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/Documentation/binfmt_misc.txt

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  1     Kernel Support for miscellaneous (your favourite) Binary Formats v1.1
  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	mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc 
 16
 17To 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
 19your needs) and echo it to /proc/sys/fs/binfmt_misc/register.
 20Here 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
 60There 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
 66To 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
 68a line "none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your
 69/etc/fstab so it auto mounts on boot.
 70
 71You may want to add the binary formats in one of your /etc/rc scripts during
 72boot-up. Read the manual of your init program to figure out how to do this
 73right.
 74
 75Think about the order of adding entries! Later added entries are matched first!
 76
 77
 78A 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
 90For java support see Documentation/java.txt
 91
 92
 93You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
 94or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name.
 95Catting the file tells you the current status of binfmt_misc/the entry.
 96
 97You can remove one entry or all entries by echoing -1 to /proc/.../the_name
 98or /proc/sys/fs/binfmt_misc/status.
 99
100
101HINTS:
102======
103
104If you want to pass special arguments to your interpreter, you can
105write a wrapper script for it. See Documentation/java.txt for an
106example.
107
108Your interpreter should NOT look in the PATH for the filename; the kernel
109passes it the full filename (or the file descriptor) to use.  Using $PATH can
110cause unexpected behaviour and can be a security hazard.
111
112
113There is a web page about binfmt_misc at
114http://www.tat.physik.uni-tuebingen.de/~rguenth/linux/binfmt_misc.html
115
116Richard G´┐Żnther <rguenth@tat.physik.uni-tuebingen.de>