src/include/overridable_function.h - llvm-project/libcxx - Git at Google

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//

 #ifndef _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H
 #define _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H

 #include <__config>
 #include <cstdint>

 #if defined(__arm64e__) && __has_feature(ptrauth_calls)
 #  include <ptrauth.h>
 #endif

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif

 //
 // This file provides the std::__is_function_overridden utility, which allows checking
 // whether an overridable function (typically a weak symbol) like `operator new`
 // has been overridden by a user or not.
 //
 // This is a low-level utility which does not work on all platforms, since it needs
 // to make assumptions about the object file format in use. Furthermore, it requires
 // the "base definition" of the function (the one we want to check whether it has been
 // overridden) to be annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
 //
 // This currently works with Mach-O files (used on Darwin) and with ELF files (used on Linux
 // and others). On platforms where we know how to implement this detection, the macro
 // _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on
 // other platforms. The _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro is defined to
 // nothing on unsupported platforms so that it can be used to decorate functions regardless
 // of whether detection is actually supported.
 //
 // How does this work?
 // -------------------
 //
 // Let's say we want to check whether a weak function `f` has been overridden by the user.
 // The general mechanism works by placing `f`'s definition (in the libc++ built library)
 // inside a special section, which we do using the `__section__` attribute via the
 // _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
 //
 // Then, when comes the time to check whether the function has been overridden, we take
 // the address of the function and we check whether it falls inside the special function
 // we created. This can be done by finding pointers to the start and the end of the section
 // (which is done differently for ELF and Mach-O), and then checking whether `f` falls
 // within those bounds. If it falls within those bounds, then `f` is still inside the
 // special section and so it is the version we defined in the libc++ built library, i.e.
 // it was not overridden. Otherwise, it was overridden by the user because it falls
 // outside of the section.
 //
 // Important note
 // --------------
 //
 // This mechanism should never be used outside of the libc++ built library. In particular,
 // attempting to use this within the libc++ headers will not work at all because we don't
 // want to be defining special sections inside user's executables which use our headers.
 // This is provided inside libc++'s include tree solely to make it easier to share with
 // libc++abi, which needs the same mechanism.
 //

 #if defined(_LIBCPP_OBJECT_FORMAT_MACHO)

 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
 #  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE                                                                 \
     __attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions")))

 _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Ret, class... _Args>
 _LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
   // Declare two dummy bytes and give them these special `__asm` values. These values are
   // defined by the linker, which means that referring to `&__lcxx_override_start` will
   // effectively refer to the address where the section starts (and same for the end).
   extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override");
   extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override");

   // Now get a uintptr_t out of these locations, and out of the function pointer.
   uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start);
   uintptr_t __end   = reinterpret_cast<uintptr_t>(&__lcxx_override_end);
   uintptr_t __ptr   = reinterpret_cast<uintptr_t>(__fptr);

 #if defined(__arm64e__) && __has_feature(ptrauth_calls)
   // We must pass a void* to ptrauth_strip since it only accepts a pointer type. Also, in particular,
   // we must NOT pass a function pointer, otherwise we will strip the function pointer, and then attempt
   // to authenticate and re-sign it when casting it to a uintptr_t again, which will fail because we just
   // stripped the function pointer. See rdar://122927845.
   __ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer));
 #endif

   // Finally, the function was overridden if it falls outside of the section's bounds.
   return __ptr < __start || __ptr > __end;
 }
 _LIBCPP_END_NAMESPACE_STD

 #elif defined(_LIBCPP_OBJECT_FORMAT_ELF)

 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
 #  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override")))

 // This is very similar to what we do for Mach-O above. The ELF linker will implicitly define
 // variables with those names corresponding to the start and the end of the section.
 //
 // See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section
 extern char __start___lcxx_override;
 extern char __stop___lcxx_override;

 _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Ret, class... _Args>
 _LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
   uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override);
   uintptr_t __end   = reinterpret_cast<uintptr_t>(&__stop___lcxx_override);
   uintptr_t __ptr   = reinterpret_cast<uintptr_t>(__fptr);

   return __ptr < __start || __ptr > __end;
 }
 _LIBCPP_END_NAMESPACE_STD

 #else

 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0
 #  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */

 #endif

 #endif // _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H
	// -- C++ --
	//===----------------------------------------------------------------------===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//

	#ifndef _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H
	#define _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H

	#include <__config>
	#include <cstdint>

	#if defined(__arm64e__) && __has_feature(ptrauth_calls)
	# include <ptrauth.h>
	#endif

	#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
	# pragma GCC system_header
	#endif

	//
	// This file provides the std::__is_function_overridden utility, which allows checking
	// whether an overridable function (typically a weak symbol) like `operator new`
	// has been overridden by a user or not.
	//
	// This is a low-level utility which does not work on all platforms, since it needs
	// to make assumptions about the object file format in use. Furthermore, it requires
	// the "base definition" of the function (the one we want to check whether it has been
	// overridden) to be annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
	//
	// This currently works with Mach-O files (used on Darwin) and with ELF files (used on Linux
	// and others). On platforms where we know how to implement this detection, the macro
	// _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on
	// other platforms. The _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro is defined to
	// nothing on unsupported platforms so that it can be used to decorate functions regardless
	// of whether detection is actually supported.
	//
	// How does this work?
	// -------------------
	//
	// Let's say we want to check whether a weak function `f` has been overridden by the user.
	// The general mechanism works by placing `f`'s definition (in the libc++ built library)
	// inside a special section, which we do using the `__section__` attribute via the
	// _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
	//
	// Then, when comes the time to check whether the function has been overridden, we take
	// the address of the function and we check whether it falls inside the special function
	// we created. This can be done by finding pointers to the start and the end of the section
	// (which is done differently for ELF and Mach-O), and then checking whether `f` falls
	// within those bounds. If it falls within those bounds, then `f` is still inside the
	// special section and so it is the version we defined in the libc++ built library, i.e.
	// it was not overridden. Otherwise, it was overridden by the user because it falls
	// outside of the section.
	//
	// Important note
	// --------------
	//
	// This mechanism should never be used outside of the libc++ built library. In particular,
	// attempting to use this within the libc++ headers will not work at all because we don't
	// want to be defining special sections inside user's executables which use our headers.
	// This is provided inside libc++'s include tree solely to make it easier to share with
	// libc++abi, which needs the same mechanism.
	//

	#if defined(_LIBCPP_OBJECT_FORMAT_MACHO)

	# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
	# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE \
	__attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions")))

	_LIBCPP_BEGIN_NAMESPACE_STD
	template <class _Ret, class... _Args>
	_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
	// Declare two dummy bytes and give them these special `__asm` values. These values are
	// defined by the linker, which means that referring to `&__lcxx_override_start` will
	// effectively refer to the address where the section starts (and same for the end).
	extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override");
	extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override");

	// Now get a uintptr_t out of these locations, and out of the function pointer.
	uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start);
	uintptr_t __end = reinterpret_cast<uintptr_t>(&__lcxx_override_end);
	uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr);

	#if defined(__arm64e__) && __has_feature(ptrauth_calls)
	// We must pass a void* to ptrauth_strip since it only accepts a pointer type. Also, in particular,
	// we must NOT pass a function pointer, otherwise we will strip the function pointer, and then attempt
	// to authenticate and re-sign it when casting it to a uintptr_t again, which will fail because we just
	// stripped the function pointer. See rdar://122927845.
	__ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer));
	#endif

	// Finally, the function was overridden if it falls outside of the section's bounds.
	return __ptr < __start \|\| __ptr > __end;
	}
	_LIBCPP_END_NAMESPACE_STD

	#elif defined(_LIBCPP_OBJECT_FORMAT_ELF)

	# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
	# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override")))

	// This is very similar to what we do for Mach-O above. The ELF linker will implicitly define
	// variables with those names corresponding to the start and the end of the section.
	//
	// See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section
	extern char __start___lcxx_override;
	extern char __stop___lcxx_override;

	_LIBCPP_BEGIN_NAMESPACE_STD
	template <class _Ret, class... _Args>
	_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
	uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override);
	uintptr_t __end = reinterpret_cast<uintptr_t>(&__stop___lcxx_override);
	uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr);

	return __ptr < __start \|\| __ptr > __end;
	}
	_LIBCPP_END_NAMESPACE_STD

	#else

	# define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0
	# define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */

	#endif

	#endif // _LIBCPP_SRC_INCLUDE_OVERRIDABLE_FUNCTION_H