[clang] Do not instrument relative vtables under hwasan

Full context in
https://bugs.fuchsia.dev/p/fuchsia/issues/detail?id=107017.

Instrumenting hwasan with globals results in a linker error under the
relative vtables abi:

```
ld.lld: error:
libunwind.cpp:(.rodata..L_ZTVN9libunwind12UnwindCursorINS_17LocalAddressSpaceENS_15Registers_arm64EEE.hwasan+0x8):
relocation R_AARCH64_PLT32 out of range: 6845471433603167792 is not in
[-2147483648, 2147483647]; references
libunwind::AbstractUnwindCursor::~AbstractUnwindCursor()
>>> defined in
libunwind/src/CMakeFiles/unwind_shared.dir/libunwind.cpp.obj
```

This is because the tag is included in the vtable address when
calculating the offset between the vtable and virtual function. A
temporary solution until we can resolve this is to just disable hwasan
instrumentation on relative vtables specifically, which can be done in
the frontend.

Differential Revision: https://reviews.llvm.org/D132425

1 files changed, 23 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
index cdd40d2..c8b29ae 100644
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -921,12 +921,33 @@ llvm::GlobalVariable *CodeGenVTables::GenerateConstructionVTable(
 
   CGM.EmitVTableTypeMetadata(RD, VTable, *VTLayout.get());
 
-  if (UsingRelativeLayout && !VTable->isDSOLocal())
-    GenerateRelativeVTableAlias(VTable, OutName);
+  if (UsingRelativeLayout) {
+    RemoveHwasanMetadata(VTable);
+    if (!VTable->isDSOLocal())
+      GenerateRelativeVTableAlias(VTable, OutName);
+  }
 
   return VTable;
 }
 
+// Ensure this vtable is not instrumented by hwasan. That is, a global alias is
+// not generated for it. This is mainly used by the relative-vtables ABI where
+// vtables instead contain 32-bit offsets between the vtable and function
+// pointers. Hwasan is disabled for these vtables for now because the tag in a
+// vtable pointer may fail the overflow check when resolving 32-bit PLT
+// relocations. A future alternative for this would be finding which usages of
+// the vtable can continue to use the untagged hwasan value without any loss of
+// value in hwasan.
+void CodeGenVTables::RemoveHwasanMetadata(llvm::GlobalValue *VTable) {
+  if (CGM.getLangOpts().Sanitize.has(SanitizerKind::HWAddress)) {
+    llvm::GlobalValue::SanitizerMetadata Meta;
+    if (VTable->hasSanitizerMetadata())
+      Meta = VTable->getSanitizerMetadata();
+    Meta.NoHWAddress = true;
+    VTable->setSanitizerMetadata(Meta);
+  }
+}
+
 // If the VTable is not dso_local, then we will not be able to indicate that
 // the VTable does not need a relocation and move into rodata. A frequent
 // time this can occur is for classes that should be made public from a DSO