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129 lines
6 KiB
C++
129 lines
6 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H
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#define _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H
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#include <__config>
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#include <cstdint>
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#if defined(__arm64e__) && __has_feature(ptrauth_calls)
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# include <ptrauth.h>
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#endif
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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#endif
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//
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// This file provides the std::__is_function_overridden utility, which allows checking
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// whether an overridable function (typically a weak symbol) like `operator new`
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// has been overridden by a user or not.
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//
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// This is a low-level utility which does not work on all platforms, since it needs
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// to make assumptions about the object file format in use. Furthermore, it requires
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// the "base definition" of the function (the one we want to check whether it has been
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// overridden) to be annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
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//
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// This currently works with Mach-O files (used on Darwin) and with ELF files (used on Linux
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// and others). On platforms where we know how to implement this detection, the macro
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// _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on
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// other platforms. The _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro is defined to
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// nothing on unsupported platforms so that it can be used to decorate functions regardless
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// of whether detection is actually supported.
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//
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// How does this work?
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// -------------------
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//
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// Let's say we want to check whether a weak function `f` has been overridden by the user.
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// The general mechanism works by placing `f`'s definition (in the libc++ built library)
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// inside a special section, which we do using the `__section__` attribute via the
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// _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
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//
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// Then, when comes the time to check whether the function has been overridden, we take
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// the address of the function and we check whether it falls inside the special function
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// we created. This can be done by finding pointers to the start and the end of the section
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// (which is done differently for ELF and Mach-O), and then checking whether `f` falls
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// within those bounds. If it falls within those bounds, then `f` is still inside the
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// special section and so it is the version we defined in the libc++ built library, i.e.
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// it was not overridden. Otherwise, it was overridden by the user because it falls
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// outside of the section.
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//
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// Important note
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// --------------
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//
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// This mechanism should never be used outside of the libc++ built library. In particular,
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// attempting to use this within the libc++ headers will not work at all because we don't
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// want to be defining special sections inside user's executables which use our headers.
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//
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#if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
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# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
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# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE \
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__attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions")))
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_LIBCPP_BEGIN_NAMESPACE_STD
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template <class _Ret, class... _Args>
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_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
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// Declare two dummy bytes and give them these special `__asm` values. These values are
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// defined by the linker, which means that referring to `&__lcxx_override_start` will
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// effectively refer to the address where the section starts (and same for the end).
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extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override");
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extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override");
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// Now get a uintptr_t out of these locations, and out of the function pointer.
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uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start);
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uintptr_t __end = reinterpret_cast<uintptr_t>(&__lcxx_override_end);
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uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr);
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#if defined(__arm64e__) && __has_feature(ptrauth_calls)
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// We must pass a void* to ptrauth_strip since it only accepts a pointer type. Also, in particular,
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// we must NOT pass a function pointer, otherwise we will strip the function pointer, and then attempt
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// to authenticate and re-sign it when casting it to a uintptr_t again, which will fail because we just
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// stripped the function pointer. See rdar://122927845.
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__ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer));
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#endif
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// Finally, the function was overridden if it falls outside of the section's bounds.
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return __ptr < __start || __ptr > __end;
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}
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_LIBCPP_END_NAMESPACE_STD
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#elif defined(_LIBCPP_OBJECT_FORMAT_ELF)
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# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
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# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override")))
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// This is very similar to what we do for Mach-O above. The ELF linker will implicitly define
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// variables with those names corresponding to the start and the end of the section.
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//
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// See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section
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extern char __start___lcxx_override;
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extern char __stop___lcxx_override;
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_LIBCPP_BEGIN_NAMESPACE_STD
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template <class _Ret, class... _Args>
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_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
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uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override);
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uintptr_t __end = reinterpret_cast<uintptr_t>(&__stop___lcxx_override);
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uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr);
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return __ptr < __start || __ptr > __end;
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}
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_LIBCPP_END_NAMESPACE_STD
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#else
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# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0
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# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */
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#endif
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#endif // _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H
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