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booting.md

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How to boot almost anywhere

Ward seeks to boot almost anywhere. To serve that purpose ward is distributed in two separate forms:

  • A multiboot and multiboot2 compliant ELF file that can co-exist with other operating systems in the /boot directory.

  • A raw disk image that can be written to a flash drive via dd, or passed as a boot drive to QEMU and other hypervisor software

MBR

Legacy x86 systems boot by copying the first 512 byte sector of the hard drive to address 0x7c00 and then jumping to that location. At this point the system is still in 16-bit mode and a considerable amount of bootstraping is required to switch to 64-bit protected mode and load the rest of the kernel into memory.

This method is mostly obsolete on bare metal systems, but is frequently required by cloud providers so it still must be supported.

UEFI

The Universal Extensible Firmware Interface defines a procedure for loading kernel binaries from disk and starting them. Unlike the MBR method, UEFI understands FAT32 filesystems and Microsoft's PE32+ executable format eliminating the need for a dedicated bootloader.

There are a couple different approaches for handling the executable format requirement:

  • For kernels that don't need to support other boot environments, they can be distributed only as PE32+ binaries. These binaries can be produced directly by a compiler or converted via objcopy. This is the approach used by Windows.

  • To support UEFI and its own bootloader format at the same time, Linux exposes a EFI Stub. This lets the Linux kernel binary masquerade as a PE32+ binary well enough to get firmware to load it.

  • The kernel can be in a completely different binary format, but get chainloaded by a UEFI compatible bootloader like GRUB2. This is the approach that Ward uses.

GRUB

GRUB2 provides several different installation methods, but they all either require being run from within a running system or make undesirable assuptions about the disk layout. Since Ward wants more flexibility, we assemble our own disk image by piecing together a couple of components.

boot.img

This file contains the initial boot code to place into the first sector of the hard drive (the MBR). It is 512 bytes long, although only the first 440 bytes should be copied to avoid overwriting partition information.

One important detail is that the GRUB installer actually replaces bytes 92-99 of this file with the offset (in 512 byte sectors) to the start of the main GRUB binary.

cp /usr/lib/grub/i386-pc/boot.img .
python3 -c "print('\x40')" | dd of=boot.img bs=1 seek=92 count=1 conv=notrunc 2> /dev/null

core.img

This file contains the main GRUB installation for MBR systems. It is generated via the grub-mkimage command and has all necessary modules compiled into it. You also have the option of passing an early config file to specify the root partition and where to look for the main config file.

grub-mkimage -O i386-pc -o core.img -p '/' -c grub/grub-early.cfg \
    biosdisk normal search part_msdos part_gpt fat multiboot multiboot2 gfxmenu echo probe

It turns out that boot.img actually only loads the first 512 bytes of core.img, so more information is required to get the rest loaded into memory. This initial prefix (called diskboot.img) contains a hardcoded offset and length for the core file. Note that some documentation describes this as a "block list", but it is actually only a single entry.

The mkimage command sets the length correctly, but it doesn't actually know the disk offset that core.image will be placed at so that must be handled separately. In particular, we must overwrite bytes 500-503 with the correct offset in sectors:

python3 -c "print('\x41')" | dd of=core.img bs=1 seek=500 count=1 conv=notrunc 2> /dev/null

grub.efi

The previous two files are sufficient for booting in MBR mode, but we would also like to support UEFI. This turns out to be a bit simpler, and no binary patching is required:

grub-mkimage -O x86_64-efi -o $@ -p '/' -c grub/grub-early.cfg \
		normal search part_msdos part_gpt fat multiboot multiboot2 gfxmenu echo video probe

Disk layout

The resulting .efi file should be renamed to bootx64.efi and placed in the /efi/boot directory of a bootable FAT32 partition on the disk.