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 | |