UNTITLED() LOCAL UNTITLED()
NAME
update-binfmts — maintain registry of executable binary formats
SYNOPSIS
update-binfmts [options] --install name path spec
update-binfmts [options] --remove name path
update-binfmts [options] --import [name]
update-binfmts [options] --unimport [name]
update-binfmts [options] --display [name]
update-binfmts [options] --enable [name]
update-binfmts [options] --disable [name]
update-binfmts [options] --find [path]
DESCRIPTION
Versions 2.1.43 and later of the Linux kernel have contained the
binfmt_misc module. This enables a system administrator to register in-
terpreters for various binary formats based on a magic number or their
file extension, and cause the appropriate interpreter to be invoked
whenever a matching file is executed. Think of it as a more flexible
version of the #! executable interpreter mechanism, or as something
which can behave a little like "associations" in certain other operating
systems (though in GNU/Linux the tendency is to keep this sort of thing
somewhere else, like your file manager). update-binfmts manages a per-
sistent database of these interpreters.
When each package providing a registered interpreter is installed,
changed, or removed, update-binfmts is called to update information
about that interpreter. update-binfmts is usually called from the
postinst or prerm scripts in Debian packages.
OPTIONS
Exactly one action must be specified; this may be accompanied by any one
of the common options.
COMMON OPTIONS
--package package-name
Specifies the name of the current package, to be used by package
post-installation and pre-removal scripts. System administrators
installing binary formats for local use should probably ignore
this option.
When installing new formats, the --import action should be used
instead. Similarly, when removing old formats, the --unimport ac-
tion should be used instead.
--admindir directory
Specifies the administrative directory, when this is to be differ-
ent from the default of /var/lib/binfmts.
--importdir directory
Specifies the directory from which packaged binary formats are im-
ported, when this is to be different from the default of
/usr/share/binfmts.
--test
Don't do anything, just demonstrate what would be done.
--help
Display some usage information.
--version
Display version information.
ACTIONS
--install name path spec
Install a binary format identified by name with interpreter path
into the database. After registration, this format will be used
when the kernel tries to execute a file matching spec (see “BINARY
FORMAT SPECIFICATIONS” below).
--install will attempt to enable this binary format in the kernel
as well as adding it to its own database; see --enable below.
You cannot install a format with any of the names ".", "..", "reg-
ister", or "status", as these are used by the filesystem or the
binfmt_misc module.
--remove name path
Remove the binary format identified by name with interpreter path
from the database. This will also attempt to disable the binary
format in the kernel; see --disable below.
--import [name]
Import a packaged format file called name, or import all format
files currently on the system if no name is given. If name is not
a full path, it is assumed to be a file in the import directory
(/usr/share/binfmts by default). See “FORMAT FILES” below for the
required contents of these files.
For packages, this is preferable to using the --install option, as
a format file can be installed without update-binfmts needing to
be available.
--unimport [name]
Unimport a packaged format file called name, or unimport all for-
mat files currently on the system if no name is given. If name is
not a full path, it is assumed to be a file in the import direc-
tory (/usr/share/binfmts by default). See “FORMAT FILES” below
for the required contents of these files.
For packages, this is preferable to using the --remove option, for
symmetry with --import.
--display [name]
Display any information held in the database about the binary for-
mat identifier name, or about all known binary formats if no name
is given. Also show whether displayed binary formats are enabled
or disabled.
--enable [name]
Enable binary format name, or all known binary formats if no name
is given, in the kernel, thus enabling direct execution of match-
ing files. You must have binfmt_misc compiled into the kernel or
loaded as a module for this to work.
--disable [name]
Disable binary format name, or all known binary formats if no name
is given, in the kernel, thus disabling direct execution of match-
ing files. You must have binfmt_misc compiled into the kernel or
loaded as a module for this to work.
--find [path]
Print the list of interpreters that will be tried in sequence when
attempting to execute path, one per line. The first one for which
execvp(3) succeeds will be used.
Note that if multiple formats match an executable, then the order
is in general not defined, and may not be preserved between
update-binfmts operations, so you should generally try to ensure
that this option prints at most one line for any given path. The
exception to this is that any format with a userspace detector
will be run before any format without a userspace detector.
BINARY FORMAT SPECIFICATIONS
--magic byte-sequence
This matches all files with the magic number byte-sequence. Hexa-
decimal escapes may be included in the byte-sequence by preceding
them with \x, for example ‘\x0a’ for a linefeed. Remember to pro-
tect such escapes with quotes or an additional backslash to pre-
vent their interpretation by the shell.
Also see --offset and --mask.
--offset offset
This is the offset of the magic/mask in the file, counted in
bytes. The default is 0. Only valid with --magic.
--mask byte-sequence
This mask will be logically-ANDed with the string to be checked
against the magic number given with --magic. The default is all
0xff, i.e. no effect. Only valid with --magic.
--extension extension
This matches all files whose names end in ".extension". Hexadeci-
mal escapes are not recognized here. Extension matching is case-
sensitive.
--detector path
If this option is used, a userspace detector program will be used
to check whether the file is suitable for this interpreter. This
may be used when the binary format is more complex than can be
handled by the kernel's format specifications alone. The program
should return an exit code of zero if the file is appropriate and
non-zero otherwise. This option cannot be used together with
--fix-binary yes.
--credentials yes, --credentials no
Whether to keep the credentials of the original binary to run the
interpreter; this is typically useful to run setuid binaries, but
has security implications.
--preserve yes, --preserve no
Whether to preserve the original argv[0] when running the inter-
preter, rather than overwriting it with the full path to the bi-
nary.
--fix-binary yes, --fix-binary no
Whether to open the interpreter binary immediately and always use
the opened image. This allows the interpreter from the host to be
used regardless of usage in chroots or different mount namespaces.
The default behaviour is no, meaning that the kernel should open
the interpreter binary lazily when needed. This option requires
Linux 4.8 or newer. It cannot be used together with --detector,
or with multiple binary formats that share the same magic number,
since the kernel will only open a single interpreter binary which
will then not be able to detect and execute the real interpreter
from inside a chroot or from a different mount namespace.
FORMAT FILES
A format file is a sequence of options, one per line, corresponding
roughly to the options given to an --install command. Each option con-
sists of a key, followed by whitespace, followed by a value.
The package option should be set to the current package. The
interpreter option should be set to the path to the interpreter that
will handle this binary format. The magic, offset, mask, extension,
detector, credentials, preserve, and fix_binary options correspond to
the command-line options of the same names.
EXIT STATUS
0 The requested action was successfully performed.
2 Problems were encountered whilst parsing the command line or per-
forming the action.
EXAMPLES
This format file can be used with an interpreter capable of handling
Java .class files:
package javawrapper
interpreter /usr/bin/javawrapper
magic \xca\xfe\xba\xbe
This corresponds roughly to the following command:
update-binfmts --package javawrapper \
--install javawrapper /usr/bin/javawrapper \
--magic '\xca\xfe\xba\xbe'
NOTES
If you're not careful, you can break your system with update-binfmts.
An easy way to do this is to register an ELF binary as a handler for
ELF, which will almost certainly cause your system to hang immediately;
even if it doesn't, you won't be able to run update-binfmts to fix it.
In the future update-binfmts may have some checks to prevent this sort
of thing happening accidentally, though of course you can still manipu-
late the binfmt_misc kernel module directly.
AUTHOR
update-binfmts is copyright (C) 2000, 2001, 2002, 2003, 2004, 2005,
2006, 2007, 2008, 2009, 2010, 2011 Colin Watson <cjwatson@debian.org>.
See the GNU General Public License version 3 or later for copying condi-
tions.
You can find the GNU GPL v3 in /usr/share/common-licenses/GPL-3 on any
modern Debian system.
Richard Guenther wrote the binfmt_misc kernel module.
THANKS
Ian Jackson wrote update-alternatives and dpkg-divert, from which this
program borrows heavily.
Debian January 24, 2011 UPDATE-BINFMTS(8)
Generated by dwww version 1.16 on Tue Dec 16 07:17:30 CET 2025.