Documentation/binfmt_misc.txt

   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
  70
  71 There 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
  77 To 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
  79 a line "none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your
  80 /etc/fstab so it auto mounts on boot.
  81
  82 You may want to add the binary formats in one of your /etc/rc scripts during
  83 boot-up. Read the manual of your init program to figure out how to do this
  84 right.
  85
  86 Think about the order of adding entries! Later added entries are matched first!
  87
  88
  89 A 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
 101 For java support see Documentation/java.txt
 102
 103
 104 You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
 105 or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name.
 106 Catting the file tells you the current status of binfmt_misc/the entry.
 107
 108 You can remove one entry or all entries by echoing -1 to /proc/.../the_name
 109 or /proc/sys/fs/binfmt_misc/status.
 110
 111
 112 HINTS:
 113 ======
 114
 115 If you want to pass special arguments to your interpreter, you can
 116 write a wrapper script for it. See Documentation/java.txt for an
 117 example.
 118
 119 Your interpreter should NOT look in the PATH for the filename; the kernel
 120 passes it the full filename (or the file descriptor) to use.  Using $PATH can
 121 cause unexpected behaviour and can be a security hazard.
 122
 123
 124 Richard Günther <rguenth@tat.physik.uni-tuebingen.de>