[clang] Do not instrument the rtti_proxies under hwasan

[leonardchan]

Summary

We run into a duplicate symbol error when instrumenting the rtti_proxies generated as part of the relative vtables ABI with hwasan:

ld.lld: error: duplicate symbol: typeinfo for icu_71::UObject (.rtti_proxy)
>>> defined at brkiter.cpp
>>>            arm64-hwasan-shared/obj/third_party/icu/source/common/libicuuc.brkiter.cpp.o:(typeinfo for icu_71::UObject (.rtti_proxy))
>>> defined at locavailable.cpp
>>>            arm64-hwasan-shared/obj/third_party/icu/source/common/libicuuc.locavailable.cpp.o:(.data.rel.ro..L_ZTIN6icu_717UObjectE.rtti_proxy.hwasan+0xE00000000000000)

The issue here is that the hwasan alias carries over the visibility and linkage of the original proxy, so we have duplicate external symbols that participate in linking. Similar to D132425 we can just disable hwasan for the proxies for now.

clang/lib/CodeGen/CGVTables.h

public:
 
   /// Generate a public facing alias for the vtable and make the vtable either
   /// hidden or private. The alias will have the original linkage and visibility
   /// of the vtable. This is used for cases under the relative vtables ABI
   /// when a vtable may not be dso_local.
   void GenerateRelativeVTableAlias(llvm::GlobalVariable *VTable,
                                    llvm::StringRef AliasNameRef);
 
-  /// Specify a vtable should not be instrumented with hwasan.
-  void RemoveHwasanMetadata(llvm::GlobalValue *VTable);
+  /// Specify a global should not be instrumented with hwasan.
+  void RemoveHwasanMetadata(llvm::GlobalValue *GV) const;
 };
 
 } // end namespace CodeGen
 } // end namespace clang
 #endif

clang/lib/CodeGen/CGVTables.cpp

if (!proxy) {
                                        /*isConstant=*/true, stubLinkage,
                                        globalVal, rttiProxyName);
       proxy->setDSOLocal(true);
       proxy->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
       if (!proxy->hasLocalLinkage()) {
         proxy->setVisibility(llvm::GlobalValue::HiddenVisibility);
         proxy->setComdat(module.getOrInsertComdat(rttiProxyName));
       }
+      // Do not instrument the rtti proxies with hwasan to avoid a duplicate
+      // symbol error. Aliases generated by hwasan will retain the same namebut
+      // the addresses they are set to may have different tags from different
+      // compilation units. We don't run into this without hwasan because the
+      // proxies are in comdat groups, but those aren't propagated to the alias.
+      RemoveHwasanMetadata(proxy);
     }
     target = proxy;
   }
 
   builder.addRelativeOffsetToPosition(CGM.Int32Ty, target,
                                       /*position=*/vtableAddressPoint);
 }
 

 // 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) {
+void CodeGenVTables::RemoveHwasanMetadata(llvm::GlobalValue *GV) const {
   if (CGM.getLangOpts().Sanitize.has(SanitizerKind::HWAddress)) {
     llvm::GlobalValue::SanitizerMetadata Meta;
-    if (VTable->hasSanitizerMetadata())
-      Meta = VTable->getSanitizerMetadata();
+    if (GV->hasSanitizerMetadata())
+      Meta = GV->getSanitizerMetadata();
     Meta.NoHWAddress = true;
-    VTable->setSanitizerMetadata(Meta);
+    GV->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
 // (like in libc++). For cases like these, we can make the vtable hidden or
 // private and create a public alias with the same visibility and linkage as

clang/test/CodeGenCXX/RelativeVTablesABI/relative-vtables-hwasan.cpp

 // RUN: %clang_cc1 %s -triple=aarch64-unknown-fuchsia -S -o - -emit-llvm -fsanitize=hwaddress | FileCheck %s
 
 /// The usual vtable will have default visibility. In this case, the actual
 /// vtable is hidden and the alias is made public. With hwasan enabled, we want
 /// to ensure the local one self-referenced in the hidden vtable is not
 /// hwasan-instrumented.
 // CHECK-DAG: @_ZTV1A.local = private unnamed_addr constant { [3 x i32] } { [3 x i32] [i32 0, i32 trunc (i64 sub (i64 ptrtoint (ptr @_ZTI1A.rtti_proxy to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1A.local, i32 0, i32 0, i32 2) to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @_ZN1A3fooEv to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1A.local, i32 0, i32 0, i32 2) to i64)) to i32)] }, no_sanitize_hwaddress, align 4
 // CHECK-DAG: @_ZTV1A = unnamed_addr alias { [3 x i32] }, ptr @_ZTV1A.local
+// CHECK-DAG: @_ZTI1A.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1A, no_sanitize_hwaddress, comdat
 
 class A {
 public:
   virtual void foo();
 };
 
 void A::foo() {}
 
 void A_foo(A *a) {
   a->foo();
 }
 
 /// If the vtable happens to be hidden, then the alias is not needed. In this
 /// case, the original vtable struct itself should be unsanitized.
 // CHECK-DAG: @_ZTV1B = hidden unnamed_addr constant { [3 x i32] } { [3 x i32] [i32 0, i32 trunc (i64 sub (i64 ptrtoint (ptr @_ZTI1B.rtti_proxy to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1B, i32 0, i32 0, i32 2) to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @_ZN1B3fooEv to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1B, i32 0, i32 0, i32 2) to i64)) to i32)] }, no_sanitize_hwaddress, align 4
+// CHECK-DAG: @_ZTI1B.rtti_proxy = hidden unnamed_addr constant ptr @_ZTI1B, no_sanitize_hwaddress, comdat
 
 class __attribute__((visibility("hidden"))) B {
 public:
   virtual void foo();
 };
 
 void B::foo() {}
 
 void B_foo(B *b) {
   b->foo();
 }