This is xnu-8019. See this file in:
/*
 * Copyright (c) 1998-2020 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
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 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */

#ifndef _IOGUARDPAGEMEMORYDESCRIPTOR_H
#define _IOGUARDPAGEMEMORYDESCRIPTOR_H

#include <IOKit/IOMemoryDescriptor.h>
#include <libkern/c++/OSPtr.h>

/*!
 *   @class IOGuardPageMemoryDescriptor
 *   @abstract Provides a memory descriptor that allows for variable size guard regions. Use with
 *             IOMultiMemoryDescriptor to surround other memory descriptors with guard pages.
 */
class IOGuardPageMemoryDescriptor : public IOGeneralMemoryDescriptor
{
	OSDeclareDefaultStructors(IOGuardPageMemoryDescriptor);

protected:
	virtual void free() APPLE_KEXT_OVERRIDE;

	vm_offset_t    _buffer;
	vm_size_t      _size;

public:

	/* @function withSize
	 *  @discussion Create a IOGuardPageMemoryDescriptor with the specified size.
	 *  @param Size of the guard region. This will be rounded up to the nearest multiple of page size.
	 *  @return IOGuardPageMemoryDescriptor instance to be released by the caller, which will free the allocated
	 *          virtual memory region.
	 */
	static OSPtr<IOGuardPageMemoryDescriptor> withSize(vm_size_t size);

	virtual bool initWithSize(vm_size_t size);

private:
	virtual IOReturn doMap(vm_map_t           addressMap,
	    IOVirtualAddress * atAddress,
	    IOOptionBits       options,
	    IOByteCount        sourceOffset = 0,
	    IOByteCount        length = 0 ) APPLE_KEXT_OVERRIDE;
};

#endif /* !_IOGUARDPAGEMEMORYDESCRIPTOR_H */