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"name below" is not part of the /proc path and should not be formatted in monospace. "doesn``t" is rendered in HTML with a double backtick. Revert it back to "doesn't". Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net> Link: https://lore.kernel.org/r/20210101211447.1021412-1-j.neuschaefer@gmx.net Signed-off-by: Jonathan Corbet <corbet@lwn.net>
151 lines
7.2 KiB
ReStructuredText
151 lines
7.2 KiB
ReStructuredText
Kernel Support for miscellaneous Binary Formats (binfmt_misc)
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=============================================================
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This Kernel feature allows you to invoke almost (for restrictions see below)
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every program by simply typing its name in the shell.
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This includes for example compiled Java(TM), Python or Emacs programs.
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To achieve this you must tell binfmt_misc which interpreter has to be invoked
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with which binary. Binfmt_misc recognises the binary-type by matching some bytes
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at the beginning of the file with a magic byte sequence (masking out specified
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bits) you have supplied. Binfmt_misc can also recognise a filename extension
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aka ``.com`` or ``.exe``.
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First you must mount binfmt_misc::
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mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
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To actually register a new binary type, you have to set up a string looking like
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``:name:type:offset:magic:mask:interpreter:flags`` (where you can choose the
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``:`` upon your needs) and echo it to ``/proc/sys/fs/binfmt_misc/register``.
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Here is what the fields mean:
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- ``name``
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is an identifier string. A new /proc file will be created with this
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name below ``/proc/sys/fs/binfmt_misc``; cannot contain slashes ``/`` for
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obvious reasons.
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- ``type``
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is the type of recognition. Give ``M`` for magic and ``E`` for extension.
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- ``offset``
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is the offset of the magic/mask in the file, counted in bytes. This
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defaults to 0 if you omit it (i.e. you write ``:name:type::magic...``).
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Ignored when using filename extension matching.
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- ``magic``
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is the byte sequence binfmt_misc is matching for. The magic string
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may contain hex-encoded characters like ``\x0a`` or ``\xA4``. Note that you
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must escape any NUL bytes; parsing halts at the first one. In a shell
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environment you might have to write ``\\x0a`` to prevent the shell from
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eating your ``\``.
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If you chose filename extension matching, this is the extension to be
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recognised (without the ``.``, the ``\x0a`` specials are not allowed).
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Extension matching is case sensitive, and slashes ``/`` are not allowed!
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- ``mask``
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is an (optional, defaults to all 0xff) mask. You can mask out some
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bits from matching by supplying a string like magic and as long as magic.
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The mask is anded with the byte sequence of the file. Note that you must
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escape any NUL bytes; parsing halts at the first one. Ignored when using
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filename extension matching.
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- ``interpreter``
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is the program that should be invoked with the binary as first
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argument (specify the full path)
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- ``flags``
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is an optional field that controls several aspects of the invocation
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of the interpreter. It is a string of capital letters, each controls a
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certain aspect. The following flags are supported:
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``P`` - preserve-argv[0]
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Legacy behavior of binfmt_misc is to overwrite
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the original argv[0] with the full path to the binary. When this
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flag is included, binfmt_misc will add an argument to the argument
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vector for this purpose, thus preserving the original ``argv[0]``.
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e.g. If your interp is set to ``/bin/foo`` and you run ``blah``
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(which is in ``/usr/local/bin``), then the kernel will execute
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``/bin/foo`` with ``argv[]`` set to ``["/bin/foo", "/usr/local/bin/blah", "blah"]``. The interp has to be aware of this so it can
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execute ``/usr/local/bin/blah``
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with ``argv[]`` set to ``["blah"]``.
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``O`` - open-binary
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Legacy behavior of binfmt_misc is to pass the full path
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of the binary to the interpreter as an argument. When this flag is
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included, binfmt_misc will open the file for reading and pass its
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descriptor as an argument, instead of the full path, thus allowing
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the interpreter to execute non-readable binaries. This feature
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should be used with care - the interpreter has to be trusted not to
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emit the contents of the non-readable binary.
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``C`` - credentials
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Currently, the behavior of binfmt_misc is to calculate
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the credentials and security token of the new process according to
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the interpreter. When this flag is included, these attributes are
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calculated according to the binary. It also implies the ``O`` flag.
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This feature should be used with care as the interpreter
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will run with root permissions when a setuid binary owned by root
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is run with binfmt_misc.
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``F`` - fix binary
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The usual behaviour of binfmt_misc is to spawn the
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binary lazily when the misc format file is invoked. However,
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this doesn't work very well in the face of mount namespaces and
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changeroots, so the ``F`` mode opens the binary as soon as the
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emulation is installed and uses the opened image to spawn the
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emulator, meaning it is always available once installed,
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regardless of how the environment changes.
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There are some restrictions:
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- the whole register string may not exceed 1920 characters
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- the magic must reside in the first 128 bytes of the file, i.e.
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offset+size(magic) has to be less than 128
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- the interpreter string may not exceed 127 characters
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To use binfmt_misc you have to mount it first. You can mount it with
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``mount -t binfmt_misc none /proc/sys/fs/binfmt_misc`` command, or you can add
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a line ``none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0`` to your
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``/etc/fstab`` so it auto mounts on boot.
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You may want to add the binary formats in one of your ``/etc/rc`` scripts during
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boot-up. Read the manual of your init program to figure out how to do this
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right.
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Think about the order of adding entries! Later added entries are matched first!
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A few examples (assumed you are in ``/proc/sys/fs/binfmt_misc``):
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- enable support for em86 (like binfmt_em86, for Alpha AXP only)::
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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
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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
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- enable support for packed DOS applications (pre-configured dosemu hdimages)::
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echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
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- enable support for Windows executables using wine::
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echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
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For java support see Documentation/admin-guide/java.rst
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You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
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or 1 (to enable) to ``/proc/sys/fs/binfmt_misc/status`` or
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``/proc/.../the_name``.
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Catting the file tells you the current status of ``binfmt_misc/the_entry``.
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You can remove one entry or all entries by echoing -1 to ``/proc/.../the_name``
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or ``/proc/sys/fs/binfmt_misc/status``.
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Hints
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-----
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If you want to pass special arguments to your interpreter, you can
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write a wrapper script for it.
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See :doc:`Documentation/admin-guide/java.rst <./java>` for an example.
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Your interpreter should NOT look in the PATH for the filename; the kernel
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passes it the full filename (or the file descriptor) to use. Using ``$PATH`` can
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cause unexpected behaviour and can be a security hazard.
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Richard Günther <rguenth@tat.physik.uni-tuebingen.de>
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