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 binfmt_misc
file-system interface (usually mounted under part of /proc). This is either done directly by sending special sequences to the register
procfs file or using a wrapper like Debian-based distributions binfmt-support package or systemd's systemd-binfmt.service
.
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 eitherE
orM
- 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
andmask
are ignored. - If it is
M
, the format is identified bymagic
number at an absoluteoffset
(defaults to0
) in the file andmask
is a bitmask (defaults to all0xFF
) indicating which bits in the number are significant.
- If it is
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 ofinterpreter
invocation:P
to preserve the original program name typed by user in command line — by adding that name toargv
; theinterpreter
has to be aware of this so it can correctly pass that additional parameter to the interpreted program as itsargv
.O
to open the program file and pass its file descriptor to theinterpreter
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 thaninterpreter
file (see setuid); impliesO
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.
- 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)- 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.
- binfmt can be used to turn some compiled languages such as Go into scripting languages, acting as a substitute for the shebang line.
See also
References
- Kernel Support for miscellaneous Binary Formats (binfmt_misc) The Linux Kernel documentation
- How programs get run, January 28, 2015, by David Drysdale, LWN.net
- Details of package binfmt-support in sid, Debian
- systemd-binfmt.service, freedesktop.org
- binfmt.d, freedesktop.org
- Java(tm) Binary Kernel Support for Linux v1.03 The Linux Kernel documentation
- Mono(tm) Binary Kernel Support for Linux The Linux Kernel documentation
- Guide:Running Mono Applications
- Official QEMU mirror, QEMU, 23 November 2022
- Using Go as a scripting language in Linux, Cloudflare, 2018-02-20, retrieved 2019-05-05
External links
- Project home page Archived on archive.org