1 // -*- C++ -*- 2 //===----------------------------------------------------------------------===// 3 // 4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 5 // See https://llvm.org/LICENSE.txt for license information. 6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 7 // 8 //===----------------------------------------------------------------------===// 9 10 #ifndef _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H 11 #define _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H 12 13 #include <__config> 14 #include <cstdint> 15 16 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 17 # pragma GCC system_header 18 #endif 19 20 // 21 // This file provides the std::__is_function_overridden utility, which allows checking 22 // whether an overridable function (typically a weak symbol) like `operator new` 23 // has been overridden by a user or not. 24 // 25 // This is a low-level utility which does not work on all platforms, since it needs 26 // to make assumptions about the object file format in use. Furthermore, it requires 27 // the "base definition" of the function (the one we want to check whether it has been 28 // overridden) to be annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro. 29 // 30 // This currently works with Mach-O files (used on Darwin) and with ELF files (used on Linux 31 // and others). On platforms where we know how to implement this detection, the macro 32 // _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on 33 // other platforms. The _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro is defined to 34 // nothing on unsupported platforms so that it can be used to decorate functions regardless 35 // of whether detection is actually supported. 36 // 37 // How does this work? 38 // ------------------- 39 // 40 // Let's say we want to check whether a weak function `f` has been overridden by the user. 41 // The general mechanism works by placing `f`'s definition (in the libc++ built library) 42 // inside a special section, which we do using the `__section__` attribute via the 43 // _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro. 44 // 45 // Then, when comes the time to check whether the function has been overridden, we take 46 // the address of the function and we check whether it falls inside the special function 47 // we created. This can be done by finding pointers to the start and the end of the section 48 // (which is done differently for ELF and Mach-O), and then checking whether `f` falls 49 // within those bounds. If it falls within those bounds, then `f` is still inside the 50 // special section and so it is the version we defined in the libc++ built library, i.e. 51 // it was not overridden. Otherwise, it was overridden by the user because it falls 52 // outside of the section. 53 // 54 // Important note 55 // -------------- 56 // 57 // This mechanism should never be used outside of the libc++ built library. In particular, 58 // attempting to use this within the libc++ headers will not work at all because we don't 59 // want to be defining special sections inside user's executables which use our headers. 60 // This is provided inside libc++'s include tree solely to make it easier to share with 61 // libc++abi, which needs the same mechanism. 62 // 63 64 #if defined(_LIBCPP_OBJECT_FORMAT_MACHO) 65 66 # define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1 67 # define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE \ 68 __attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions"))) 69 70 _LIBCPP_BEGIN_NAMESPACE_STD 71 template <class _Ret, class... _Args> 72 _LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept { 73 // Declare two dummy bytes and give them these special `__asm` values. These values are 74 // defined by the linker, which means that referring to `&__lcxx_override_start` will 75 // effectively refer to the address where the section starts (and same for the end). 76 extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override"); 77 extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override"); 78 79 // Now get a uintptr_t out of these locations, and out of the function pointer. 80 uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start); 81 uintptr_t __end = reinterpret_cast<uintptr_t>(&__lcxx_override_end); 82 uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr); 83 84 // Finally, the function was overridden if it falls outside of the section's bounds. 85 return __ptr < __start || __ptr > __end; 86 } 87 _LIBCPP_END_NAMESPACE_STD 88 89 #elif defined(_LIBCPP_OBJECT_FORMAT_ELF) 90 91 # define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1 92 # define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override"))) 93 94 // This is very similar to what we do for Mach-O above. The ELF linker will implicitly define 95 // variables with those names corresponding to the start and the end of the section. 96 // 97 // See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section 98 extern char __start___lcxx_override; 99 extern char __stop___lcxx_override; 100 101 _LIBCPP_BEGIN_NAMESPACE_STD 102 template <class _Ret, class... _Args> 103 _LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept { 104 uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override); 105 uintptr_t __end = reinterpret_cast<uintptr_t>(&__stop___lcxx_override); 106 uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr); 107 108 return __ptr < __start || __ptr > __end; 109 } 110 _LIBCPP_END_NAMESPACE_STD 111 112 #else 113 114 # define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0 115 # define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */ 116 117 #endif 118 119 #endif // _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H 120