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| 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(__arm64e__) && __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 | +// This is provided inside libc++'s include tree solely to make it easier to share with |
| 65 | +// libc++abi, which needs the same mechanism. |
| 66 | +// |
| 67 | + |
| 68 | +#if defined(_LIBCPP_OBJECT_FORMAT_MACHO) |
| 69 | + |
| 70 | +# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1 |
| 71 | +# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE \ |
| 72 | + __attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions"))) |
| 73 | + |
| 74 | +_LIBCPP_BEGIN_NAMESPACE_STD |
| 75 | +template <class _Ret, class... _Args> |
| 76 | +_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept { |
| 77 | + // Declare two dummy bytes and give them these special `__asm` values. These values are |
| 78 | + // defined by the linker, which means that referring to `&__lcxx_override_start` will |
| 79 | + // effectively refer to the address where the section starts (and same for the end). |
| 80 | + extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override"); |
| 81 | + extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override"); |
| 82 | + |
| 83 | + // Now get a uintptr_t out of these locations, and out of the function pointer. |
| 84 | + uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start); |
| 85 | + uintptr_t __end = reinterpret_cast<uintptr_t>(&__lcxx_override_end); |
| 86 | + uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr); |
| 87 | + |
| 88 | +#if defined(__arm64e__) && __has_feature(ptrauth_calls) |
| 89 | + // We must pass a void* to ptrauth_strip since it only accepts a pointer type. Also, in particular, |
| 90 | + // we must NOT pass a function pointer, otherwise we will strip the function pointer, and then attempt |
| 91 | + // to authenticate and re-sign it when casting it to a uintptr_t again, which will fail because we just |
| 92 | + // stripped the function pointer. See rdar://122927845. |
| 93 | + __ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer)); |
| 94 | +#endif |
| 95 | + |
| 96 | + // Finally, the function was overridden if it falls outside of the section's bounds. |
| 97 | + return __ptr < __start || __ptr > __end; |
| 98 | +} |
| 99 | +_LIBCPP_END_NAMESPACE_STD |
| 100 | + |
| 101 | +#elif defined(_LIBCPP_OBJECT_FORMAT_ELF) |
| 102 | + |
| 103 | +# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1 |
| 104 | +# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override"))) |
| 105 | + |
| 106 | +// This is very similar to what we do for Mach-O above. The ELF linker will implicitly define |
| 107 | +// variables with those names corresponding to the start and the end of the section. |
| 108 | +// |
| 109 | +// See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section |
| 110 | +extern char __start___lcxx_override; |
| 111 | +extern char __stop___lcxx_override; |
| 112 | + |
| 113 | +_LIBCPP_BEGIN_NAMESPACE_STD |
| 114 | +template <class _Ret, class... _Args> |
| 115 | +_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept { |
| 116 | + uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override); |
| 117 | + uintptr_t __end = reinterpret_cast<uintptr_t>(&__stop___lcxx_override); |
| 118 | + uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr); |
| 119 | + |
| 120 | + return __ptr < __start || __ptr > __end; |
| 121 | +} |
| 122 | +_LIBCPP_END_NAMESPACE_STD |
| 123 | + |
| 124 | +#else |
| 125 | + |
| 126 | +# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0 |
| 127 | +# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */ |
| 128 | + |
| 129 | +#endif |
| 130 | + |
| 131 | +#endif // _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H |
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