Revert "[libcxx] Use alias for detecting overriden function" (#124431) Reverts llvm/llvm-project#120805 This change while desirable has two issues we discovered: - It is incompatible with `-funique-internal-linkage-names`, see https://github.com/llvm/llvm-project/pull/120805#discussion_r1913709817 - It is incompatible with `-fvisibility-global-new-delete=force-hidden`, see https://github.com/llvm/llvm-project/issues/123224#issuecomment-2607963878 We were hoping to address both of these issues with https://github.com/llvm/llvm-project/pull/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. GitOrigin-RevId: 4167ea2cb082a2acb00b8b1dc09aa780dc0e3110
diff --git a/src/include/overridable_function.h b/src/include/overridable_function.h index 7372e34..6c70f62 100644 --- a/src/include/overridable_function.h +++ b/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(); - -_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) +# 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"); -template <auto _Func> -_LIBCPP_HIDE_FROM_ABI constexpr bool __is_function_overridden(); + // 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__) + # 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 +# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override"))) + +// 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 #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/src/new.cpp b/src/new.cpp index b14b522..e010fe4 100644 --- a/src/new.cpp +++ b/src/new.cpp
@@ -43,7 +43,7 @@ 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 @@ # 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 " @@ -74,7 +74,7 @@ # 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); } @@ -82,7 +82,7 @@ # 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 @@ 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 @@ # 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 @@ # 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 "