Documentation/binfmt_misc.txt - arm/linux - Git at Google


            
                 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.

            
            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/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.


            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/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

            
            cause unexpected behaviour and can be a security hazard.


            Richard Günther <rguenth@tat.physik.uni-tuebingen.de>
	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.
	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/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.

	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/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
	cause unexpected behaviour and can be a security hazard.


	Richard Günther <rguenth@tat.physik.uni-tuebingen.de>