Revert "[libcxx] Use alias for detecting overriden function" (#124431)

petrhosek · web-flow · commit 4167ea2cb082 · 2025-01-27T22:26:15.000-08:00
Reverts #120805 This change while desirable has two issues we discovered: - It is incompatible with `-funique-internal-linkage-names`, see #120805 (comment) - It is incompatible with `-fvisibility-global-new-delete=force-hidden`, see #123224 (comment) We were hoping to address both of these issues with #122983, but that change has other issues we haven't yet managed to resolve. For now, we have decided to revert the change to avoid shipping a broken feature in LLVM 20, and we plan to follow up with a new approach post branch.
diff --git a/libcxx/src/include/overridable_function.h b/libcxx/src/include/overridable_function.h
@@ -29,81 +29,106 @@
 // This is a low-level utility which does not work on all platforms, since it needs
 // to make assumptions about the object file format in use. Furthermore, it requires
 // the "base definition" of the function (the one we want to check whether it has been
-// overridden) to be defined using the _LIBCPP_OVERRIDABLE_FUNCTION macro.
+// overridden) to be annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
 //
 // This currently works with Mach-O files (used on Darwin) and with ELF files (used on Linux
 // and others). On platforms where we know how to implement this detection, the macro
 // _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on
-// other platforms. The _LIBCPP_OVERRIDABLE_FUNCTION macro expands to regular function
-// definition on unsupported platforms so that it can be used to decorate functions
-// regardless of whether detection is actually supported.
+// other platforms. The _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro is defined to
+// nothing on unsupported platforms so that it can be used to decorate functions regardless
+// of whether detection is actually supported.
 //
 // How does this work?
 // -------------------
 //
 // Let's say we want to check whether a weak function `f` has been overridden by the user.
-// The general mechanism works by defining a symbol `f_impl__` and a weak alias `f` via the
-// _LIBCPP_OVERRIDABLE_FUNCTION macro.
+// The general mechanism works by placing `f`'s definition (in the libc++ built library)
+// inside a special section, which we do using the `__section__` attribute via the
+// _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
 //
 // Then, when comes the time to check whether the function has been overridden, we take
-// the address of the function `f` and we check whether it is different from `f_impl__`.
-// If so it means the function was overriden by the user.
+// the address of the function and we check whether it falls inside the special function
+// we created. This can be done by finding pointers to the start and the end of the section
+// (which is done differently for ELF and Mach-O), and then checking whether `f` falls
+// within those bounds. If it falls within those bounds, then `f` is still inside the
+// special section and so it is the version we defined in the libc++ built library, i.e.
+// it was not overridden. Otherwise, it was overridden by the user because it falls
+// outside of the section.
 //
 // Important note
 // --------------
 //
-// This mechanism should never be used outside of the libc++ built library. Functions defined
-// with this macro must be defined at global scope.
+// This mechanism should never be used outside of the libc++ built library. In particular,
+// attempting to use this within the libc++ headers will not work at all because we don't
+// want to be defining special sections inside user's executables which use our headers.
 //
 
 #if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
 
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-template <auto _Func>
-_LIBCPP_HIDE_FROM_ABI constexpr bool __is_function_overridden();
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
+#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE                                                                 \
+    __attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions")))
 
+_LIBCPP_BEGIN_NAMESPACE_STD
+template <class _Ret, class... _Args>
+_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) 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>(__fptr);
+
+#  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
+
+  // Finally, the function was overridden if it falls outside of the section's bounds.
+  return __ptr < __start || __ptr > __end;
+}
 _LIBCPP_END_NAMESPACE_STD
 
-#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
-#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist)                                                    \
-    static __attribute__((used)) type symbol##_impl__ arglist __asm__("_" _LIBCPP_TOSTRING(symbol));                   \
-    __asm__(".globl _" _LIBCPP_TOSTRING(symbol));                                                                      \
-    __asm__(".weak_definition _" _LIBCPP_TOSTRING(symbol));                                                            \
-    extern __typeof(symbol##_impl__) name __attribute__((weak_import));                                                \
-    _LIBCPP_BEGIN_NAMESPACE_STD                                                                                        \
-    template <>                                                                                                        \
-    inline bool __is_function_overridden<static_cast<type(*) arglist>(name)>() {                                       \
-      return static_cast<type(*) arglist>(name) != symbol##_impl__;                                                    \
-    }                                                                                                                  \
-    _LIBCPP_END_NAMESPACE_STD                                                                                          \
-    static type symbol##_impl__ arglist
-
-#elif defined(_LIBCPP_OBJECT_FORMAT_ELF)
+// The NVPTX linker cannot create '__start/__stop' sections.
+#elif defined(_LIBCPP_OBJECT_FORMAT_ELF) && !defined(__NVPTX__)
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
+#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override")))
 
-template <auto _Func>
-_LIBCPP_HIDE_FROM_ABI constexpr bool __is_function_overridden();
+// This is very similar to what we do for Mach-O above. The ELF linker will implicitly define
+// variables with those names corresponding to the start and the end of the section.
+//
+// See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section
+extern char __start___lcxx_override;
+extern char __stop___lcxx_override;
 
+_LIBCPP_BEGIN_NAMESPACE_STD
+template <class _Ret, class... _Args>
+_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
+  uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override);
+  uintptr_t __end   = reinterpret_cast<uintptr_t>(&__stop___lcxx_override);
+  uintptr_t __ptr   = reinterpret_cast<uintptr_t>(__fptr);
+
+#  if __has_feature(ptrauth_calls)
+  // We must pass a void* to ptrauth_strip since it only accepts a pointer type. See full explanation above.
+  __ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer));
+#  endif
+
+  return __ptr < __start || __ptr > __end;
+}
 _LIBCPP_END_NAMESPACE_STD
 
-#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
-#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist)                                                    \
-    static type symbol##_impl__ arglist __asm__(_LIBCPP_TOSTRING(symbol##_impl__));                                    \
-    [[gnu::weak, gnu::alias(_LIBCPP_TOSTRING(symbol##_impl__))]] type name arglist;                                    \
-    _LIBCPP_BEGIN_NAMESPACE_STD                                                                                        \
-    template <>                                                                                                        \
-    inline bool __is_function_overridden<static_cast<type(*) arglist>(name)>() {                                       \
-      return static_cast<type(*) arglist>(name) != symbol##_impl__;                                                    \
-    }                                                                                                                  \
-    _LIBCPP_END_NAMESPACE_STD                                                                                          \
-    static type symbol##_impl__ arglist
-
 #else
 
 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0
-#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist) _LIBCPP_WEAK type name arglist
+#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */
 
 #endif
 
diff --git a/libcxx/src/new.cpp b/libcxx/src/new.cpp
@@ -43,7 +43,7 @@ static void* operator_new_impl(std::size_t size) {
   return p;
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(_Znwm, void*, operator new, (std::size_t size)) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new(std::size_t size) _THROW_BAD_ALLOC {
   void* p = operator_new_impl(size);
   if (p == nullptr)
     __throw_bad_alloc_shim();
@@ -54,7 +54,7 @@ _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
 #  if !_LIBCPP_HAS_EXCEPTIONS
 #    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden<static_cast<void* (*)(std::size_t)>(&operator new)>(),
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new)),
       "libc++ was configured with exceptions disabled and `operator new(size_t)` has been overridden, "
       "but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case "
@@ -74,15 +74,15 @@ _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
 #  endif
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(_Znam, void*, operator new[], (size_t size)) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new[](size_t size) _THROW_BAD_ALLOC {
   return ::operator new(size);
 }
 
 _LIBCPP_WEAK void* operator new[](size_t size, const std::nothrow_t&) noexcept {
 #  if !_LIBCPP_HAS_EXCEPTIONS
 #    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden<static_cast<void* (*)(std::size_t)>(&operator new[])>(),
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new[])),
       "libc++ was configured with exceptions disabled and `operator new[](size_t)` has been overridden, "
       "but `operator new[](size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, nothrow_t)` must call `operator new[](size_t)`, which will terminate in case "
@@ -136,8 +136,8 @@ static void* operator_new_aligned_impl(std::size_t size, std::align_val_t alignm
   return p;
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(_ZnwmSt11align_val_t, void*, operator new, (std::size_t size, std::align_val_t alignment))
-_THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
+operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
   void* p = operator_new_aligned_impl(size, alignment);
   if (p == nullptr)
     __throw_bad_alloc_shim();
@@ -148,7 +148,7 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
 #    if !_LIBCPP_HAS_EXCEPTIONS
 #      if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden<static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new)>(),
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new)),
       "libc++ was configured with exceptions disabled and `operator new(size_t, align_val_t)` has been overridden, "
       "but `operator new(size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, align_val_t, nothrow_t)` must call `operator new(size_t, align_val_t)`, which will "
@@ -168,14 +168,16 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
 #    endif
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(_ZnamSt11align_val_t, void*, operator new[], (size_t size, std::align_val_t alignment))
-_THROW_BAD_ALLOC { return ::operator new(size, alignment); }
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
+operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
+  return ::operator new(size, alignment);
+}
 
 _LIBCPP_WEAK void* operator new[](size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
 #    if !_LIBCPP_HAS_EXCEPTIONS
 #      if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden<static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new[])>(),
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new[])),
       "libc++ was configured with exceptions disabled and `operator new[](size_t, align_val_t)` has been overridden, "
       "but `operator new[](size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, align_val_t, nothrow_t)` must call `operator new[](size_t, align_val_t)`, which will "
diff --git a/libcxxabi/src/stdlib_new_delete.cpp b/libcxxabi/src/stdlib_new_delete.cpp
@@ -63,7 +63,7 @@ static void* operator_new_impl(std::size_t size) {
   return p;
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(_Znwm, void*, operator new, (std::size_t size)) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new(std::size_t size) _THROW_BAD_ALLOC {
   void* p = operator_new_impl(size);
   if (p == nullptr)
     __throw_bad_alloc_shim();
@@ -74,7 +74,7 @@ _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
 #if !_LIBCPP_HAS_EXCEPTIONS
 #  if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden<static_cast<void* (*)(std::size_t)>(&operator new)>(),
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new)),
       "libc++ was configured with exceptions disabled and `operator new(size_t)` has been overridden, "
       "but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case "
@@ -94,15 +94,15 @@ _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
 #endif
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(_Znam, void*, operator new[], (size_t size)) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new[](size_t size) _THROW_BAD_ALLOC {
   return ::operator new(size);
 }
 
 _LIBCPP_WEAK void* operator new[](size_t size, const std::nothrow_t&) noexcept {
 #if !_LIBCPP_HAS_EXCEPTIONS
 #  if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden<static_cast<void* (*)(std::size_t)>(&operator new[])>(),
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new[])),
       "libc++ was configured with exceptions disabled and `operator new[](size_t)` has been overridden, "
       "but `operator new[](size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, nothrow_t)` must call `operator new[](size_t)`, which will terminate in case "
@@ -156,8 +156,8 @@ static void* operator_new_aligned_impl(std::size_t size, std::align_val_t alignm
   return p;
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(_ZnwmSt11align_val_t, void*, operator new, (std::size_t size, std::align_val_t alignment))
-_THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
+operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
   void* p = operator_new_aligned_impl(size, alignment);
   if (p == nullptr)
     __throw_bad_alloc_shim();
@@ -168,7 +168,7 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
 #  if !_LIBCPP_HAS_EXCEPTIONS
 #    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden<static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new)>(),
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new)),
       "libc++ was configured with exceptions disabled and `operator new(size_t, align_val_t)` has been overridden, "
       "but `operator new(size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, align_val_t, nothrow_t)` must call `operator new(size_t, align_val_t)`, which will "
@@ -188,14 +188,16 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
 #  endif
 }
 
-_LIBCPP_OVERRIDABLE_FUNCTION(_ZnamSt11align_val_t, void*, operator new[], (size_t size, std::align_val_t alignment))
-_THROW_BAD_ALLOC { return ::operator new(size, alignment); }
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
+operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
+  return ::operator new(size, alignment);
+}
 
 _LIBCPP_WEAK void* operator new[](size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
 #  if !_LIBCPP_HAS_EXCEPTIONS
 #    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
   _LIBCPP_ASSERT_SHIM(
-      !std::__is_function_overridden<static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new[])>(),
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new[])),
       "libc++ was configured with exceptions disabled and `operator new[](size_t, align_val_t)` has been overridden, "
       "but `operator new[](size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, align_val_t, nothrow_t)` must call `operator new[](size_t, align_val_t)`, which will "
