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1Kernel Support for miscellaneous Binary Formats (binfmt_misc)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 invoked9with which binary. Binfmt_misc recognises the binary-type by matching some bytes10at the beginning of the file with a magic byte sequence (masking out specified11bits) you have supplied. Binfmt_misc can also recognise a filename extension12aka ``.com`` or ``.exe``.13 14First you must mount binfmt_misc::15 16 mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc17 18To actually register a new binary type, you have to set up a string looking like19``:name:type:offset:magic:mask:interpreter:flags`` (where you can choose the20``:`` upon your needs) and echo it to ``/proc/sys/fs/binfmt_misc/register``.21 22Here is what the fields mean:23 24- ``name``25 is an identifier string. A new /proc file will be created with this26 name below ``/proc/sys/fs/binfmt_misc``; cannot contain slashes ``/`` for27 obvious reasons.28- ``type``29 is the type of recognition. Give ``M`` for magic and ``E`` for extension.30- ``offset``31 is the offset of the magic/mask in the file, counted in bytes. This32 defaults to 0 if you omit it (i.e. you write ``:name:type::magic...``).33 Ignored when using filename extension matching.34- ``magic``35 is the byte sequence binfmt_misc is matching for. The magic string36 may contain hex-encoded characters like ``\x0a`` or ``\xA4``. Note that you37 must escape any NUL bytes; parsing halts at the first one. In a shell38 environment you might have to write ``\\x0a`` to prevent the shell from39 eating your ``\``.40 If you chose filename extension matching, this is the extension to be41 recognised (without the ``.``, the ``\x0a`` specials are not allowed).42 Extension matching is case sensitive, and slashes ``/`` are not allowed!43- ``mask``44 is an (optional, defaults to all 0xff) mask. You can mask out some45 bits from matching by supplying a string like magic and as long as magic.46 The mask is anded with the byte sequence of the file. Note that you must47 escape any NUL bytes; parsing halts at the first one. Ignored when using48 filename extension matching.49- ``interpreter``50 is the program that should be invoked with the binary as first51 argument (specify the full path)52- ``flags``53 is an optional field that controls several aspects of the invocation54 of the interpreter. It is a string of capital letters, each controls a55 certain aspect. The following flags are supported:56 57 ``P`` - preserve-argv[0]58 Legacy behavior of binfmt_misc is to overwrite59 the original argv[0] with the full path to the binary. When this60 flag is included, binfmt_misc will add an argument to the argument61 vector for this purpose, thus preserving the original ``argv[0]``.62 e.g. If your interp is set to ``/bin/foo`` and you run ``blah``63 (which is in ``/usr/local/bin``), then the kernel will execute64 ``/bin/foo`` with ``argv[]`` set to ``["/bin/foo", "/usr/local/bin/blah", "blah"]``. The interp has to be aware of this so it can65 execute ``/usr/local/bin/blah``66 with ``argv[]`` set to ``["blah"]``.67 ``O`` - open-binary68 Legacy behavior of binfmt_misc is to pass the full path69 of the binary to the interpreter as an argument. When this flag is70 included, binfmt_misc will open the file for reading and pass its71 descriptor as an argument, instead of the full path, thus allowing72 the interpreter to execute non-readable binaries. This feature73 should be used with care - the interpreter has to be trusted not to74 emit the contents of the non-readable binary.75 ``C`` - credentials76 Currently, the behavior of binfmt_misc is to calculate77 the credentials and security token of the new process according to78 the interpreter. When this flag is included, these attributes are79 calculated according to the binary. It also implies the ``O`` flag.80 This feature should be used with care as the interpreter81 will run with root permissions when a setuid binary owned by root82 is run with binfmt_misc.83 ``F`` - fix binary84 The usual behaviour of binfmt_misc is to spawn the85 binary lazily when the misc format file is invoked. However,86 this doesn't work very well in the face of mount namespaces and87 changeroots, so the ``F`` mode opens the binary as soon as the88 emulation is installed and uses the opened image to spawn the89 emulator, meaning it is always available once installed,90 regardless of how the environment changes.91 92 93There are some restrictions:94 95 - the whole register string may not exceed 1920 characters96 - the magic must reside in the first 128 bytes of the file, i.e.97 offset+size(magic) has to be less than 12898 - the interpreter string may not exceed 127 characters99 100To use binfmt_misc you have to mount it first. You can mount it with101``mount -t binfmt_misc none /proc/sys/fs/binfmt_misc`` command, or you can add102a line ``none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0`` to your103``/etc/fstab`` so it auto mounts on boot.104 105You may want to add the binary formats in one of your ``/etc/rc`` scripts during106boot-up. Read the manual of your init program to figure out how to do this107right.108 109Think about the order of adding entries! Later added entries are matched first!110 111 112A few examples (assumed you are in ``/proc/sys/fs/binfmt_misc``):113 114- enable support for em86 (like binfmt_em86, for Alpha AXP only)::115 116 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:' > register117 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:' > register118 119- enable support for packed DOS applications (pre-configured dosemu hdimages)::120 121 echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register122 123- enable support for Windows executables using wine::124 125 echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register126 127For java support see Documentation/admin-guide/java.rst128 129 130You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)131or 1 (to enable) to ``/proc/sys/fs/binfmt_misc/status`` or132``/proc/.../the_name``.133Catting the file tells you the current status of ``binfmt_misc/the_entry``.134 135You can remove one entry or all entries by echoing -1 to ``/proc/.../the_name``136or ``/proc/sys/fs/binfmt_misc/status``.137 138 139Hints140-----141 142If you want to pass special arguments to your interpreter, you can143write a wrapper script for it.144See :doc:`Documentation/admin-guide/java.rst <./java>` for an example.145 146Your interpreter should NOT look in the PATH for the filename; the kernel147passes it the full filename (or the file descriptor) to use. Using ``$PATH`` can148cause unexpected behaviour and can be a security hazard.149 150 151Richard Günther <rguenth@tat.physik.uni-tuebingen.de>152