Binfmt misc

binfmt_misc (Miscellaneous Binary Format) is a capability of the Linux kernel which allows arbitrary executable file formats to be recognized and passed to certain user space applications, such as emulators and virtual machines. It is one of a number of binary format handlers in the kernel that are involved in preparing a user-space program to run.

The executable formats are registered through the special purpose file system  file-system interface (usually mounted under part of /proc). This is either done directly by sending special sequences to the  procfs file or using a wrapper like Debian-based distributions binfmt-support package or systemd's.

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


 * is the name of the binary format.
 * is either  or
 * If it is, the executable file format is identified by its filename extension:   is the file extension to be associated with the binary format;   and   are ignored.
 * If it is, the format is identified by   number at an absolute   (defaults to  ) in the file and   is a bitmask (defaults to all  ) indicating which bits in the number are significant.
 * is a program that is to be run with the matching file as an argument.
 * (optional) is a string of letters, each controlling a certain aspect of  invocation:
 * to preserve the original program name typed by user in command line — by adding that name to ; the   has to be aware of this so it can correctly pass that additional parameter to the interpreted program as its.
 * to open the program file and pass its file descriptor to the  so it could read an otherwise unreadable file (for which the user does not have the "Read" permission).
 * to determine new process credentials based on program file rather than  file (see setuid); implies   flag.
 * 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  directory which can be read to get information about a given file format.

Common usage

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