The loli Bootloader

A bootloader meant to be small and useful for experienced users. Just like systemd-boot, most functionaility is implemented through UEFI API.

Currently WIP. Please wait for future releases, unless you're experienced with UEFI and Linux bootflow.

Limitation

• Could read boot-files from ESP partition only. We could support misc parititions in the future, but are still limited by the types supported by UEFI firmware (most of the time this just means FAT-family).
• Loads EFI binaries only (for Linux kernels, EFIstub must be enabled).
• For devicetree systems, EFI_DT_FIXUP_PROTOCOL_GUID must be implemented in firwmare to correctly fix up devicetrees, since we don't carry a libfdt.
• initrd is passed through LINUX_EFI_INITRD_MEDIA_GUID configuration table, which is only supported by Linux 6.1 or later.
• Building involves a Python script taken from systemd-boot, there's a plan to rewrite it in Lua or C.
• Extlinux-style configuration. I don't want to create one more new format.

Supported Platforms

• x86_64 (amd64): ACPI
• riscv64: devicetree and ACPI
• loongarch64: devicetree (untested) and ACPI
• aarch64: devicetree and ACPI

Configuration File

The loli bootloader searches for /loli.cfg in ESP as its configuration file. Its format is extlinux-like, for example,

timeout 3
default l2

label l1
	menu title Linux RISC-V Boot Debug
	kernel  /vmlinux-riscv
	initrd  /initramfs.gz
	append  earlycon rootwait console=ttyS0 loglevel=9 memblock=debug

label l2
	menu title Linux RISC-V Boot Quiet
	kernel  /vmlinux-riscv
	initrd  /initramfs.gz
	append  quiet rootwait console=ttyS0

Supported keys outside an entry

• default: Specify the default entry to boot. Its value should be the label of the default entry. Without specifying, the first entry is booted automatically.
• timeout: Specify timeout before booting the first entry. 0 means no timeout and is the default value.

Supported keys inside a label

• kernel
• initrd: Optional
• append: Optional, command arguments to be passed to the kernel.
• menu title: Optional, pretty description of the entry. When unspecified, the entry's label is shown in boot menu instead.

Note that white space characters are permited in labels, so it's usually unnecessary to use menu title in hand-written configuration.

Treatment to malformed entries

As long as the entry cannot be understood by loli, it's automatically skipped.

How to Build

You need to install

• GCC or Clang
• Python 3
• Python package pyelftools

Simply run make in the top directory, which should soon complete with loli.efi produced.

Useful Targets

• loli.efi: The UEFI application in PE format. This is the default.
• loli.elf: The ELF application, should be converted to a PE binary (by using tools/elf2efi.py) before booting.
• clean: Clean the project up.

Useful Makefile Variables

• ARCH: Specify the architecture to build. This variable could be automatically detected with uname -m. Possible values:
  • x86_64
  • riscv64
  • loongarch64
  • aarch64
• CC: Target compiler.
• CCAS: Target assembler.
• CCLD: Target linker.
• CFLAGS: Add extra flags when building C files.
• CCASFLAGS: Add extra flags when assembling assembly.
• LDFLAGS: Add extra flags when linking the ELF file.
• PYTHON: Should point to a Python-3 compatible Python interpreter.
• DEBUG: When set, loli-loader is built with optimization disabled (instead of the default -O2) and debug info enabled.

For cross-compilation, it's usually necessary to adjust ARCH, CC, CCAS and CCLD. An exception is building with Clang and LLD, where you could alternate ARCH, CFLAGS and LDFLAGS only, LLVM will take care rest of the work.

Note

This is a relatively small project, I don't even bother writing proper dependency tracking rules. Please run a clean build after making breaking changes in the headers.

License

This project is distributed under Mozilla Public License Version 2.0 (SPDX-License-Identifier: MPL-2.0).

Warnings about UEFI

Long-term exposure to this code may cause loss of sanity, nightmares about Windows-style API, or any other number other debilitating side effect. This code is known to cause cancer, birth defects, and reproductive harm.