[libcxx] Use alias for detecting overriden function

This mechanism is preferable in environments like embedded since it
doesn't require special handling of the custom section.

Author: petrhosek

libcxx/src/include/overridable_function.h

@@ -29,7 +29,7 @@
 // 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 annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
+// overridden) to be defined using the _LIBCPP_OVERRIDABLE_FUNCTION 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
@@ -42,93 +42,74 @@
 // -------------------
 //
 // Let's say we want to check whether a weak function `f` has been overridden by the user.
-// 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.
+// The general mechanism works by defining a symbol `f_impl__` and a weak alias `f` via the
+// _LIBCPP_OVERRIDABLE_FUNCTION macro.
 //
 // Then, when comes the time to check whether the function has been overridden, we take
-// 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.
+// 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.
 //
 // Important note
 // --------------
 //
-// 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.
+// This mechanism should never be used outside of the libc++ built library.
 //
 
 #if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
 
-#  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
 
-// The NVPTX linker cannot create '__start/__stop' sections.
-#elif defined(_LIBCPP_OBJECT_FORMAT_ELF) && !defined(__NVPTX__)
+template <typename _Func>
+constexpr _Func* __overload_of(_Func* f) { return f; }
 
-#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
-#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override")))
+template <auto _Func>
+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_END_NAMESPACE_STD
+
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
+#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist) \
+  extern "C" type symbol##_impl__ arglist; \
+  __asm__(".globl _" _LIBCPP_TOSTRING(symbol)); \
+  __asm__(".set _" _LIBCPP_TOSTRING(symbol) ", _" _LIBCPP_TOSTRING(symbol##_impl__)); \
+  extern __typeof(symbol##_impl__) name __attribute__((weak_import)); \
+  _LIBCPP_BEGIN_NAMESPACE_STD \
+  template <> \
+  constexpr bool __is_function_overridden<__overload_of<type arglist>(name)>() { \
+    return __overload_of<type arglist>(name) != symbol##_impl__; \
+  } \
+  _LIBCPP_END_NAMESPACE_STD \
+  type symbol##_impl__ arglist
+
+#elif defined(_LIBCPP_OBJECT_FORMAT_ELF)
 
 _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;
-}
+
+template <typename _Func>
+constexpr _Func* __overload_of(_Func* f) { return f; }
+
+template <auto _Func>
+constexpr bool __is_function_overridden();
+
 _LIBCPP_END_NAMESPACE_STD
 
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
+#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist) \
+  extern "C" type symbol##_impl__ arglist; \
+  [[gnu::weak, gnu::alias(_LIBCPP_TOSTRING(symbol##_impl__))]] type name arglist; \
+  _LIBCPP_BEGIN_NAMESPACE_STD \
+  template <> \
+  constexpr bool __is_function_overridden<__overload_of<type arglist>(name)>() { \
+    return __overload_of<type arglist>(name) != symbol##_impl__; \
+  } \
+  _LIBCPP_END_NAMESPACE_STD \
+  type symbol##_impl__ arglist
+
 #else
 
 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0
-#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */
+#  define _LIBCPP_OVERRIDABLE_FUNCTION(symbol, type, name, arglist) \
+  _LIBCPP_WEAK type name arglist
 
 #endif
 

libcxx/src/new.cpp

@@ -43,7 +43,7 @@ static void* operator_new_impl(std::size_t size) {
   return p;
 }
 
-_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new(std::size_t size) _THROW_BAD_ALLOC {
+_LIBCPP_OVERRIDABLE_FUNCTION(_Znmw, 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 {
 #  ifdef _LIBCPP_HAS_NO_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<std::__overload_of<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_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new[](size_t size) _THROW_BAD_ALLOC {
+_LIBCPP_OVERRIDABLE_FUNCTION(_Znam, 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 {
 #  ifdef _LIBCPP_HAS_NO_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<std::__overload_of<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 +136,9 @@ static void* operator_new_aligned_impl(std::size_t size, std::align_val_t alignm
   return p;
 }
 
-_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK 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 {
+_LIBCPP_OVERRIDABLE_FUNCTION(_ZnwmSt11align_val_t, 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 +149,7 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
 #    ifdef _LIBCPP_HAS_NO_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<std::__overload_of<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 "
@@ -168,16 +169,15 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
 #    endif
 }
 
-_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
-operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
+_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_WEAK void* operator new[](size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
 #    ifdef _LIBCPP_HAS_NO_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<std::__overload_of<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 "