Open Source Summit + Embedded Linux Conference + OSPOCon 2021

File ownership is a global property on most systems that have a uid and gid concept. On POSIXy systems the chown*() syscall family allows to change the owner of a file or directory. If the ownership of a file is changed it will be changed for each user on the systems equally. But various use-cases exist where this can be problematic: - Portable home directories that are used on different computers where the user is assigned a different uid and gid. - Filesystems that allow to merge or unionize multiple filesystems are often shared between different users. - On Linux user namespaces used in containers also affect file ownership. - Chowning files on really large filesystems is costly. Idmapped mounts available with Linux 5.13 solve these problems and others by making it possible to change ownership on a per-mount point basis. In this talk we will take a close look technical at the idmapped mount feature, the use-cases it is intended to solve, and we will demo it's capabilities.