binfmtmisc

binfmt_misc is a Linux kernel facility that allows the registration of miscellaneous executable formats so that the kernel can automatically invoke an interpreter or loader to run non-native binaries. It exposes a runtime interface at /proc/sys/fs/binfmt_misc and uses a simple registration mechanism to associate a format with an interpreter or loader. When a program is executed, the kernel checks the registered rules and, if a match is found, delegates execution to the specified interpreter, supplying the binary as an argument. This enables transparent execution of foreign-architecture binaries and other formats without requiring each application to provide its own launcher.

Registration is performed by writing a line to /proc/sys/fs/binfmt_misc/register. A format definition consists of fields such as a name, type, offset, magic or extension, optional mask, the path to the interpreter or loader, and optional flags. The type can indicate a magic-based match or a filename extension match, among other options. A common use is to enable QEMU user-mode emulation so that binaries for architectures like ARM can be run on an x86 host, with the kernel automatically invoking qemu when needed.

binfmt_misc has been part of the Linux kernel as a generic mechanism for handling non-native executable formats for many years. It is typically enabled by loading the binfmt_misc module and can be configured at runtime, offering a lightweight alternative to per-application emulation layers. Security considerations include the fact that any registered format can alter process startup behavior and should be managed carefully.