binfmt_misc

binfmt_misc (Miscellaneous Binary Format) is a capability of the

Registration

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The register file contains lines which define executable types to be handled. Each line is of the form:

:name:type:offset:magic:mask:interpreter:flags

• name is the name of the binary format.
• type is either E or M
  • If it is E, the executable file format is identified by its filename extension: magic is the file extension to be associated with the binary format; offset and mask are ignored.
  • If it is M, the format is identified by
    bitmask (defaults to all 0x
    FF) indicating which bits in the number are significant.
• interpreter is a program that is to be run with the matching file as an argument.
• flags (optional) is a string of letters, each controlling a certain aspect of interpreter invocation:
  • P to preserve the original program name typed by user in command line — by adding that name to argv; the interpreter has to be aware of this so it can correctly pass that additional parameter to the interpreted program as its argv[0].
  • O to open the program file and pass its file descriptor to the interpreter so it could read an otherwise unreadable file (for which the user does not have the "Read"
    permission
    ).
  • C to determine new process credentials based on program file rather than interpreter file (see setuid); implies O flag.
  • F to make the kernel open the binary at configuration time instead of lazily at startup time, so that it is available inside other mount namespaces and chroots as well.

Each format has a corresponding file entry in the /proc/sys/fs/binfmt_misc directory which can be read to get information about a given file format.

Deregistration

$ echo -1 >/proc/sys/fs/binfmt_misc/status # all entries
$ echo -1 >/proc/sys/fs/binfmt_misc/qemu-xtensa # single entry

Common usage

• binfmt_misc allows Java programs to be passed directly to the Java virtual machine.^[6]
• binfmt_misc allows identification of
  PE executables
  using the magic number "MZ". Examples:
  • :DOSWin:M::MZ::/usr/bin/wine: will hand all such files to Wine (thus assuming they are ordinary MS-DOS or Microsoft Windows executable)
  • :CLR:M::MZ::/usr/bin/mono: will hand such files to Mono (thus assuming they are .NET executables)^[7][8]
  • Microsoft's WSL1 use binfmt to allow for calling Windows programs from Linux. The line is equivalent to :WSLInterOP:M::MZ::/init:P.
• binfmt_misc can also be combined with QEMU or Box86 to execute programs for other processor architectures as if they were native binaries.^[9]
• binfmt can be used to turn some compiled languages such as Go into scripting languages, acting as a substitute for the
  shebang line.^[10]

• Shebang (Unix)

References

External links

• Project home page Archived on archive.org