Limine BIOS chainloading enhancements

July 27, 2022 — BarryK

A couple of days ago, was working on what was planned to become Limine Installer version 1.2, when hit a problem with BIOS chainloading. Here is an overview of the situation so far, leading up to a solution for the problem...

Introducing MBR_ID

Github member mintsuki has been very helpful, implementing enhancements when I have reported issues. When I first got involved with Limine, drive chainloading on a BIOS-firmware computer was this:

Chainload protocol on BIOS:

DRIVE - The 1-based drive to chainload, if omitted, assume boot drive.
PARTITION - The 1-based partition to chainload, if omitted, or set to 0, chainload drive (MBR).

A problem with DRIVE number is that it may change. I raised this as an issue, and mintsuki introduced MBR_ID:

    DRIVE - The 1-based drive to chainload, if omitted, assume boot drive.
    PARTITION - The 1-based partition to chainload, if omitted, or set to 0, chainload drive (MBR).
    MBR_ID - Optional. If passed, use an MBR ID (32-bit hex value) to identify the drive containing the volume to chainload.

In a MBR drive, you can obtain that 32-bit number like this:

# lsblk --pairs -o PTUUID,PTTYPE --nodeps /dev/sdc
PTUUID="c5e0f2d2" PTTYPE="dos"

PTUUID is a unique number identifying that drive.

VBR and PARTITION number

Good, but how is that PARTITION parameter used? We need to know about the Volume Boot Record (VBR), also known as the Partition Boot Record (PBR), see explanations here:

https://whereismydata.wordpress.com/2008/08/23/file-systems-mbr-and-volume-boot-record-basic/

http://glennastory.net/boot/pbr.html

This topic of the VBR arose because I couldn't see how to chainload Windows XP or 7, as they require executing binary files 'ntldr' or 'bootmgr', respectively.

Note, if the BIOS computer has been upgraded to Windows 10, that will also require executing 'bootmgr'. Windows 10+ on UEFI computers is a different situation, requiring launching a EFI file, that Limine can do.

Limine does not have to execute those binaries directly, it is done via the VBR. If 'ntldr' or 'bootmgr' is found in a vfat partition, say partition 1, then Limine Installer will create an entry, say:

:Windows XP
    COMMENT=Windows bootup
    PROTOCOL=chainload
    MBR_ID=c5e0f2d2
    PARTITION=1

A likely scenario is that the computer has one drive, with Windows XP or 7 (or 10), and you have installed Limine to the MBR, thus supplanting the previous Windows bootup. So, this chainloading brings back Windows as an entry in the Limine menu.

Introducing GPT_UUID

Now, the problem mentioned in first paragraph of this blog post...

On a legacy-BIOS computer, the drive most likely has a MBR; however, not necessarily, it may have a GPT (GUUID Partition Table) -- which supports drives greater than 2TB, as well as other benefits. I raised an issue about this:

https://github.com/limine-bootloader/limine/issues/201

mintsuki added an enhancement:

Chainload protocol on BIOS:

    DRIVE - The 1-based drive to chainload, if omitted, assume boot drive.
    PARTITION - The 1-based partition to chainload, if omitted, or set to 0, chainload drive (MBR).
    MBR_ID - Optional. If passed, use an MBR ID (32-bit hex value) to identify the drive containing the volume to chainload. Overrides DRIVE, if present, but does not override PARTITION.
    GPT_UUID or GPT_GUID - Optional. If passed, use the GPT GUID to identify the drive containing the volume to chainload. Overrides DRIVE and MBR_ID, if present, but does not override PARTITION.

That GPT_UUID is obtained with lsblk, same as before:

# lsblk --pairs -o PTUUID,PTTYPE --nodeps /dev/sda
PTUUID="91da59bf-979c-4d2c-b0da-a758cb1ba74f" PTTYPE="gpt"

It is very unlikely that a computer with Windows XP has a GPT drive, but a Windows 7 PC might, or it could have been a user-upgrade. So, the Limine entry, if 'bootmgr' binary is in partition 1:

:Windows 7
    COMMENT=Windows bootup
    PROTOCOL=chainload
    GPT_UUID=91da59bf-979c-4d2c-b0da-a758cb1ba74f
    PARTITION=1

GPT_UUID has been introduced in Limine version 3.13, so I have compiled that in OpenEmbedded. Uploaded here:

https://distro.ibiblio.org/easyos/amd64/packages/compat/oe/dunfell/limine-3.13-r9-nocona-64.tar.xz

Introducing chainload_next

To round off this post, mintsuki also implemented "chainload_next" protocol, after discussion here:

https://github.com/limine-bootloader/limine/issues/191

The documentation:

chainload_next protocol: This protocol does not specify any locally assignable key on BIOS. Will boot the next bootable drive found in the system, if there is one.

In a BIOS computer, with two drives, if you had installed the second drive with Limine bootloader, if the first drive has Windows, Grub, or whatever, you can chainload to it with this protocol.

Tags: easy