[libcxx-commits] [libcxx] [libc++] Replace the of use custom sections for detecting overriden functions (PR #208330)

[Nikolas Klauser via libcxx-commits]

================
@@ -63,64 +51,53 @@
 // want to be defining special sections inside user's executables which use our headers.
 //
 
-#if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
+#if defined(_LIBCPP_OBJECT_FORMAT_MACHO) || (defined(_LIBCPP_OBJECT_FORMAT_ELF) && !defined(__NVPTX__))
 
 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
-#  define OVERRIDABLE_FUNCTION [[gnu::weak, gnu::section("__TEXT,__lcxx_override,regular,pure_instructions")]]
-
-_LIBCPP_BEGIN_NAMESPACE_STD template <typename T, T* _Func>
-_LIBCPP_HIDE_FROM_ABI inline bool __is_function_overridden() noexcept {
-  // Declare two dummy bytes and give them these special `__asm` values. These values are
-  // defined by the linker, which means that referring to `&__lcxx_override_start` will
-  // effectively refer to the address where the section starts (and same for the end).
-  extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override");
-  extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override");
-
-  // Now get a uintptr_t out of these locations, and out of the function pointer.
-  uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start);
-  uintptr_t __end   = reinterpret_cast<uintptr_t>(&__lcxx_override_end);
-  uintptr_t __ptr   = reinterpret_cast<uintptr_t>(_Func);
-
-#  if __has_feature(ptrauth_calls)
-  // We must pass a void* to ptrauth_strip since it only accepts a pointer type. Also, in particular,
-  // we must NOT pass a function pointer, otherwise we will strip the function pointer, and then attempt
-  // to authenticate and re-sign it when casting it to a uintptr_t again, which will fail because we just
-  // stripped the function pointer. See rdar://122927845.
-  __ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer));
-#  endif
+#  define OVERRIDABLE_FUNCTION [[gnu::weak]]
 
-  // Finally, the function was overridden if it falls outside of the section's bounds.
-  return __ptr < __start || __ptr > __end;
-}
-_LIBCPP_END_NAMESPACE_STD
+_LIBCPP_BEGIN_NAMESPACE_STD
 
-// The NVPTX linker cannot create '__start/__stop' sections.
-#elif defined(_LIBCPP_OBJECT_FORMAT_ELF) && !defined(__NVPTX__)
+namespace {
 
-#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
-#  define OVERRIDABLE_FUNCTION [[gnu::weak, gnu::section("__lcxx_override")]]
+template <typename T>
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI T* __libcpp_launder(T* __ptr) noexcept {
+  __asm__ volatile("" : "+r"(__ptr));
+  return __ptr;
+}
----------------
philnik777 wrote:

I think I'd be happy to make that explicit by naming it e.g. `launder_function_pointer`.

https://github.com/llvm/llvm-project/pull/208330