Documentation / admin-guide / binfmt-misc.rst


Based on kernel version 5.8. Page generated on 2020-08-08 17:40 EST.

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Kernel Support for miscellaneous Binary Formats (binfmt_misc)
=============================================================

This Kernel feature allows you to invoke almost (for restrictions see below)
every program by simply typing its name in the shell.
This includes for example compiled Java(TM), Python or Emacs programs.

To achieve this you must tell binfmt_misc which interpreter has to be invoked
with which binary. Binfmt_misc recognises the binary-type by matching some bytes
at the beginning of the file with a magic byte sequence (masking out specified
bits) you have supplied. Binfmt_misc can also recognise a filename extension
aka ``.com`` or ``.exe``.

First you must mount binfmt_misc::

	mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc

To actually register a new binary type, you have to set up a string looking like
``:name:type:offset:magic:mask:interpreter:flags`` (where you can choose the
``:`` upon your needs) and echo it to ``/proc/sys/fs/binfmt_misc/register``.

Here is what the fields mean:

- ``name``
   is an identifier string. A new /proc file will be created with this
   ``name below /proc/sys/fs/binfmt_misc``; cannot contain slashes ``/`` for
   obvious reasons.
- ``type``
   is the type of recognition. Give ``M`` for magic and ``E`` for extension.
- ``offset``
   is the offset of the magic/mask in the file, counted in bytes. This
   defaults to 0 if you omit it (i.e. you write ``:name:type::magic...``).
   Ignored when using filename extension matching.
- ``magic``
   is the byte sequence binfmt_misc is matching for. The magic string
   may contain hex-encoded characters like ``\x0a`` or ``\xA4``. Note that you
   must escape any NUL bytes; parsing halts at the first one. In a shell
   environment you might have to write ``\\x0a`` to prevent the shell from
   eating your ``\``.
   If you chose filename extension matching, this is the extension to be
   recognised (without the ``.``, the ``\x0a`` specials are not allowed).
   Extension    matching is case sensitive, and slashes ``/`` are not allowed!
- ``mask``
   is an (optional, defaults to all 0xff) mask. You can mask out some
   bits from matching by supplying a string like magic and as long as magic.
   The mask is anded with the byte sequence of the file. Note that you must
   escape any NUL bytes; parsing halts at the first one. Ignored when using
   filename extension matching.
- ``interpreter``
   is the program that should be invoked with the binary as first
   argument (specify the full path)
- ``flags``
   is an optional field that controls several aspects of the invocation
   of the interpreter. It is a string of capital letters, each controls a
   certain aspect. The following flags are supported:

      ``P`` - preserve-argv[0]
            Legacy behavior of binfmt_misc is to overwrite
            the original argv[0] with the full path to the binary. When this
            flag is included, binfmt_misc will add an argument to the argument
            vector for this purpose, thus preserving the original ``argv[0]``.
            e.g. If your interp is set to ``/bin/foo`` and you run ``blah``
            (which is in ``/usr/local/bin``), then the kernel will execute
            ``/bin/foo`` with ``argv[]`` set to ``["/bin/foo", "/usr/local/bin/blah", "blah"]``.  The interp has to be aware of this so it can
            execute ``/usr/local/bin/blah``
            with ``argv[]`` set to ``["blah"]``.
      ``O`` - open-binary
	    Legacy behavior of binfmt_misc is to pass the full path
            of the binary to the interpreter as an argument. When this flag is
            included, binfmt_misc will open the file for reading and pass its
            descriptor as an argument, instead of the full path, thus allowing
            the interpreter to execute non-readable binaries. This feature
            should be used with care - the interpreter has to be trusted not to
            emit the contents of the non-readable binary.
      ``C`` - credentials
            Currently, the behavior of binfmt_misc is to calculate
            the credentials and security token of the new process according to
            the interpreter. When this flag is included, these attributes are
            calculated according to the binary. It also implies the ``O`` flag.
            This feature should be used with care as the interpreter
            will run with root permissions when a setuid binary owned by root
            is run with binfmt_misc.
      ``F`` - fix binary
            The usual behaviour of binfmt_misc is to spawn the
	    binary lazily when the misc format file is invoked.  However,
	    this doesn``t work very well in the face of mount namespaces and
	    changeroots, so the ``F`` mode opens the binary as soon as the
	    emulation is installed and uses the opened image to spawn the
	    emulator, meaning it is always available once installed,
	    regardless of how the environment changes.


There are some restrictions:

 - the whole register string may not exceed 1920 characters
 - the magic must reside in the first 128 bytes of the file, i.e.
   offset+size(magic) has to be less than 128
 - the interpreter string may not exceed 127 characters

To use binfmt_misc you have to mount it first. You can mount it with
``mount -t binfmt_misc none /proc/sys/fs/binfmt_misc`` command, or you can add
a line ``none  /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0`` to your
``/etc/fstab`` so it auto mounts on boot.

You may want to add the binary formats in one of your ``/etc/rc`` scripts during
boot-up. Read the manual of your init program to figure out how to do this
right.

Think about the order of adding entries! Later added entries are matched first!


A few examples (assumed you are in ``/proc/sys/fs/binfmt_misc``):

- enable support for em86 (like binfmt_em86, for Alpha AXP only)::

    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
    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

- enable support for packed DOS applications (pre-configured dosemu hdimages)::

    echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register

- enable support for Windows executables using wine::

    echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register

For java support see Documentation/admin-guide/java.rst


You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
or 1 (to enable) to ``/proc/sys/fs/binfmt_misc/status`` or
``/proc/.../the_name``.
Catting the file tells you the current status of ``binfmt_misc/the_entry``.

You can remove one entry or all entries by echoing -1 to ``/proc/.../the_name``
or ``/proc/sys/fs/binfmt_misc/status``.


Hints
-----

If you want to pass special arguments to your interpreter, you can
write a wrapper script for it.
See :doc:`Documentation/admin-guide/java.rst <./java>` for an example.

Your interpreter should NOT look in the PATH for the filename; the kernel
passes it the full filename (or the file descriptor) to use.  Using ``$PATH`` can
cause unexpected behaviour and can be a security hazard.


Richard Günther <rguenth@tat.physik.uni-tuebingen.de>