binfmt-misc.rst 7.2 KB

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  1. Kernel Support for miscellaneous Binary Formats (binfmt_misc)
  2. =============================================================
  3. This Kernel feature allows you to invoke almost (for restrictions see below)
  4. every program by simply typing its name in the shell.
  5. This includes for example compiled Java(TM), Python or Emacs programs.
  6. To achieve this you must tell binfmt_misc which interpreter has to be invoked
  7. with which binary. Binfmt_misc recognises the binary-type by matching some bytes
  8. at the beginning of the file with a magic byte sequence (masking out specified
  9. bits) you have supplied. Binfmt_misc can also recognise a filename extension
  10. aka ``.com`` or ``.exe``.
  11. First you must mount binfmt_misc::
  12. mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
  13. To actually register a new binary type, you have to set up a string looking like
  14. ``:name:type:offset:magic:mask:interpreter:flags`` (where you can choose the
  15. ``:`` upon your needs) and echo it to ``/proc/sys/fs/binfmt_misc/register``.
  16. Here is what the fields mean:
  17. - ``name``
  18. is an identifier string. A new /proc file will be created with this
  19. ``name below /proc/sys/fs/binfmt_misc``; cannot contain slashes ``/`` for
  20. obvious reasons.
  21. - ``type``
  22. is the type of recognition. Give ``M`` for magic and ``E`` for extension.
  23. - ``offset``
  24. 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. Ignored when using filename extension matching.
  27. - ``magic``
  28. is the byte sequence binfmt_misc is matching for. The magic string
  29. may contain hex-encoded characters like ``\x0a`` or ``\xA4``. Note that you
  30. must escape any NUL bytes; parsing halts at the first one. In a shell
  31. environment you might have to write ``\\x0a`` to prevent the shell from
  32. 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).
  35. Extension matching is case sensitive, and slashes ``/`` are not allowed!
  36. - ``mask``
  37. is an (optional, defaults to all 0xff) mask. You can mask out some
  38. bits from matching by supplying a string like magic and as long as magic.
  39. The mask is anded with the byte sequence of the file. Note that you must
  40. escape any NUL bytes; parsing halts at the first one. Ignored when using
  41. filename extension matching.
  42. - ``interpreter``
  43. is the program that should be invoked with the binary as first
  44. argument (specify the full path)
  45. - ``flags``
  46. is an optional field that controls several aspects of the invocation
  47. of the interpreter. It is a string of capital letters, each controls a
  48. certain aspect. The following flags are supported:
  49. ``P`` - preserve-argv[0]
  50. Legacy behavior of binfmt_misc is to overwrite
  51. the original argv[0] with the full path to the binary. When this
  52. flag is included, binfmt_misc will add an argument to the argument
  53. vector for this purpose, thus preserving the original ``argv[0]``.
  54. e.g. If your interp is set to ``/bin/foo`` and you run ``blah``
  55. (which is in ``/usr/local/bin``), then the kernel will execute
  56. ``/bin/foo`` with ``argv[]`` set to ``["/bin/foo", "/usr/local/bin/blah", "blah"]``. The interp has to be aware of this so it can
  57. execute ``/usr/local/bin/blah``
  58. with ``argv[]`` set to ``["blah"]``.
  59. ``O`` - open-binary
  60. Legacy behavior of binfmt_misc is to pass the full path
  61. of the binary to the interpreter as an argument. When this flag is
  62. included, binfmt_misc will open the file for reading and pass its
  63. descriptor as an argument, instead of the full path, thus allowing
  64. the interpreter to execute non-readable binaries. This feature
  65. should be used with care - the interpreter has to be trusted not to
  66. emit the contents of the non-readable binary.
  67. ``C`` - credentials
  68. Currently, the behavior of binfmt_misc is to calculate
  69. the credentials and security token of the new process according to
  70. the interpreter. When this flag is included, these attributes are
  71. calculated according to the binary. It also implies the ``O`` flag.
  72. This feature should be used with care as the interpreter
  73. will run with root permissions when a setuid binary owned by root
  74. is run with binfmt_misc.
  75. ``F`` - fix binary
  76. The usual behaviour of binfmt_misc is to spawn the
  77. binary lazily when the misc format file is invoked. However,
  78. this doesn``t work very well in the face of mount namespaces and
  79. changeroots, so the ``F`` mode opens the binary as soon as the
  80. emulation is installed and uses the opened image to spawn the
  81. emulator, meaning it is always available once installed,
  82. regardless of how the environment changes.
  83. There are some restrictions:
  84. - the whole register string may not exceed 1920 characters
  85. - the magic must reside in the first 128 bytes of the file, i.e.
  86. offset+size(magic) has to be less than 128
  87. - the interpreter string may not exceed 127 characters
  88. To use binfmt_misc you have to mount it first. You can mount it with
  89. ``mount -t binfmt_misc none /proc/sys/fs/binfmt_misc`` command, or you can add
  90. a line ``none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0`` to your
  91. ``/etc/fstab`` so it auto mounts on boot.
  92. You may want to add the binary formats in one of your ``/etc/rc`` scripts during
  93. boot-up. Read the manual of your init program to figure out how to do this
  94. right.
  95. Think about the order of adding entries! Later added entries are matched first!
  96. A few examples (assumed you are in ``/proc/sys/fs/binfmt_misc``):
  97. - enable support for em86 (like binfmt_em86, for Alpha AXP only)::
  98. 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
  99. 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
  100. - enable support for packed DOS applications (pre-configured dosemu hdimages)::
  101. echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
  102. - enable support for Windows executables using wine::
  103. echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
  104. For java support see Documentation/admin-guide/java.rst
  105. You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
  106. or 1 (to enable) to ``/proc/sys/fs/binfmt_misc/status`` or
  107. ``/proc/.../the_name``.
  108. Catting the file tells you the current status of ``binfmt_misc/the_entry``.
  109. You can remove one entry or all entries by echoing -1 to ``/proc/.../the_name``
  110. or ``/proc/sys/fs/binfmt_misc/status``.
  111. Hints
  112. -----
  113. If you want to pass special arguments to your interpreter, you can
  114. write a wrapper script for it.
  115. See :doc:`Documentation/admin-guide/java.rst <./java>` for an example.
  116. Your interpreter should NOT look in the PATH for the filename; the kernel
  117. passes it the full filename (or the file descriptor) to use. Using ``$PATH`` can
  118. cause unexpected behaviour and can be a security hazard.
  119. Richard Günther <rguenth@tat.physik.uni-tuebingen.de>