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 invoked 9with which binary. Binfmt_misc recognises the binary-type by matching some bytes 10at the beginning of the file with a magic byte sequence (masking out specified 11bits) you have supplied. Binfmt_misc can also recognise a filename extension 12aka ``.com`` or ``.exe``. 13 14First you must mount binfmt_misc:: 15 16 mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc 17 18To actually register a new binary type, you have to set up a string looking like 19``:name:type:offset:magic:mask:interpreter:flags`` (where you can choose the 20``:`` 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 this 26 name below ``/proc/sys/fs/binfmt_misc``; cannot contain slashes ``/`` for 27 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. This 32 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 string 36 may contain hex-encoded characters like ``\x0a`` or ``\xA4``. Note that you 37 must escape any NUL bytes; parsing halts at the first one. In a shell 38 environment you might have to write ``\\x0a`` to prevent the shell from 39 eating your ``\``. 40 If you chose filename extension matching, this is the extension to be 41 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 some 45 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 must 47 escape any NUL bytes; parsing halts at the first one. Ignored when using 48 filename extension matching. 49- ``interpreter`` 50 is the program that should be invoked with the binary as first 51 argument (specify the full path) 52- ``flags`` 53 is an optional field that controls several aspects of the invocation 54 of the interpreter. It is a string of capital letters, each controls a 55 certain aspect. The following flags are supported: 56 57 ``P`` - preserve-argv[0] 58 Legacy behavior of binfmt_misc is to overwrite 59 the original argv[0] with the full path to the binary. When this 60 flag is included, binfmt_misc will add an argument to the argument 61 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 execute 64 ``/bin/foo`` with ``argv[]`` set to ``["/bin/foo", "/usr/local/bin/blah", "blah"]``. The interp has to be aware of this so it can 65 execute ``/usr/local/bin/blah`` 66 with ``argv[]`` set to ``["blah"]``. 67 ``O`` - open-binary 68 Legacy behavior of binfmt_misc is to pass the full path 69 of the binary to the interpreter as an argument. When this flag is 70 included, binfmt_misc will open the file for reading and pass its 71 descriptor as an argument, instead of the full path, thus allowing 72 the interpreter to execute non-readable binaries. This feature 73 should be used with care - the interpreter has to be trusted not to 74 emit the contents of the non-readable binary. 75 ``C`` - credentials 76 Currently, the behavior of binfmt_misc is to calculate 77 the credentials and security token of the new process according to 78 the interpreter. When this flag is included, these attributes are 79 calculated according to the binary. It also implies the ``O`` flag. 80 This feature should be used with care as the interpreter 81 will run with root permissions when a setuid binary owned by root 82 is run with binfmt_misc. 83 ``F`` - fix binary 84 The usual behaviour of binfmt_misc is to spawn the 85 binary lazily when the misc format file is invoked. However, 86 this doesn't work very well in the face of mount namespaces and 87 changeroots, so the ``F`` mode opens the binary as soon as the 88 emulation is installed and uses the opened image to spawn the 89 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 characters 96 - the magic must reside in the first 128 bytes of the file, i.e. 97 offset+size(magic) has to be less than 128 98 - the interpreter string may not exceed 127 characters 99 100To use binfmt_misc you have to mount it first. You can mount it with 101``mount -t binfmt_misc none /proc/sys/fs/binfmt_misc`` command, or you can add 102a line ``none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0`` to your 103``/etc/fstab`` so it auto mounts on boot. 104 105You may want to add the binary formats in one of your ``/etc/rc`` scripts during 106boot-up. Read the manual of your init program to figure out how to do this 107right. 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:' > register 117 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 118 119- enable support for packed DOS applications (pre-configured dosemu hdimages):: 120 121 echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register 122 123- enable support for Windows executables using wine:: 124 125 echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register 126 127For java support see Documentation/admin-guide/java.rst 128 129 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`` or 132``/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 139Hints 140----- 141 142If you want to pass special arguments to your interpreter, you can 143write 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 kernel 147passes it the full filename (or the file descriptor) to use. Using ``$PATH`` can 148cause unexpected behaviour and can be a security hazard. 149 150 151Richard Günther <rguenth@tat.physik.uni-tuebingen.de> 152