Documentation/admin-guide/binfmt-misc.rst

   1 Kernel Support for miscellaneous (your favourite) Binary Formats v1.1
   2 =====================================================================
   3
   4 This Kernel feature allows you to invoke almost (for restrictions see below)
   5 every program by simply typing its name in the shell.
   6 This includes for example compiled Java(TM), Python or Emacs programs.
   7
   8 To achieve this you must tell binfmt_misc which interpreter has to be invoked
   9 with which binary. Binfmt_misc recognises the binary-type by matching some bytes
  10 at the beginning of the file with a magic byte sequence (masking out specified
  11 bits) you have supplied. Binfmt_misc can also recognise a filename extension
  12 aka ``.com`` or ``.exe``.
  13
  14 First you must mount binfmt_misc::
  15
  16         mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
  17
  18 To actually register a new binary type, you have to set up a string looking like
  19 ``:name:type:offset:magic:mask:interpreter:flags`` (where you can choose the
  20 ``:`` upon your needs) and echo it to ``/proc/sys/fs/binfmt_misc/register``.
  21
  22 Here is what the fields mean:
  23
  24 - ``name``
  25    is an identifier string. A new /proc file will be created with this
  26    ``name below /proc/sys/fs/binfmt_misc``; cannot contain slashes ``/`` for
  27    obvious reasons.
  28 - ``type``
  29    is the type of recognition. Give ``M`` for magic and ``E`` for extension.
  30 - ``offset``
  31    is the offset of the magic/mask in the file, counted in bytes. This
  32    defaults to 0 if you omit it (i.e. you write ``:name:type::magic...``).
  33    Ignored when using filename extension matching.
  34 - ``magic``
  35    is the byte sequence binfmt_misc is matching for. The magic string
  36    may contain hex-encoded characters like ``\x0a`` or ``\xA4``. Note that you
  37    must escape any NUL bytes; parsing halts at the first one. In a shell
  38    environment you might have to write ``\\x0a`` to prevent the shell from
  39    eating your ``\``.
  40    If you chose filename extension matching, this is the extension to be
  41    recognised (without the ``.``, the ``\x0a`` specials are not allowed).
  42    Extension    matching is case sensitive, and slashes ``/`` are not allowed!
  43 - ``mask``
  44    is an (optional, defaults to all 0xff) mask. You can mask out some
  45    bits from matching by supplying a string like magic and as long as magic.
  46    The mask is anded with the byte sequence of the file. Note that you must
  47    escape any NUL bytes; parsing halts at the first one. Ignored when using
  48    filename extension matching.
  49 - ``interpreter``
  50    is the program that should be invoked with the binary as first
  51    argument (specify the full path)
  52 - ``flags``
  53    is an optional field that controls several aspects of the invocation
  54    of the interpreter. It is a string of capital letters, each controls a
  55    certain aspect. The following flags are supported:
  56
  57       ``P`` - preserve-argv[0]
  58             Legacy behavior of binfmt_misc is to overwrite
  59             the original argv[0] with the full path to the binary. When this
  60             flag is included, binfmt_misc will add an argument to the argument
  61             vector for this purpose, thus preserving the original ``argv[0]``.
  62             e.g. If your interp is set to ``/bin/foo`` and you run ``blah``
  63             (which is in ``/usr/local/bin``), then the kernel will execute
  64             ``/bin/foo`` with ``argv[]`` set to ``["/bin/foo", "/usr/local/bin/blah", "blah"]``.  The interp has to be aware of this so it can
  65             execute ``/usr/local/bin/blah``
  66             with ``argv[]`` set to ``["blah"]``.
  67       ``O`` - open-binary
  68             Legacy behavior of binfmt_misc is to pass the full path
  69             of the binary to the interpreter as an argument. When this flag is
  70             included, binfmt_misc will open the file for reading and pass its
  71             descriptor as an argument, instead of the full path, thus allowing
  72             the interpreter to execute non-readable binaries. This feature
  73             should be used with care - the interpreter has to be trusted not to
  74             emit the contents of the non-readable binary.
  75       ``C`` - credentials
  76             Currently, the behavior of binfmt_misc is to calculate
  77             the credentials and security token of the new process according to
  78             the interpreter. When this flag is included, these attributes are
  79             calculated according to the binary. It also implies the ``O`` flag.
  80             This feature should be used with care as the interpreter
  81             will run with root permissions when a setuid binary owned by root
  82             is run with binfmt_misc.
  83       ``F`` - fix binary
  84             The usual behaviour of binfmt_misc is to spawn the
  85             binary lazily when the misc format file is invoked.  However,
  86             this doesn``t work very well in the face of mount namespaces and
  87             changeroots, so the ``F`` mode opens the binary as soon as the
  88             emulation is installed and uses the opened image to spawn the
  89             emulator, meaning it is always available once installed,
  90             regardless of how the environment changes.
  91
  92
  93 There are some restrictions:
  94
  95  - the whole register string may not exceed 1920 characters
  96  - the magic must reside in the first 128 bytes of the file, i.e.
  97    offset+size(magic) has to be less than 128
  98  - the interpreter string may not exceed 127 characters
  99
 100 To use binfmt_misc you have to mount it first. You can mount it with
 101 ``mount -t binfmt_misc none /proc/sys/fs/binfmt_misc`` command, or you can add
 102 a line ``none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0`` to your
 103 ``/etc/fstab`` so it auto mounts on boot.
 104
 105 You may want to add the binary formats in one of your ``/etc/rc`` scripts during
 106 boot-up. Read the manual of your init program to figure out how to do this
 107 right.
 108
 109 Think about the order of adding entries! Later added entries are matched first!
 110
 111
 112 A few examples (assumed you are in ``/proc/sys/fs/binfmt_misc``):
 113
 114 - enable support for em86 (like binfmt_em86, for Alpha AXP only)::
 115
 116     echo ':i386:M::\x7fELF\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x03:\xff\xff\xff\xff\xff\xfe\xfe\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfb\xff\xff:/bin/em86:' > register
 117     echo ':i486:M::\x7fELF\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x06:\xff\xff\xff\xff\xff\xfe\xfe\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfb\xff\xff:/bin/em86:' > register
 118
 119 - enable support for packed DOS applications (pre-configured dosemu hdimages)::
 120
 121     echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
 122
 123 - enable support for Windows executables using wine::
 124
 125     echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
 126
 127 For java support see Documentation/admin-guide/java.rst
 128
 129
 130 You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
 131 or 1 (to enable) to ``/proc/sys/fs/binfmt_misc/status`` or
 132 ``/proc/.../the_name``.
 133 Catting the file tells you the current status of ``binfmt_misc/the_entry``.
 134
 135 You can remove one entry or all entries by echoing -1 to ``/proc/.../the_name``
 136 or ``/proc/sys/fs/binfmt_misc/status``.
 137
 138
 139 Hints
 140 -----
 141
 142 If you want to pass special arguments to your interpreter, you can
 143 write a wrapper script for it. See Documentation/admin-guide/java.rst for an
 144 example.
 145
 146 Your interpreter should NOT look in the PATH for the filename; the kernel
 147 passes it the full filename (or the file descriptor) to use.  Using ``$PATH`` can
 148 cause unexpected behaviour and can be a security hazard.
 149
 150
 151 Richard Günther <rguenth@tat.physik.uni-tuebingen.de>