linux/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 ':'
  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      '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
  78There 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
  84To 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
  86a line "none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your
  87/etc/fstab so it auto mounts on boot.
  88
  89You may want to add the binary formats in one of your /etc/rc scripts during
  90boot-up. Read the manual of your init program to figure out how to do this
  91right.
  92
  93Think about the order of adding entries! Later added entries are matched first!
  94
  95
  96A 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
 108For java support see Documentation/java.txt
 109
 110
 111You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
 112or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name.
 113Catting the file tells you the current status of binfmt_misc/the entry.
 114
 115You can remove one entry or all entries by echoing -1 to /proc/.../the_name
 116or /proc/sys/fs/binfmt_misc/status.
 117
 118
 119HINTS:
 120======
 121
 122If you want to pass special arguments to your interpreter, you can
 123write a wrapper script for it. See Documentation/java.txt for an
 124example.
 125
 126Your interpreter should NOT look in the PATH for the filename; the kernel
 127passes it the full filename (or the file descriptor) to use.  Using $PATH can
 128cause unexpected behaviour and can be a security hazard.
 129
 130
 131Richard G√ľnther <rguenth@tat.physik.uni-tuebingen.de>
 132