求助：关于隐藏文件头的读写偏移的问题

小弟正在学习楚狂人的《寒江独钓：windows内核安全编程》，遇到一事很不明了，希望得到各位高手的帮助，感激万分。直接进入正题：
相关代码如下：
void CFIrpReadPre(PIRP irp,PIO_STACK_LOCATION irpsp)
{
PLARGE_INTEGER offset;
PFCB fcb = (PFCB)irpsp->FileObject->FsContext;
offset = &irpsp->Parameters.Read.ByteOffset;
if(offset->LowPart == FILE_USE_FILE_POINTER_POSITION && offset->HighPart == -1)
{
// 请问当这种情况下怎样修改文件的偏移？
ASSERT(FALSE);
}
// 偏移必须修改为增加4k。
offset->QuadPart += CF_FILE_HEADER_SIZE;
KdPrint(("CFIrpReadPre: offset = %8x\r\n",
offset->LowPart));
}
从WDK中找到的相关资料：
IrpSp->Parameters.Read.ByteOffset
Pointer to a LARGE_INTEGER variable that specifies the starting byte offset within the file of the data to be read.
Under certain circumstances, this parameter might validly contain a NULL or negative value. For example, if the file object was opened for synchronous I/O, and one of the following conditions is true, this indicates that the current file position should be used instead of an explicit file offset value:

IrpSp->Parameters.Read.ByteOffset.QuadPart == 0

IrpSp->Parameters.Read.ByteOffset.LowPart == FILE_USE_FILE_POINTER_POSITION and
IrpSp->Parameters.Read.ByteOffset.HighPart == -1.

恳请各位高手的赐教！