131 lines
7 KiB
Text
131 lines
7 KiB
Text
Kernel Support for miscellaneous (your favourite) Binary Formats v1.1
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=====================================================================
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This Kernel feature allows you to invoke almost (for restrictions see below)
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every program by simply typing its name in the shell.
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This includes for example compiled Java(TM), Python or Emacs programs.
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To achieve this you must tell binfmt_misc which interpreter has to be invoked
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with which binary. Binfmt_misc recognises the binary-type by matching some bytes
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at the beginning of the file with a magic byte sequence (masking out specified
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bits) you have supplied. Binfmt_misc can also recognise a filename extension
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aka '.com' or '.exe'.
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First you must mount binfmt_misc:
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mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
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To actually register a new binary type, you have to set up a string looking like
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:name:type:offset:magic:mask:interpreter:flags (where you can choose the ':'
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upon your needs) and echo it to /proc/sys/fs/binfmt_misc/register.
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Here is what the fields mean:
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- 'name' is an identifier string. A new /proc file will be created with this
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name below /proc/sys/fs/binfmt_misc; cannot contain slashes '/' for obvious
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reasons.
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- 'type' is the type of recognition. Give 'M' for magic and 'E' for extension.
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- 'offset' is the offset of the magic/mask in the file, counted in bytes. This
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defaults to 0 if you omit it (i.e. you write ':name:type::magic...'). Ignored
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when using filename extension matching.
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- 'magic' is the byte sequence binfmt_misc is matching for. The magic string
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may contain hex-encoded characters like \x0a or \xA4. Note that you must
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escape any NUL bytes; parsing halts at the first one. In a shell environment
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you might have to write \\x0a to prevent the shell from eating your \.
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If you chose filename extension matching, this is the extension to be
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recognised (without the '.', the \x0a specials are not allowed). Extension
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matching is case sensitive, and slashes '/' are not allowed!
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- 'mask' is an (optional, defaults to all 0xff) mask. You can mask out some
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bits from matching by supplying a string like magic and as long as magic.
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The mask is anded with the byte sequence of the file. Note that you must
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escape any NUL bytes; parsing halts at the first one. Ignored when using
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filename extension matching.
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- 'interpreter' is the program that should be invoked with the binary as first
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argument (specify the full path)
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- 'flags' is an optional field that controls several aspects of the invocation
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of the interpreter. It is a string of capital letters, each controls a
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certain aspect. The following flags are supported -
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'P' - preserve-argv[0]. Legacy behavior of binfmt_misc is to overwrite
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the original argv[0] with the full path to the binary. When this
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flag is included, binfmt_misc will add an argument to the argument
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vector for this purpose, thus preserving the original argv[0].
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e.g. If your interp is set to /bin/foo and you run `blah` (which is
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in /usr/local/bin), then the kernel will execute /bin/foo with
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argv[] set to ["/bin/foo", "/usr/local/bin/blah", "blah"]. The
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interp has to be aware of this so it can execute /usr/local/bin/blah
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with argv[] set to ["blah"].
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'O' - open-binary. Legacy behavior of binfmt_misc is to pass the full path
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of the binary to the interpreter as an argument. When this flag is
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included, binfmt_misc will open the file for reading and pass its
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descriptor as an argument, instead of the full path, thus allowing
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the interpreter to execute non-readable binaries. This feature
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should be used with care - the interpreter has to be trusted not to
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emit the contents of the non-readable binary.
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'C' - credentials. Currently, the behavior of binfmt_misc is to calculate
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the credentials and security token of the new process according to
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the interpreter. When this flag is included, these attributes are
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calculated according to the binary. It also implies the 'O' flag.
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This feature should be used with care as the interpreter
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will run with root permissions when a setuid binary owned by root
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is run with binfmt_misc.
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'F' - fix binary. The usual behaviour of binfmt_misc is to spawn the
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binary lazily when the misc format file is invoked. However,
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this doesn't work very well in the face of mount namespaces and
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changeroots, so the F mode opens the binary as soon as the
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emulation is installed and uses the opened image to spawn the
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emulator, meaning it is always available once installed,
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regardless of how the environment changes.
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There are some restrictions:
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- the whole register string may not exceed 1920 characters
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- the magic must reside in the first 128 bytes of the file, i.e.
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offset+size(magic) has to be less than 128
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- the interpreter string may not exceed 127 characters
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To use binfmt_misc you have to mount it first. You can mount it with
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"mount -t binfmt_misc none /proc/sys/fs/binfmt_misc" command, or you can add
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a line "none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your
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/etc/fstab so it auto mounts on boot.
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You may want to add the binary formats in one of your /etc/rc scripts during
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boot-up. Read the manual of your init program to figure out how to do this
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right.
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Think about the order of adding entries! Later added entries are matched first!
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A few examples (assumed you are in /proc/sys/fs/binfmt_misc):
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- enable support for em86 (like binfmt_em86, for Alpha AXP only):
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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
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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
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- enable support for packed DOS applications (pre-configured dosemu hdimages):
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echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register
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- enable support for Windows executables using wine:
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echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
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For java support see Documentation/java.txt
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You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable)
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or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name.
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Catting the file tells you the current status of binfmt_misc/the entry.
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You can remove one entry or all entries by echoing -1 to /proc/.../the_name
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or /proc/sys/fs/binfmt_misc/status.
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HINTS:
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======
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If you want to pass special arguments to your interpreter, you can
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write a wrapper script for it. See Documentation/java.txt for an
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example.
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Your interpreter should NOT look in the PATH for the filename; the kernel
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passes it the full filename (or the file descriptor) to use. Using $PATH can
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cause unexpected behaviour and can be a security hazard.
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Richard Günther <rguenth@tat.physik.uni-tuebingen.de>
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