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 __has_feature(ptrauth_calls) 17 # include <ptrauth.h> 18 #endif 19 20 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 21 # pragma GCC system_header 22 #endif 23 24 // 25 // This file provides the std::__is_function_overridden utility, which allows checking 26 // whether an overridable function (typically a weak symbol) like `operator new` 27 // has been overridden by a user or not. 28 // 29 // This is a low-level utility which does not work on all platforms, since it needs 30 // to make assumptions about the object file format in use. Furthermore, it requires 31 // the "base definition" of the function (the one we want to check whether it has been 32 // overridden) to be annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro. 33 // 34 // This currently works with Mach-O files (used on Darwin) and with ELF files (used on Linux 35 // and others). On platforms where we know how to implement this detection, the macro 36 // _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on 37 // other platforms. The _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro is defined to 38 // nothing on unsupported platforms so that it can be used to decorate functions regardless 39 // of whether detection is actually supported. 40 // 41 // How does this work? 42 // ------------------- 43 // 44 // Let's say we want to check whether a weak function `f` has been overridden by the user. 45 // The general mechanism works by placing `f`'s definition (in the libc++ built library) 46 // inside a special section, which we do using the `__section__` attribute via the 47 // _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro. 48 // 49 // Then, when comes the time to check whether the function has been overridden, we take 50 // the address of the function and we check whether it falls inside the special function 51 // we created. This can be done by finding pointers to the start and the end of the section 52 // (which is done differently for ELF and Mach-O), and then checking whether `f` falls 53 // within those bounds. If it falls within those bounds, then `f` is still inside the 54 // special section and so it is the version we defined in the libc++ built library, i.e. 55 // it was not overridden. Otherwise, it was overridden by the user because it falls 56 // outside of the section. 57 // 58 // Important note 59 // -------------- 60 // 61 // This mechanism should never be used outside of the libc++ built library. In particular, 62 // attempting to use this within the libc++ headers will not work at all because we don't 63 // want to be defining special sections inside user's executables which use our headers. 64 // 65 66 #if defined(_LIBCPP_OBJECT_FORMAT_MACHO) 67 68 # define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1 69 # define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE \ 70 __attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions"))) 71 72 _LIBCPP_BEGIN_NAMESPACE_STD 73 template <class _Ret, class... _Args> 74 _LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept { 75 // Declare two dummy bytes and give them these special `__asm` values. These values are 76 // defined by the linker, which means that referring to `&__lcxx_override_start` will 77 // effectively refer to the address where the section starts (and same for the end). 78 extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override"); 79 extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override"); 80 81 // Now get a uintptr_t out of these locations, and out of the function pointer. 82 uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start); 83 uintptr_t __end = reinterpret_cast<uintptr_t>(&__lcxx_override_end); 84 uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr); 85 86 # if __has_feature(ptrauth_calls) 87 // We must pass a void* to ptrauth_strip since it only accepts a pointer type. Also, in particular, 88 // we must NOT pass a function pointer, otherwise we will strip the function pointer, and then attempt 89 // to authenticate and re-sign it when casting it to a uintptr_t again, which will fail because we just 90 // stripped the function pointer. See rdar://122927845. 91 __ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer)); 92 # endif 93 94 // Finally, the function was overridden if it falls outside of the section's bounds. 95 return __ptr < __start || __ptr > __end; 96 } 97 _LIBCPP_END_NAMESPACE_STD 98 99 #elif defined(_LIBCPP_OBJECT_FORMAT_ELF) 100 101 # define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1 102 # define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override"))) 103 104 // This is very similar to what we do for Mach-O above. The ELF linker will implicitly define 105 // variables with those names corresponding to the start and the end of the section. 106 // 107 // See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section 108 extern char __start___lcxx_override; 109 extern char __stop___lcxx_override; 110 111 _LIBCPP_BEGIN_NAMESPACE_STD 112 template <class _Ret, class... _Args> 113 _LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept { 114 uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override); 115 uintptr_t __end = reinterpret_cast<uintptr_t>(&__stop___lcxx_override); 116 uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr); 117 118 return __ptr < __start || __ptr > __end; 119 } 120 _LIBCPP_END_NAMESPACE_STD 121 122 #else 123 124 # define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0 125 # define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */ 126 127 #endif 128 129 #endif // _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H 130