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>
__is_function_overridden(_Ret (* __fptr)(_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