1Kernel Support for miscellaneous Binary Formats (binfmt_misc)
2=============================================================
3
4This Kernel feature allows you to invoke almost (for restrictions see below)
5every program by simply typing its name in the shell.
6This includes for example compiled Java(TM), Python or Emacs programs.
7
8To achieve this you must tell binfmt_misc which interpreter has to be invoked
9with which binary. Binfmt_misc recognises the binary-type by matching some bytes
10at the beginning of the file with a magic byte sequence (masking out specified
11bits) you have supplied. Binfmt_misc can also recognise a filename extension
12aka ``.com`` or ``.exe``.
13
14First you must mount binfmt_misc::
15
16	mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
17
18To 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
22Here 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
93There 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
100To 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
102a line ``none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0`` to your
103``/etc/fstab`` so it auto mounts on boot.
104
105You may want to add the binary formats in one of your ``/etc/rc`` scripts during
106boot-up. Read the manual of your init program to figure out how to do this
107right.
108
109Think about the order of adding entries! Later added entries are matched first!
110
111
112A 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
127For java support see Documentation/admin-guide/java.rst
128
129
130You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
131or 1 (to enable) to ``/proc/sys/fs/binfmt_misc/status`` or
132``/proc/.../the_name``.
133Catting the file tells you the current status of ``binfmt_misc/the_entry``.
134
135You can remove one entry or all entries by echoing -1 to ``/proc/.../the_name``
136or ``/proc/sys/fs/binfmt_misc/status``.
137
138
139Hints
140-----
141
142If you want to pass special arguments to your interpreter, you can
143write a wrapper script for it.
144See :doc:`Documentation/admin-guide/java.rst <./java>` for an example.
145
146Your interpreter should NOT look in the PATH for the filename; the kernel
147passes it the full filename (or the file descriptor) to use.  Using ``$PATH`` can
148cause unexpected behaviour and can be a security hazard.
149
150
151Richard Günther <rguenth@tat.physik.uni-tuebingen.de>
152