/dev/secret: Minix Character Device Driver

Implemented a character device driver for Minix 3 that exposes a /dev/secret interface for storing and retrieving a secret between processes. The device enforces ownership: only one user can hold the secret at a time, and only the owner can read it. Ownership can be transferred to another user via an ioctl call. The driver integrates with the Minix System Event Framework (SEF) and preserves its full state across live update events.

The trickiest part of the driver was getting the secret reset condition right. The secret should persist after the writer closes their file descriptor, so another process can come along and read it. But once any read file descriptor has been opened, the secret should wipe itself when the last fd closes. That required tracking not just an open count, but whether a read fd had ever been opened during the current ownership. Getting the ordering of those state transitions correct took the most careful thought of anything in the project.

This project gave me a concrete feel for what it means to work below the filesystem. You cannot use normal libc IO, you cannot seek, and when a user process hands you a buffer address it is a virtual address in their address space, not yours. The kernel has to mediate every byte transfer, which is what sys_safecopyto and sys_safecopyfrom are for. That constraint also forced me to treat the secret as raw bytes rather than a string, since there is no guarantee the contents are null-terminated or printable. More broadly, the project gave me exposure to how a real OS structures its driver framework, how services register and communicate, and how live updates can be supported without losing state.