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
71template <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
98extern char __start___lcxx_override;
99extern char __stop___lcxx_override;
100
101_LIBCPP_BEGIN_NAMESPACE_STD
102template <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

source code of libcxx/src/include/overridable_function.h