wikkyk / mlb

Minimal Linux Bootloader, adapted/adopted from Sebastian Plotz

Minimal Linux Bootloader

Minimal Linux Bootloader, adapted/adopted from Sebastian Plotz.

This is a single stage x86 bootloader that can boot a single Linux kernel. There is no support for initrd and only one kernel can be configured to boot at any time.

While I have no plans for adding initrd support, I'm willing to consider it if there is interest.

This bootloader is probably aimed at enthusiasts who are not afraid of breaking things and know how the Linux boot process works. I am not responsible if you render your machine unbootable with MLB.

MLB was originally written by Sebastian Plotz. I've changed the code very slightly and wrote the Makefile and mlbinstall.

http://sebastian-plotz.blogspot.de/

Dependencies

• a C99 compiler (tested with GCC 4.7.3)
• an x86 machine (duh)
• make (tested with GNU Make 3.82)
• nasm (tested with NASM 2.10.07)
• xxd (tested with xxd 1.10)

Building

Simply run make. You will get a binary called mlbinstall which you can use to install the bootloader. If you want to, stick it in /bin or something.

Installing

Don't forget to backup your MBR!

dd if=/dev/sdXN of=/boot/mbr.bak bs=512 count=1

To install MLB, simply invoke

mlbinstall <target> <kernel> <command line> [-vbr]

where:

• <target> is where you want your bootloader installed (e.g. a file, a block device, whatever)
• <kernel> is the kernel file to boot
• <command line> is the command line to pass to the kernel - don't forget to pass root= as mlbinstall will not calculate the root device. Currently the command line cannot be longer than 80 bytes (more just won't fit in the MBR).
• [-vbr] will make mlbinstall ignore the space for the partition table, giving you 144 bytes for the command line. You can safely use it if you're installing to a Volume Boot Record (partition), e.g. for using with another bootloader in multi-OS setups.

mlbinstall is quite noisy if it detects any problems. Restore your MBR from backup, fix the problems, and try again. If it succeeds, it won't say anything.

Error codes

If MLB fails to boot your kernel, it will print a one-letter error code. Currently, there are two error codes:

Error What it means
----- ----------------------------------------
  R   Failed reading data from disk
  M   Failed moving data to its final location

Contact

Email patches, pull requests, help requests to wiktor /at\ brodlo \dot/ net

Alternatively, send pull requests and/or open issues on github: https://github.com/wiktor-b/mlb

Thanks!

• wiktor

mlb's Issues

chainloading issue

read_from_hdd assumes that mlb was installed on the first hard disk (0x80). This assumption breaks when mlb was chainloaded by another bootloader from another disk. BIOS assigns 0x80 to the first hdd according to boot order.

I am not familiar with real mode programming, it might be possible to enumerate all disks and compare MBR disk signature at 0x1B8 to find out which hdd.

Disk Address Packet

You could move the disk address packet onto the stack, to save some more bytes in the binary / allow a longer commandline. I saw that being done in the freebsd boot0 loader.

EDIT0: should save 16 Bytes, but needs two instructions to reserve and free stack space....
EDIT1: here's the link: https://github.com/freebsd/freebsd/blob/master/sys/boot/i386/boot0/boot0.S#L561

Feature Request: initramfs support

Hi,

since I am looking for a replacement for syslinux to save space on my 3,5" 1440kB FDD based Linux Distribution I was kinda directed to this project...

As you said, there is currently no initramfs support but you'd consider it if asked for.

And I'd like to make that request since CONFIG_INITRAMFS_SOURCE would really make it a lot harder and slower to build OS/1337.

Also it would allow the Minimal Linux Bootloader or mlb to be a much easier drop-in replacement for syslinux.

Thanks in advance for your time and feedback.

Yours Faithfully,

Kevin Karhan
OS/1337 Maintainer