| Issue |
101716
|
| Summary |
[PAC][clang] Fix address discrimination for type_info vtable pointers
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
kovdan01
|
In #99726, `-fptrauth-type-info-vtable-pointer-discrimination` was introduced, which is intended to enable type and address discrimination for type_info vtable pointers.
However, some codegen logic for actually enabling address discrimination is missing. Particularly, in `ItaniumRTTIBuilder::BuildVTablePointer` (clang/lib/CodeGen/ItaniumCXXABI.cpp), there is the following piece of code:
```
if (auto &Schema = CGM.getCodeGenOpts().PointerAuth.CXXTypeInfoVTablePointer)
VTable = CGM.getConstantSignedPointer(VTable, Schema, nullptr, GlobalDecl(),
QualType(Ty, 0));
```
Here, `nullptr` is used as `StorageAddress` unconditionally, so, address discrimination is not actually enabled even if requested. It caused test-suite failures in several EH-related tests.
I was able to fix that locally by just using a dummy `ptr inttoptr (i64 1 to ptr)` value as `StorageAddress` (just like I did with init/fini, see https://github.com/llvm/llvm-project/pull/96478#issuecomment-2196819332), and tests became passing. I'm not sure how to get a proper `StorageAddress` here, so I've used that dummy placeholder which actually seems to do the job.
An existing test clang/test/CodeGenCXX/ptrauth-type-info-vtable.cpp shows such incorrect behavior. Particularly, in line 55, we have
```
ptr ptrauth (ptr getelementptr inbounds (ptr, ptr @_ZTVN10__cxxabiv117__class_type_infoE, i64 2), i32 2, i64 [[STDTYPEINFO_DISC]])
```
This has constant discriminator, but does not have address discrimination, while it should be enabled with `-fptrauth-type-info-vtable-pointer-discrimination`. The correct output should be smth like (if we use a placeholder value `ptr inttoptr (i64 1 to ptr)` as storage address)
```
ptr ptrauth (ptr getelementptr inbounds (ptr, ptr @_ZTVN10__cxxabiv117__class_type_infoE, i64 2), i32 2, i64 [[STDTYPEINFO_DISC]], ptr inttoptr (i64 1 to ptr))
```
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