linux/Documentation/admin-guide/binfmt-misc.rst

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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:
binfmt_misc: expand the register format limit to 1920 bytes The current code places a 256 byte limit on the registration format. This ends up being fairly limited when you try to do matching against a binary format like ELF: - the magic & mask formats cannot have any embedded NUL chars (string_unescape_inplace halts at the first NUL) - each escape sequence quadruples the size: \x00 is needed for NUL - trying to match bytes at the start of the file as well as further on leads to a lot of \x00 sequences in the mask - magic & mask have to be the same length (when decoded) - still need bytes for the other fields - impossible! Let's look at a concrete (and common) example: using QEMU to run MIPS ELFs. The name field uses 11 bytes "qemu-mipsel". The interp uses 20 bytes "/usr/bin/qemu-mipsel". The type & flags takes up 4 bytes. We need 7 bytes for the delimiter (usually ":"). We can skip offset. So already we're down to 107 bytes to use with the magic/mask instead of the real limit of 128 (BINPRM_BUF_SIZE). If people use shell code to register (which they do the majority of the time), they're down to ~26 possible bytes since the escape sequence must be \x##. The ELF format looks like (both 32 & 64 bit): e_ident: 16 bytes e_type: 2 bytes e_machine: 2 bytes Those 20 bytes are enough for most architectures because they have so few formats in the first place, thus they can be uniquely identified. That also means for shell users, since 20 is smaller than 26, they can sanely register a handler. But for some targets (like MIPS), we need to poke further. The ELF fields continue on: e_entry: 4 or 8 bytes e_phoff: 4 or 8 bytes e_shoff: 4 or 8 bytes e_flags: 4 bytes We only care about e_flags here as that includes the bits to identify whether the ELF is O32/N32/N64. But now we have to consume another 16 bytes (for 32 bit ELFs) or 28 bytes (for 64 bit ELFs) just to match the flags. If every byte is escaped, we send 288 more bytes to the kernel ((20 {e_ident,e_type,e_machine} + 12 {e_entry,e_phoff,e_shoff} + 4 {e_flags}) * 2 {mask,magic} * 4 {escape}) and we've clearly blown our budget. Even if we try to be clever and do the decoding ourselves (rather than relying on the kernel to process \x##), we still can't hit the mark -- string_unescape_inplace treats mask & magic as C strings so NUL cannot be embedded. That leaves us with having to pass \x00 for the 12/24 entry/phoff/shoff bytes (as those will be completely random addresses), and that is a minimum requirement of 48/96 bytes for the mask alone. Add up the rest and we blow through it (this is for 64 bit ELFs): magic: 20 {e_ident,e_type,e_machine} + 24 {e_entry,e_phoff,e_shoff} + 4 {e_flags} = 48 # ^^ See note below. mask: 20 {e_ident,e_type,e_machine} + 96 {e_entry,e_phoff,e_shoff} + 4 {e_flags} = 120 Remember above we had 107 left over, and now we're at 168. This is of course the *best* case scenario -- you'll also want to have NUL bytes in the magic & mask too to match literal zeros. Note: the reason we can use 24 in the magic is that we can work off of the fact that for bytes the mask would clobber, we can stuff any value into magic that we want. So when mask is \x00, we don't need the magic to also be \x00, it can be an unescaped raw byte like '!'. This lets us handle more formats (barely) under the current 256 limit, but that's a pretty tall hoop to force people to jump through. With all that said, let's bump the limit from 256 bytes to 1920. This way we support escaping every byte of the mask & magic field (which is 1024 bytes by themselves -- 128 * 4 * 2), and we leave plenty of room for other fields. Like long paths to the interpreter (when you have source in your /really/long/homedir/qemu/foo). Since the current code stuffs more than one structure into the same buffer, we leave a bit of space to easily round up to 2k. 1920 is just as arbitrary as 256 ;). Signed-off-by: Mike Frysinger <vapier@gentoo.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2014-10-13 22:52:03 +00:00
- 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>