http://wiki.przemoc.net/tips/linux#mounting_partition_from_vdi_fixed-size_image

vdiwrap.c

/* SPDX-License-Identifier: MIT */
/*
 * Copyright (C) 2009  Przemyslaw Pawelczyk <przemoc@gmail.com>
 *
 ***** Description ************************************************************
 *
 * Simple (and incomplete) syscalls interposition for accessing fixed-size Sun
 * xVM VirtualBox Virtual Disk Images (.vdi files), especially using sfdisk.
 *
 ***** License ****************************************************************
 *
 * MIT License
 *
 ***** Tested on **************************************************************
 *
 * - Debian 5.0.3 (x64)  w/ util-linux 2.13.1.1-1
 * - Slackware 12.1      w/ util-linux-ng 2.13.1
 * - Ubuntu 8.10 (x64)   w/ util-linux-ng 2.14
 * - Ubuntu 9.10 (x64)   w/ util-linux-ng 2.16
 *
 ***** Compilation ************************************************************
 *
 * $ gcc -fPIC -c               -o vdiwrap.o  vdiwrap.c &&
 *   gcc -nostdlib -shared -ldl -o vdiwrap.so vdiwrap.o
 *
 ***** Usage ******************************************************************
 *
 * # LD_PRELOAD="./vdiwrap.so" sfdisk -qluS image.vdi
 *
 ******************************************************************************
 *
 * vdiwrap.c  @ https://gist.github.com/przemoc/571086  (version 0.2b)
 *
 */

#define _GNU_SOURCE
#define _LARGEFILE64_SOURCE

#include <dlfcn.h>
#include <stdarg.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define LOAD(name) orig_##name = dlsym(RTLD_NEXT, #name)
#if __LP64__
# define N(name) name
# define O(name) orig_##name
# define OFF_T off_t
# define STAT  stat
#else
# define N(name) name ## 64
# define O(name) orig_##name ## 64
# define OFF_T off64_t
# define STAT  stat64
#endif

#define OPEN_SIGNATURE   const char *__file, int __oflag, ...
#define LSEEK_SIGNATURE  int __fd, OFF_T __offset, int __whence
#define FXSTAT_SIGNATURE int __ver, int __fd, struct STAT *__stat_buf


static int vdi_fd = -1;
static int vdi_type = 0;
static int vdi_offset = 0;

static int   (*O(open))(OPEN_SIGNATURE);
static OFF_T (*O(lseek))(LSEEK_SIGNATURE);
static int   (*O(__fxstat))(FXSTAT_SIGNATURE);

/* Functions' prototypes */

int   N(open)(OPEN_SIGNATURE);
OFF_T N(lseek)(LSEEK_SIGNATURE);
int   N(__fxstat)(FXSTAT_SIGNATURE);

/* Initialization */

void _init()
{
	LOAD(open);
	LOAD(lseek);
	LOAD(__fxstat);
}

/* Functions' bodies */

int N(open)(OPEN_SIGNATURE)
{
	int result;
	int namelen;
	unsigned mode;
	va_list ap;

	if (__oflag & O_CREAT) {
		va_start(ap, __oflag);
		mode = va_arg(ap, unsigned);
		result = O(open)(__file, __oflag, mode);
		va_end(ap);
		return result;
	}

	result = O(open)(__file, __oflag);

	if (result > 0 && (namelen = strlen(__file)) && !strncasecmp(&__file[namelen - 4], ".vdi", 4)) {
		O(lseek)(result, 76, SEEK_SET);
		read(result, &vdi_type, 4);
		if (vdi_type == 2) {
			O(lseek)(result, 344, SEEK_SET);
			read(result, &vdi_offset, 4);
			O(lseek)(result, vdi_offset, SEEK_SET);
			vdi_fd = result;
			dprintf(STDERR_FILENO, "--- VDI data offset == %d bytes ---\n", vdi_offset);
		} else
			vdi_fd = -1;
	}

	return result;
}


OFF_T N(lseek)(LSEEK_SIGNATURE)
{
	OFF_T result;

	if (__fd == vdi_fd && __whence == SEEK_SET)
		__offset += vdi_offset;

	result = O(lseek)(__fd, __offset, __whence);

	if (__fd == vdi_fd && result > 0)
		result -= vdi_offset;

	return result;
}

int N(__fxstat)(FXSTAT_SIGNATURE)
{
	int result = O(__fxstat)(__ver, __fd, __stat_buf);

	if (__fd == vdi_fd && result == 0)
		__stat_buf->st_size -= vdi_offset;

	return result;
}

commit 64d29b98c6e7504bdcc1dac41e21a594761f53af
Author: Przemyslaw Pawelczyk <przemoc@gmail.com>
Date:   2018-02-06 16:08:20 +0100

    vdiwrap.c: Fix canonical location of the source file.

    My old wiki.przemoc.net is not available for many years.

commit 925aca5fc8fdf30effe84af8c6e817fd0e903329
Author: Przemyslaw Pawelczyk <przemoc@gmail.com>
Date:   2018-02-06 16:09:12 +0100

    vdiwrap.c: Relicense to MIT.

commit b6f0e15073b74a4d6b7f71229f8b1d3152bbf2b6
Author: Przemyslaw Pawelczyk <przemoc@gmail.com>
Date:   2018-02-06 16:22:48 +0100

    Add SPDX License Identifier.

    The Software Package Data Exchange (SPDX) is a good initiative, it has
    matured over time and deserves accelerated adoption in open-source.

    https://spdx.org/learn
    https://spdx.org/using-spdx
    https://spdx.org/license-list