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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

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>
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