Documentation/filesystems/adfs.rst

.. SPDX-License-Identifier: GPL-2.0

===============================
Acorn Disc Filing System - ADFS
===============================

Filesystems supported by ADFS
-----------------------------

The ADFS module supports the following Filecore formats which have:

- new maps
- new directories or big directories

In terms of the named formats, this means we support:

- E and E+, with or without boot block
- F and F+

We fully support reading files from these filesystems, and writing to
existing files within their existing allocation.  Essentially, we do
not support changing any of the filesystem metadata.

This is intended to support loopback mounted Linux native filesystems
on a RISC OS Filecore filesystem, but will allow the data within files
to be changed.

If write support (ADFS_FS_RW) is configured, we allow rudimentary
directory updates, specifically updating the access mode and timestamp.

Mount options for ADFS
----------------------

  ============  ======================================================
  uid=nnn	All files in the partition will be owned by
		user id nnn.  Default 0 (root).
  gid=nnn	All files in the partition will be in group
		nnn.  Default 0 (root).
  ownmask=nnn	The permission mask for ADFS 'owner' permissions
		will be nnn.  Default 0700.
  othmask=nnn	The permission mask for ADFS 'other' permissions
		will be nnn.  Default 0077.
  ftsuffix=n	When ftsuffix=0, no file type suffix will be applied.
		When ftsuffix=1, a hexadecimal suffix corresponding to
		the RISC OS file type will be added.  Default 0.
  ============  ======================================================

Mapping of ADFS permissions to Linux permissions
------------------------------------------------

  ADFS permissions consist of the following:

	- Owner read
	- Owner write
	- Other read
	- Other write

  (In older versions, an 'execute' permission did exist, but this
  does not hold the same meaning as the Linux 'execute' permission
  and is now obsolete).

  The mapping is performed as follows::

	Owner read				-> -r--r--r--
	Owner write				-> --w--w---w
	Owner read and filetype UnixExec	-> ---x--x--x
    These are then masked by ownmask, eg 700	-> -rwx------
	Possible owner mode permissions		-> -rwx------

	Other read				-> -r--r--r--
	Other write				-> --w--w--w-
	Other read and filetype UnixExec	-> ---x--x--x
    These are then masked by othmask, eg 077	-> ----rwxrwx
	Possible other mode permissions		-> ----rwxrwx

  Hence, with the default masks, if a file is owner read/write, and
  not a UnixExec filetype, then the permissions will be::

			-rw-------

  However, if the masks were ownmask=0770,othmask=0007, then this would
  be modified to::

			-rw-rw----

  There is no restriction on what you can do with these masks.  You may
  wish that either read bits give read access to the file for all, but
  keep the default write protection (ownmask=0755,othmask=0577)::

			-rw-r--r--

  You can therefore tailor the permission translation to whatever you
  desire the permissions should be under Linux.

RISC OS file type suffix
------------------------

  RISC OS file types are stored in bits 19..8 of the file load address.

  To enable non-RISC OS systems to be used to store files without losing
  file type information, a file naming convention was devised (initially
  for use with NFS) such that a hexadecimal suffix of the form ,xyz
  denoted the file type: e.g. BasicFile,ffb is a BASIC (0xffb) file.  This
  naming convention is now also used by RISC OS emulators such as RPCEmu.

  Mounting an ADFS disc with option ftsuffix=1 will cause appropriate file
  type suffixes to be appended to file names read from a directory.  If the
  ftsuffix option is zero or omitted, no file type suffixes will be added.