.\" update-binfmts.8
.Dd January 24, 2011
.Os Debian
.ds volume-operating-system Debian
.Dt UPDATE\-BINFMTS 8
.Sh NAME
.Nm update\-binfmts
.Nd maintain registry of executable binary formats
.Sh SYNOPSIS
.Nm
.Op Ar options
.Fl Fl install
.Ar name path spec
.br
.Nm
.Op Ar options
.Fl Fl remove
.Ar name path
.br
.Nm
.Op Ar options
.Fl Fl import
.Op Ar name
.br
.Nm
.Op Ar options
.Fl Fl display
.Op Ar name
.br
.Nm
.Op Ar options
.Fl Fl enable
.Op Ar name
.br
.Nm
.Op Ar options
.Fl Fl disable
.Op Ar name
.br
.Nm
.Op Ar options
.Fl Fl find
.Op Ar path
.Sh DESCRIPTION
Versions 2.1.43 and later of the Linux kernel have contained the binfmt_misc
module.
This enables a system administrator to register interpreters 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).
.Nm
manages a persistent database of these interpreters.
.Pp
When each package providing a registered interpreter is installed, changed,
or removed,
.Nm
is called to update information about that interpreter.
.Nm
is usually called from the
.Pa postinst
or
.Pa prerm
scripts in Debian packages.
.Sh OPTIONS
Exactly one action must be specified; this may be accompanied by any one of
the common options.
.Ss COMMON OPTIONS
.Bl -tag -width 4n
.It Fl Fl package Ar 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.
.Pp
When installing new formats, the
.Fl Fl import
action should be used instead.
.It Fl Fl admindir Ar directory
Specifies the administrative directory, when this is to be different from
the default of
.Pa /var/lib/binfmts .
.It Fl Fl importdir Ar directory
Specifies the directory from which packaged binary formats are imported,
when this is to be different from the default of
.Pa /usr/share/binfmts .
.It Fl Fl test
Don't do anything, just demonstrate what would be done.
.It Fl Fl help
Display some usage information.
.It Fl Fl version
Display version information.
.El
.Ss ACTIONS
.Bl -tag -width 4n
.It Fl Fl install Ar name path spec
Install a binary format identified by
.Ar name
with interpreter
.Ar path
into the database.
After registration, this format will be used when the kernel tries to
execute a file matching
.Ar spec
(see
.Sx BINARY FORMAT SPECIFICATIONS
below).
.Pp
.Fl Fl install
will attempt to enable this binary format in the kernel as well as adding it
to its own database; see
.Fl Fl enable
below.
.Pp
You cannot install a format with any of the names ".", "..", "register", or
"status", as these are used by the filesystem or the binfmt_misc module.
.It Fl Fl remove Ar name path
Remove the binary format identified by
.Ar name
with interpreter
.Ar path
from the database.
This will also attempt to disable the binary format in the kernel; see
.Fl Fl disable
below.
.It Fl Fl import Op Ar name
Import a packaged format file called
.Ar name ,
or import all format files currently on the system if no
.Ar name
is given.
If
.Ar name
is not a full path, it is assumed to be a file in the import directory
.Pf ( Pa /usr/share/binfmts
by default).
See
.Sx FORMAT FILES
below for the required contents of these files.
.Pp
For packages, this is preferable to using the
.Fl Fl install
option, as a format file can be installed without
.Nm
needing to be available.
.It Fl Fl display Op Ar name
Display any information held in the database about the binary format
identifier
.Ar name ,
or about all known binary formats if no
.Ar name
is given.
Also show whether displayed binary formats are enabled or disabled.
.It Fl Fl enable Op Ar name
Enable binary format
.Ar name ,
or all known binary formats if no
.Ar name
is given, in the kernel, thus enabling direct execution of matching files.
You must have binfmt_misc compiled into the kernel or loaded as a module for
this to work.
.It Fl Fl disable Op Ar name
Disable binary format
.Ar name ,
or all known binary formats if no
.Ar name
is given, in the kernel, thus disabling direct execution of matching files.
You must have binfmt_misc compiled into the kernel or loaded as a module for
this to work.
.It Fl Fl find Op Ar path
Print the list of interpreters that will be tried in sequence when
attempting to execute
.Ar path ,
one per line.
The first one for which
.Xr execvp 3
succeeds will be used.
.Pp
Note that if multiple formats match an executable, then the order is in
general not defined, and may not be preserved between
.Nm
operations, so you should generally try to ensure that this option prints at
most one line for any given
.Ar path .
The exception to this is that any format with a userspace detector will be
run before any format without a userspace detector.
.El
.Ss BINARY FORMAT SPECIFICATIONS
.Bl -tag -width 4n
.It Fl Fl magic Ar byte-sequence
This matches all files with the magic number
.Ar byte-sequence .
Hexadecimal escapes may be included in the
.Ar byte-sequence
by preceding them with \ex, for example
.Sq \ex0a
for a linefeed.
Remember to protect such escapes with quotes or an additional backslash to
prevent their interpretation by the shell.
.Pp
Also see
.Fl Fl offset
and
.Fl Fl mask .
.It Fl Fl offset Ar offset
This is the offset of the magic/mask in the file, counted in bytes.
The default is 0.
Only valid with
.Fl Fl magic .
.It Fl Fl mask Ar byte-sequence
This mask will be logically-ANDed with the string to be checked against the
magic number given with
.Fl Fl magic .
The default is all 0xff, i.e. no effect.
Only valid with
.Fl Fl magic .
.It Fl Fl extension Ar extension
This matches all files whose names end in
.Qq Pf . Ar extension .
Hexadecimal escapes are not recognized here.
Extension matching is case-sensitive.
.It Fl Fl detector Ar 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.
.It Fl Fl credentials Cm yes , Fl Fl credentials Cm 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.
.El
.Ss FORMAT FILES
A format file is a sequence of options, one per line, corresponding roughly
to the options given to an
.Fl Fl install
command.
Each option consists of a key, followed by whitespace, followed by a value.
.Pp
The
.Ar package
option should be set to the current package.
The
.Ar interpreter
option should be set to the path to the interpreter that will handle this
binary format.
The
.Ar magic ,
.Ar offset ,
.Ar mask ,
.Ar extension ,
.Ar detector ,
and
.Ar credentials
options correspond to the command-line options of the same names.
.Sh EXIT STATUS
.Bl -tag -width 4n
.It 0
The requested action was successfully performed.
.It 2
Problems were encountered whilst parsing the command line or performing the
action.
.El
.Sh EXAMPLES
This format file can be used with an interpreter capable of handling Java
\&.class files:
.Bd -literal
    package javawrapper
    interpreter /usr/bin/javawrapper
    magic \exca\exfe\exba\exbe
.Ed
.Pp
This corresponds roughly to the following command:
.Bd -literal
    update\-binfmts \-\-package javawrapper \e
        \-\-install javawrapper /usr/bin/javawrapper \e
        \-\-magic \(aq\exca\exfe\exba\exbe\(aq
.Ed
.Sh NOTES
If you're not careful, you can break your system with
.Nm .
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
.Nm
to fix it.
In the future
.Nm
may have some checks to prevent this sort of thing happening accidentally,
though of course you can still manipulate the binfmt_misc kernel module
directly.
.Sh AUTHOR
.An -nosplit
.Nm
is copyright (C) 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
2010, 2011
.An Colin Watson Aq cjwatson@debian.org .
See the GNU General Public License version 3 or later for copying
conditions.
.Pp
You can find the GNU GPL v3 in
.Pa /usr/share/common\-licenses/GPL\-3
on any modern Debian system.
.Pp
Richard Guenther wrote the binfmt_misc kernel module.
.Sh THANKS
Ian Jackson wrote
.Ic update\-alternatives
and
.Ic dpkg\-divert ,
from which this program borrows heavily.