[libcxx-commits] [libcxx] [libcxx] Use alias for detecting overriden function (PR #114961)
Louis Dionne via libcxx-commits
libcxx-commits at lists.llvm.org
Wed Nov 6 05:46:03 PST 2024
================
@@ -29,106 +29,91 @@
// 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
// _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on
-// 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.
+// 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.
//
// 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 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. Functions defined
+// with this macro must be defined at global scope.
//
#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;
+
+template <class _Func>
+_LIBCPP_HIDE_FROM_ABI constexpr _Func* __overload_of(_Func* f) {
+ return f;
}
-_LIBCPP_END_NAMESPACE_STD
-// The NVPTX linker cannot create '__start/__stop' sections.
-#elif defined(_LIBCPP_OBJECT_FORMAT_ELF) && !defined(__NVPTX__)
+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__("__lcxx_override")))
+_LIBCPP_END_NAMESPACE_STD
-// 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;
+# 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
----------------
ldionne wrote:
Can the `_impl` symbol be made static? After all it doesn't need to be externally visible, right? Same goes for the ELF version.
https://github.com/llvm/llvm-project/pull/114961
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