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Documentation / binfmt_misc.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 22:55 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		mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc 
16	
17	To actually register a new binary type, you have to set up a string looking like
18	:name:type:offset:magic:mask:interpreter:flags (where you can choose the ':' upon
19	your needs) and echo it to /proc/sys/fs/binfmt_misc/register.
20	Here is what the fields mean:
21	 - 'name' is an identifier string. A new /proc file will be created with this
22	   name below /proc/sys/fs/binfmt_misc
23	 - 'type' is the type of recognition. Give 'M' for magic and 'E' for extension.
24	 - 'offset' is the offset of the magic/mask in the file, counted in bytes. This
25	   defaults to 0 if you omit it (i.e. you write ':name:type::magic...')
26	 - 'magic' is the byte sequence binfmt_misc is matching for. The magic string
27	   may contain hex-encoded characters like \x0a or \xA4. In a shell environment
28	   you will have to write \\x0a to prevent the shell from eating your \.
29	   If you chose filename extension matching, this is the extension to be
30	   recognised (without the '.', the \x0a specials are not allowed). Extension
31	   matching is case sensitive!
32	 - 'mask' is an (optional, defaults to all 0xff) mask. You can mask out some
33	   bits from matching by supplying a string like magic and as long as magic.
34	   The mask is anded with the byte sequence of the file.
35	 - 'interpreter' is the program that should be invoked with the binary as first
36	   argument (specify the full path)
37	 - 'flags' is an optional field that controls several aspects of the invocation
38	   of the interpreter. It is a string of capital letters, each controls a certain
39	   aspect. The following flags are supported -
40	      'P' - preserve-argv[0].  Legacy behavior of binfmt_misc is to overwrite the
41	            original argv[0] with the full path to the binary.  When this flag is
42	            included, binfmt_misc will add an argument to the argument vector for
43	            this purpose, thus preserving the original argv[0].
44	      'O' - open-binary. Legacy behavior of binfmt_misc is to pass the full path
45	            of the binary to the interpreter as an argument. When this flag is
46	            included, binfmt_misc will open the file for reading and pass its
47	            descriptor as an argument, instead of the full path, thus allowing
48	            the interpreter to execute non-readable binaries. This feature should
49	            be used with care - the interpreter has to be trusted not to emit
50	            the contents of the non-readable binary.
51	      'C' - credentials. Currently, the behavior of binfmt_misc is to calculate
52	            the credentials and security token of the new process according to
53	            the interpreter. When this flag is included, these attributes are
54	            calculated according to the binary. It also implies the 'O' flag.
55	            This feature should be used with care as the interpreter
56	            will run with root permissions when a setuid binary owned by root
57	            is run with binfmt_misc.
58	
59	
60	There are some restrictions:
61	 - the whole register string may not exceed 255 characters
62	 - the magic must reside in the first 128 bytes of the file, i.e.
63	   offset+size(magic) has to be less than 128
64	 - the interpreter string may not exceed 127 characters
65	
66	To use binfmt_misc you have to mount it first. You can mount it with
67	"mount -t binfmt_misc none /proc/sys/fs/binfmt_misc" command, or you can add
68	a line "none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your
69	/etc/fstab so it auto mounts on boot.
70	
71	You may want to add the binary formats in one of your /etc/rc scripts during
72	boot-up. Read the manual of your init program to figure out how to do this
73	right.
74	
75	Think about the order of adding entries! Later added entries are matched first!
76	
77	
78	A few examples (assumed you are in /proc/sys/fs/binfmt_misc):
79	
80	- enable support for em86 (like binfmt_em86, for Alpha AXP only):
81	  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
82	  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
83	
84	- enable support for packed DOS applications (pre-configured dosemu hdimages):
85	  echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
86	
87	- enable support for Windows executables using wine:
88	  echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
89	
90	For java support see Documentation/java.txt
91	
92	
93	You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
94	or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name.
95	Catting the file tells you the current status of binfmt_misc/the entry.
96	
97	You can remove one entry or all entries by echoing -1 to /proc/.../the_name
98	or /proc/sys/fs/binfmt_misc/status.
99	
100	
101	HINTS:
102	======
103	
104	If you want to pass special arguments to your interpreter, you can
105	write a wrapper script for it. See Documentation/java.txt for an
106	example.
107	
108	Your interpreter should NOT look in the PATH for the filename; the kernel
109	passes it the full filename (or the file descriptor) to use.  Using $PATH can
110	cause unexpected behaviour and can be a security hazard.
111	
112	
113	There is a web page about binfmt_misc at
114	http://www.tat.physik.uni-tuebingen.de
115	
116	Richard Günther <rguenth@tat.physik.uni-tuebingen.de>
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