linux/Documentation/filesystems/adfs.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3===============================
   4Acorn Disc Filing System - ADFS
   5===============================
   6
   7Filesystems supported by ADFS
   8-----------------------------
   9
  10The ADFS module supports the following Filecore formats which have:
  11
  12- new maps
  13- new directories or big directories
  14
  15In terms of the named formats, this means we support:
  16
  17- E and E+, with or without boot block
  18- F and F+
  19
  20We fully support reading files from these filesystems, and writing to
  21existing files within their existing allocation.  Essentially, we do
  22not support changing any of the filesystem metadata.
  23
  24This is intended to support loopback mounted Linux native filesystems
  25on a RISC OS Filecore filesystem, but will allow the data within files
  26to be changed.
  27
  28If write support (ADFS_FS_RW) is configured, we allow rudimentary
  29directory updates, specifically updating the access mode and timestamp.
  30
  31Mount options for ADFS
  32----------------------
  33
  34  ============  ======================================================
  35  uid=nnn       All files in the partition will be owned by
  36                user id nnn.  Default 0 (root).
  37  gid=nnn       All files in the partition will be in group
  38                nnn.  Default 0 (root).
  39  ownmask=nnn   The permission mask for ADFS 'owner' permissions
  40                will be nnn.  Default 0700.
  41  othmask=nnn   The permission mask for ADFS 'other' permissions
  42                will be nnn.  Default 0077.
  43  ftsuffix=n    When ftsuffix=0, no file type suffix will be applied.
  44                When ftsuffix=1, a hexadecimal suffix corresponding to
  45                the RISC OS file type will be added.  Default 0.
  46  ============  ======================================================
  47
  48Mapping of ADFS permissions to Linux permissions
  49------------------------------------------------
  50
  51  ADFS permissions consist of the following:
  52
  53        - Owner read
  54        - Owner write
  55        - Other read
  56        - Other write
  57
  58  (In older versions, an 'execute' permission did exist, but this
  59  does not hold the same meaning as the Linux 'execute' permission
  60  and is now obsolete).
  61
  62  The mapping is performed as follows::
  63
  64        Owner read                              -> -r--r--r--
  65        Owner write                             -> --w--w---w
  66        Owner read and filetype UnixExec        -> ---x--x--x
  67    These are then masked by ownmask, eg 700    -> -rwx------
  68        Possible owner mode permissions         -> -rwx------
  69
  70        Other read                              -> -r--r--r--
  71        Other write                             -> --w--w--w-
  72        Other read and filetype UnixExec        -> ---x--x--x
  73    These are then masked by othmask, eg 077    -> ----rwxrwx
  74        Possible other mode permissions         -> ----rwxrwx
  75
  76  Hence, with the default masks, if a file is owner read/write, and
  77  not a UnixExec filetype, then the permissions will be::
  78
  79                        -rw-------
  80
  81  However, if the masks were ownmask=0770,othmask=0007, then this would
  82  be modified to::
  83
  84                        -rw-rw----
  85
  86  There is no restriction on what you can do with these masks.  You may
  87  wish that either read bits give read access to the file for all, but
  88  keep the default write protection (ownmask=0755,othmask=0577)::
  89
  90                        -rw-r--r--
  91
  92  You can therefore tailor the permission translation to whatever you
  93  desire the permissions should be under Linux.
  94
  95RISC OS file type suffix
  96------------------------
  97
  98  RISC OS file types are stored in bits 19..8 of the file load address.
  99
 100  To enable non-RISC OS systems to be used to store files without losing
 101  file type information, a file naming convention was devised (initially
 102  for use with NFS) such that a hexadecimal suffix of the form ,xyz
 103  denoted the file type: e.g. BasicFile,ffb is a BASIC (0xffb) file.  This
 104  naming convention is now also used by RISC OS emulators such as RPCEmu.
 105
 106  Mounting an ADFS disc with option ftsuffix=1 will cause appropriate file
 107  type suffixes to be appended to file names read from a directory.  If the
 108  ftsuffix option is zero or omitted, no file type suffixes will be added.
 109