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 ':'
 19upon your needs) and echo it to /proc/sys/fs/binfmt_misc/register.
 20
 21Here 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
 70
 71There are some restrictions:
 72 - the whole register string may not exceed 1920 characters
 73 - the magic must reside in the first 128 bytes of the file, i.e.
 74   offset+size(magic) has to be less than 128
 75 - the interpreter string may not exceed 127 characters
 76
 77To use binfmt_misc you have to mount it first. You can mount it with
 78"mount -t binfmt_misc none /proc/sys/fs/binfmt_misc" command, or you can add
 79a line "none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your
 80/etc/fstab so it auto mounts on boot.
 81
 82You may want to add the binary formats in one of your /etc/rc scripts during
 83boot-up. Read the manual of your init program to figure out how to do this
 84right.
 85
 86Think about the order of adding entries! Later added entries are matched first!
 87
 88
 89A few examples (assumed you are in /proc/sys/fs/binfmt_misc):
 90
 91- enable support for em86 (like binfmt_em86, for Alpha AXP only):
 92  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
 93  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
 94
 95- enable support for packed DOS applications (pre-configured dosemu hdimages):
 96  echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
 97
 98- enable support for Windows executables using wine:
 99  echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
100
101For java support see Documentation/java.txt
102
103
104You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
105or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name.
106Catting the file tells you the current status of binfmt_misc/the entry.
107
108You can remove one entry or all entries by echoing -1 to /proc/.../the_name
109or /proc/sys/fs/binfmt_misc/status.
110
111
112HINTS:
113======
114
115If you want to pass special arguments to your interpreter, you can
116write a wrapper script for it. See Documentation/java.txt for an
117example.
118
119Your interpreter should NOT look in the PATH for the filename; the kernel
120passes it the full filename (or the file descriptor) to use.  Using $PATH can
121cause unexpected behaviour and can be a security hazard.
122
123
124Richard Günther <rguenth@tat.physik.uni-tuebingen.de>