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Documentation / binfmt_misc.txt




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Based on kernel version 4.8. Page generated on 2016-10-06 23:10 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 ':'
19	upon your needs) and echo it to /proc/sys/fs/binfmt_misc/register.
20	
21	Here is what the fields mean:
22	 - 'name' is an identifier string. A new /proc file will be created with this
23	   name below /proc/sys/fs/binfmt_misc; cannot contain slashes '/' for obvious
24	   reasons.
25	 - 'type' is the type of recognition. Give 'M' for magic and 'E' for extension.
26	 - 'offset' is the offset of the magic/mask in the file, counted in bytes. This
27	   defaults to 0 if you omit it (i.e. you write ':name:type::magic...'). Ignored
28	   when using filename extension matching.
29	 - 'magic' is the byte sequence binfmt_misc is matching for. The magic string
30	   may contain hex-encoded characters like \x0a or \xA4. Note that you must
31	   escape any NUL bytes; parsing halts at the first one. In a shell environment
32	   you might have to write \\x0a to prevent the shell from eating your \.
33	   If you chose filename extension matching, this is the extension to be
34	   recognised (without the '.', the \x0a specials are not allowed). Extension
35	   matching is case sensitive, and slashes '/' are not allowed!
36	 - 'mask' is an (optional, defaults to all 0xff) mask. You can mask out some
37	   bits from matching by supplying a string like magic and as long as magic.
38	   The mask is anded with the byte sequence of the file. Note that you must
39	   escape any NUL bytes; parsing halts at the first one. Ignored when using
40	   filename extension matching.
41	 - 'interpreter' is the program that should be invoked with the binary as first
42	   argument (specify the full path)
43	 - 'flags' is an optional field that controls several aspects of the invocation
44	   of the interpreter. It is a string of capital letters, each controls a
45	   certain aspect. The following flags are supported -
46	      'P' - preserve-argv[0]. Legacy behavior of binfmt_misc is to overwrite
47	            the original argv[0] with the full path to the binary. When this
48	            flag is included, binfmt_misc will add an argument to the argument
49	            vector for this purpose, thus preserving the original argv[0].
50	            e.g. If your interp is set to /bin/foo and you run `blah` (which is
51	            in /usr/local/bin), then the kernel will execute /bin/foo with
52	            argv[] set to ["/bin/foo", "/usr/local/bin/blah", "blah"].  The
53	            interp has to be aware of this so it can execute /usr/local/bin/blah
54	            with argv[] set to ["blah"].
55	      'O' - open-binary. Legacy behavior of binfmt_misc is to pass the full path
56	            of the binary to the interpreter as an argument. When this flag is
57	            included, binfmt_misc will open the file for reading and pass its
58	            descriptor as an argument, instead of the full path, thus allowing
59	            the interpreter to execute non-readable binaries. This feature
60	            should be used with care - the interpreter has to be trusted not to
61	            emit the contents of the non-readable binary.
62	      'C' - credentials. Currently, the behavior of binfmt_misc is to calculate
63	            the credentials and security token of the new process according to
64	            the interpreter. When this flag is included, these attributes are
65	            calculated according to the binary. It also implies the 'O' flag.
66	            This feature should be used with care as the interpreter
67	            will run with root permissions when a setuid binary owned by root
68	            is run with binfmt_misc.
69	      'F' - fix binary.  The usual behaviour of binfmt_misc is to spawn the
70	      	    binary lazily when the misc format file is invoked.  However,
71		    this doesn't work very well in the face of mount namespaces and
72		    changeroots, so the F mode opens the binary as soon as the
73		    emulation is installed and uses the opened image to spawn the
74		    emulator, meaning it is always available once installed,
75		    regardless of how the environment changes.
76	
77	
78	There are some restrictions:
79	 - the whole register string may not exceed 1920 characters
80	 - the magic must reside in the first 128 bytes of the file, i.e.
81	   offset+size(magic) has to be less than 128
82	 - the interpreter string may not exceed 127 characters
83	
84	To use binfmt_misc you have to mount it first. You can mount it with
85	"mount -t binfmt_misc none /proc/sys/fs/binfmt_misc" command, or you can add
86	a line "none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your
87	/etc/fstab so it auto mounts on boot.
88	
89	You may want to add the binary formats in one of your /etc/rc scripts during
90	boot-up. Read the manual of your init program to figure out how to do this
91	right.
92	
93	Think about the order of adding entries! Later added entries are matched first!
94	
95	
96	A few examples (assumed you are in /proc/sys/fs/binfmt_misc):
97	
98	- enable support for em86 (like binfmt_em86, for Alpha AXP only):
99	  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
100	  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
101	
102	- enable support for packed DOS applications (pre-configured dosemu hdimages):
103	  echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
104	
105	- enable support for Windows executables using wine:
106	  echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
107	
108	For java support see Documentation/java.txt
109	
110	
111	You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
112	or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name.
113	Catting the file tells you the current status of binfmt_misc/the entry.
114	
115	You can remove one entry or all entries by echoing -1 to /proc/.../the_name
116	or /proc/sys/fs/binfmt_misc/status.
117	
118	
119	HINTS:
120	======
121	
122	If you want to pass special arguments to your interpreter, you can
123	write a wrapper script for it. See Documentation/java.txt for an
124	example.
125	
126	Your interpreter should NOT look in the PATH for the filename; the kernel
127	passes it the full filename (or the file descriptor) to use.  Using $PATH can
128	cause unexpected behaviour and can be a security hazard.
129	
130	
131	Richard Günther <rguenth@tat.physik.uni-tuebingen.de>
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