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:

  • 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 Documentation/admin-guide/java.rst 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>