How to do IO across hardsector boundries

[Thomas S. Iversen]

Hi There

As part of an assignment I am trying to port a piece of software from 
FreeBSD to linux. Essentially this software (crypto) makes a virtual 
blockdevice with "virtual" sectors on top. Under FreeBSD these virtual 
sectors are just read/written using a simple command:

buf=g_read(dev, offset, len)
error=g_write(dev,offset,buf,len)

In linux however I have only seen the BIO layer which operates on IO on 
hardsector boundaries.

So my question really is, how do I go about updating for instance the 
512 bytes located for at byte 64 to 64+511 on the actual media without 
getting in trouble regarding the data from offset 0-63 and 64+512->1023?

Regards Thomas
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