsing the `cc` operand modifier and alias it to the `c` modifier (#GH127719).
 - Added `__builtin_elementwise_exp10`.
 - For AMDPGU targets, added `__builtin_v_cvt_off_f32_i4` that maps to the `v_cvt_off_f32_i4` instruction.
+- Support authenticated ``type_info`` vtable pointers in Objective-C++
 
 New Compiler Flags
 ------------------
diff --git a/clang/include/clang/CodeGen/ConstantInitBuilder.h b/clang/include/clang/CodeGen/ConstantInitBuilder.h
index 28d4764b6d60b..a7424abbc07eb 100644
--- a/clang/include/clang/CodeGen/ConstantInitBuilder.h
+++ b/clang/include/clang/CodeGen/ConstantInitBuilder.h
@@ -206,6 +206,9 @@ class ConstantAggregateBuilderBase {
   void addSignedPointer(llvm::Constant *Pointer,
                         const PointerAuthSchema &Schema, GlobalDecl CalleeDecl,
                         QualType CalleeType);
+  void addSignedPointer(llvm::Constant *Pointer, unsigned Key,
+                        bool UseAddressDiscrimination,
+                        llvm::ConstantInt *OtherDiscriminator);
 
   /// Add a null pointer of a specific type.
   void addNullPointer(llvm::PointerType *ptrTy) {
diff --git a/clang/lib/CodeGen/CGObjCMac.cpp b/clang/lib/CodeGen/CGObjCMac.cpp
index 1f11347b81411..e854f6b5920c8 100644
--- a/clang/lib/CodeGen/CGObjCMac.cpp
+++ b/clang/lib/CodeGen/CGObjCMac.cpp
@@ -7676,10 +7676,26 @@ CGObjCNonFragileABIMac::GetInterfaceEHType(const ObjCInterfaceDecl *ID,
   }
 
   llvm::Value *VTableIdx = llvm::ConstantInt::get(CGM.Int32Ty, 2);
+  llvm::Constant *VTablePtr = llvm::ConstantExpr::getInBoundsGetElementPtr(
+      VTableGV->getValueType(), VTableGV, VTableIdx);
+
   ConstantInitBuilder builder(CGM);
   auto values = builder.beginStruct(ObjCTypes.EHTypeTy);
-  values.add(llvm::ConstantExpr::getInBoundsGetElementPtr(
-      VTableGV->getValueType(), VTableGV, VTableIdx));
+
+  if (auto &Schema =
+          CGM.getCodeGenOpts().PointerAuth.CXXTypeInfoVTablePointer) {
+    uint32_t discrimination = 0;
+    if (Schema.hasOtherDiscrimination()) {
+      assert(Schema.getOtherDiscrimination() ==
+             PointerAuthSchema::Discrimination::Constant);
+      discrimination = Schema.getConstantDiscrimination();
+    }
+    values.addSignedPointer(
+        VTablePtr, Schema.getKey(), Schema.isAddressDiscriminated(),
+        llvm::ConstantInt::get(CGM.IntPtrTy, discrimination));
+  } else {
+    values.add(VTablePtr);
+  }
   values.add(GetClassName(ClassName));
   values.add(GetClassGlobal(ID, /*metaclass*/ false, NotForDefinition));
 
diff --git a/clang/lib/CodeGen/ConstantInitBuilder.cpp b/clang/lib/CodeGen/ConstantInitBuilder.cpp
index ce1fe137c1919..c5b1ad3a07235 100644
--- a/clang/lib/CodeGen/ConstantInitBuilder.cpp
+++ b/clang/lib/CodeGen/ConstantInitBuilder.cpp
@@ -314,3 +314,15 @@ void ConstantAggregateBuilderBase::addSignedPointer(
       Pointer, Schema, StorageAddress, CalleeDecl, CalleeType);
   add(SignedPointer);
 }
+
+void ConstantAggregateBuilderBase::addSignedPointer(
+    llvm::Constant *Pointer, unsigned Key, bool UseAddressDiscrimination,
+    llvm::ConstantInt *OtherDiscriminator) {
+  llvm::Constant *StorageAddress = nullptr;
+  if (UseAddressDiscrimination)
+    StorageAddress = getAddrOfCurrentPosition(Pointer->getType());
+
+  llvm::Constant *SignedPointer = Builder.CGM.getConstantSignedPointer(
+      Pointer, Key, StorageAddress, OtherDiscriminator);
+  add(SignedPointer);
+}
diff --git a/clang/test/CodeGenObjC/ptrauth-attr-exception.m b/clang/test/CodeGenObjC/ptrauth-attr-exception.m
new file mode 100644
index 0000000000000..1a17828b927da
--- /dev/null
+++ b/clang/test/CodeGenObjC/ptrauth-attr-exception.m
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -triple arm64-apple-ios -fptrauth-calls -emit-llvm -fexceptions -fobjc-exceptions -o - %s | FileCheck %s
+
+__attribute__((objc_root_class))
+@interface Root {
+  Class isa;
+}
+@end
+
+__attribute__((objc_exception))
+@interface A : Root
+@end
+
+@implementation A
+@end
+
+// CHECK: @"OBJC_EHTYPE_$_A" = global {{%.*}} { ptr ptrauth (ptr getelementptr inbounds (ptr, ptr @objc_ehtype_vtable, i32 2), i32 2),

[ojhunt]

@cor3ntin ping :D

[rjmccall]

I think you can just call the addSignedPointer variant that takes a schema and pass null values for the decl and type.

[ojhunt]

50/50 this code predates us having that function \o/

[ojhunt]

Ah, we can't use the general schema version as it requires the type we're constructing a field for, but we construct the type_info vtable discriminator manually from constants because we don't necessarily actually have the actually type_info type available.

[rjmccall]

I understand that we can't use the generic v-table pointer schema. But CXXTypeInfoVTablePointer has a hard-coded constant in it, which should be fine to use without a decl or type, and I'd like to avoid having an extra place that destructures a PointerAuthSchema the way this code is doing.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[rjmccall]

LGTM, but I've fixed the release note: this is about the Objective-C exceptions ABI, not a general type_info thing for Objective-C++.