Tosaithe

Tosaithe is a minimalistic UEFI-firmware menu/bootloader. It can chain-load other EFI programs and loaders, including Linux kernels, and has basic support for the Stivale2 boot protocol. It is currently x86-64 only.

This is free software, with as few restrictions as legally possible. It comes with no warranty and will not be supported by the original author. Use it at your own risk.

I began writing Tosaithe when I was experimenting with writing a "toy" OS kernel. Using Stivale2, via its flagship Limine bootloader, allowed me to get off the ground quickly. However, Limine did not (at the time) work properly on my desktop machine and had design flaws that bothered me greatly (eg it wrote to disks at boot time when trying to identify boot partition), which have been fixed now as far as I know. There's little reason to use Tosaithe in preference to Limine now.

Tosaithe mainly serves now as:

An example UEFI bootloader
... written in C++, exceptions and all
... that can be built using standard Linux toolchain (GCC and GNU binutils)
... that doesn't require EDK II or GNU-EFI

It also serves as an example client of the bmcxxabi, bmunwind, and libbmcxx libraries which together provide a C++ runtime and standard library.

Limitations of Stivale2 implementation

For Stivale2, only 64-bit ELF kernels are supported, and most optional features are not implemented (no terminal is provided, no PMRs, only 4-level page tables are supported). High-half kernels are supported but pointers in the information tags given to the kernel will not be adjusted to the high half.

Position independence is not supported (the kernel will be loaded at its nominal address).

The ELF segments must be contiguous and their virtual address must not be less than their file offset. (Both requirements are met by most common ELF files, and it would take a fairly funky linker script or linker to produce an ELF file that Tosaithe won't handle).

The Stivale2 requirement that PIC and APIC IRQs are disabled is not currently implemented. The kernel should ensure IRQs are masked itself before enabling interrupts.

No text mode is available in UEFI. The kernel will be booted regardless of whether it supports support from framebuffer. A framebuffer tag will be provided to the kernel if possible.

Building Tosaithe

Requires GCC and Binutils (may or may not work with Clang/LLVM/LLD). I have built with GCC 9.4.0 and Binutils 2.37. I recommend not trying to use older Binutils as there have been bugs with the PE+ output format support. Binutils must have been built with appropriate support (this is usually the case with distro-provided Binutils, use --enable-targets=x86_64-none-pe,x86_64-none-pep when configuring if building it yourself).

sh clone-libs.sh to clone the dependencies
sh rebuild-libs.sh to build the dependencies (in-tree)
make to build Tosaithe.

Installing Tosaithe

Copy tosaithe.efi to your EFI system partition. You can copy it over \EFI\BOOT\bootx64.efi in order to boot with it, but I highly recommend you don't do that until you're sure that it works; copy it somewhere else and run it via Grub or the UEFI shell for example. (Check your motherboard manual for access to see if UEFI shell access is possible; check web for help using it).

You will need a tosaithe.conf text file (UTF-8) in the root directory of the same partition. This should look something like:

entry: {
    description = 'Linux - 5.10.47'
    type = chain
    exec = '\vmlinuz-5.10.47'
    cmdline = 'linux root=/dev/sda1'
}

entry: {
    description = 'EFI Shell'
    type = chain
    exec = '\EFI\Shell.efi'
    cmdline = 'Shell.efi'
}

entry: {
    description = 'My Stivale2 Kernel'
    type = stivale2
    exec = '\mykernel.elf'
    cmdline = ''
}

Note that when chaining to an EFI program (when type = chain) the command line should include the program name as first argument. Paths are on the boot partition (or rather the partition that Tosaithe itself is run from).

Using Tosaithe

When Tosaithe runs it reads the configuration file, prints a banner and lists the entries found in order. The entries are numbered and to select an entry you press the corresponding number on the keyboard.

This is of course very limited - it only properly supports up to 9 entries.