- Kernel Support for miscellaneous (your favourite) Binary Formats v1.1
- =====================================================================
+Kernel Support for miscellaneous (your favourite) Binary Formats v1.1
+=====================================================================
This Kernel feature allows you to invoke almost (for restrictions see below)
every program by simply typing its name in the shell.
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'.
+aka ``.com`` or ``.exe``.
-First you must mount binfmt_misc:
- mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
+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.
+``: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 \.
+
+- ``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
+ 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
+- ``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
+- ``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
+ 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
+ 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
+ ``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.
+ 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
+ ``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.
+``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
+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):
+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)::
-- 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
+ echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > 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::
-- enable support for Windows executables using wine:
- echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
+ echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
For java support see Documentation/java.txt
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.
+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.
+You can remove one entry or all entries by echoing -1 to ``/proc/.../the_name``
+or ``/proc/sys/fs/binfmt_misc/status``.
-HINTS:
-======
+Hints
+-----
If you want to pass special arguments to your interpreter, you can
write a wrapper script for it. See Documentation/java.txt 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 $PATH can
+passes it the full filename (or the file descriptor) to use. Using ``$PATH`` can
cause unexpected behaviour and can be a security hazard.
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