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