Documentation/admin-guide/binfmt-misc.rst - linux - Git at Google

 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>
	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>