The Root Task

After initializing itself, the seL4 kernel passes control to a userspace program called the root task, whose image is provided to the kernel at boot-time. The root task's capability space contains capabilities for all memory and boot-time objects. That is to say, the root task spawns with the maximum privilege that a userspace program can have. The kernel provides the root task with a map of its initial capability space in the form of the BootInfo frame. You can read more about the root task's environment in seL4 Reference Manual § 9.1.

Part I of this training will focus on writing from-scratch root tasks in Rust. Some of the code in Part I will be quite low-level, interacting with the language runtime, the linker, and some of the finer details related to bootstrapping an seL4-based system. The only seL4-related crates we will be using are the language runtime and bindings for the seL4 API. Unlike situations where higher-level seL4-related libraries (such as seL4_libs) are used, we will be allocating objects and manage virtual address spaces from scratch.

At boot time, the seL4 kernel and root task are loaded into memory by a kind of bootloader stub referred to as a seL4 kernel loader. For Part I, we will use the kernel loader from the rust-sel4 project. We won't cover how it works or how to use it explicitly in this text. It is built at docker/Dockerfile:94:108, and used at mk/root-task.mk:31:36.