Linux boot

Boot examples with Linux. A small Linux kernel with initrd is loaded in a Qemu VM with different disk partitions and bootloaders.

Development and contributions are described below.

Most tasks can be done with the admin.sh script:

./admin.sh                     # Help printout
./admin.sh env                 # Current env
./admin.sh versions            # Used SW
./admin.sh setup               # Builds everything needed

After setup you can test boot with different images:

./admin.sh image_build --type=gpt   # default
./admin.sh qemu --uefi
./admin.sh image_build --type=fat   # requires sudo
./admin.sh qemu
./admin.sh image_build --type=mbr   # requires sudo
./admin.sh qemu

Tools:

sfdisk - Handle partitions from script
sgdisk - Command-line GPT manipulator
udisksctl - Mount images without root or sudo

Disks

Disks are created as "raw" images. This is simply a file, created for example with:

#cd to/temp/dir
truncate -s 16MiB hd.img
ls -lsh hd.img
0 -rw-rw-r-- 1 uablrek uablrek 16.0M Aug 13 12:16 hd.img

This is a sparse file. It has a size of 16MiB, but occupy 0 (zero) bytes on disk. That's why truncate is preferred before fallocate (dd is the worst choice, and should be the last resort).

The disks can be partitioned with MBR (Master Boot Record), or with GPT (GUID Partition Table). The sfdisk tool is used to manipulate partitions. Example:

sfdisk hd.img <<EOF
label: gpt
,4MiB,U,
,,L,
EOF
Device       Start   End Sectors Size Type
hd.img1     2048 10239    8192   4M EFI System
hd.img2    10240 30719   20480  10M Linux filesystem

Create a FAT file system and loop-back mount:

mkfs.fat --offset 2048 hd.img 8192
udisksctl loop-setup -f hd.img
dev=(from the printout above)
udisksctl mount -b ${dev}p1 
# copy files (as your user) to the directory in the printout
udisksctl unmount -b ${dev}p1
udisksctl loop-delete -b $dev

MBR

Master Boot Record is the older way of booting a PC. It is used for both hard-disks with partitions and media without partitions, originally floppys, but now USB-sticks, SD-cards, etc.

Media without partitions

This is the simplest case:

Firmware (BIOS) loads the first 512 bytes of the disk. This is the primary bootloader, and is unique for any secondary bootloader. It is file-system unaware
BIOS exectutes the primary bootloader. It's only job is to load and start the secondary bootloader.
The secondary bootloader is not limited in size and is likely file-system aware. It's primary task is to load and start the Linux kernel. It may present a load menu to the user

As an example we take syslinux. Syslinux works on FAT file-systems.

truncate -s 16MiB hd.img
mkfs.fat hd.img
sudo syslinux -i hd.img
dd if=hd.img bs=512 count=1 status=none | od -t x1 -Ax --endian=big

The syslinux -i installs the primary bootloader in the MBR can copies the secondary bootloader to the file system. It consists of two files: ldlinux.sys and ldlinux.c32.

Media with partitions

The MBR now holds a partition table at the end (0x1be-0x1fd). The table holds 4 partition entries of 16 byte. Let's check it out!

rm -f hd.img
truncate -s 32MiB hd.img
sfdisk --no-reread --no-tell-kernel hd.img <<EOF
label: mbr
,8MiB,c,*
,,L,
EOF
sfdisk -V -l hd.img
sfdisk -g hd.img
hd.img: 4 cylinders, 255 heads, 63 sectors/track
dd if=hd.img bs=512 count=1 status=none | od -t x1 -Ax --endian=big
0001b0 00 00 00 00 00 00 00 00 bc ff 72 1c 00 00 80 20
0001c0 21 00 0c 25 24 01 00 08 00 00 00 40 00 00 00 25
0001d0 25 01 83 14 10 04 00 48 00 00 00 b8 00 00 00 00
0001e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa

The first partition:

0001be: 80 20 21 00 0c 25 24 01 00 08 00 00 00 40 00 00
80 - active
202100 - CHS-start = 00/20/21, lba = 32 * 63 + (33 - 1) = 2048
0c - Partition = W95 FAT32 (LBA)
252401 - CHS-end = 01/25/24, lba = (255 + 37) * 63 + (36 - 1) = 18431
00 08 00 00 - LBA-start=0x800 (little endian!!)
00 40 00 00 - sectors = 0x4000

LBA = Logical Block Addressing
CHS = Cylinder Head Sector

GPT

The GUID Partition Table (GPT) replaces MBR partitioning. It allows more partitions and larger disks. Let's check it out!

rm -f hd.img
truncate -s 128MiB hd.img
sfdisk --no-reread --no-tell-kernel hd.img <<EOF
label: gpt
,8MiB,U,
,8MiB,L,*
EOF
sfdisk -V -l hd.img
Device     Start   End Sectors Size Type
hd.img1     2048 18431   16384   8M EFI System
hd.img2    18432 34815   16384   8M Linux filesystem
dd if=hd.img bs=512 status=none | od -t x1 -Ax --endian=big
000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0001c0 02 00 ee ff ff ff 01 00 00 00 ff ff 03 00 00 00
0001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa
000200 45 46 49 20 50 41 52 54 00 00 01 00 5c 00 00 00
000210 cf 2e fa 50 00 00 00 00 01 00 00 00 00 00 00 00
000220 ff ff 03 00 00 00 00 00 00 08 00 00 00 00 00 00
000230 de ff 03 00 00 00 00 00 ef 38 9c 97 d2 03 11 4c
000240 80 12 43 fa 71 52 7d f3 02 00 00 00 00 00 00 00
000250 80 00 00 00 80 00 00 00 f3 71 67 54 00 00 00 00
000260 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
000400 28 73 2a c1 1f f8 d2 11 ba 4b 00 a0 c9 3e c9 3b
000410 c7 31 a0 8e f8 6e 00 43 84 c5 da 09 21 61 21 bf
000420 00 08 00 00 00 00 00 00 ff 47 00 00 00 00 00 00
000430 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
000480 af 3d c6 0f 83 84 72 47 8e 79 3d 69 d8 47 7d e4
000490 4b 50 63 6e 86 f8 85 40 bf a1 87 b5 21 79 cb a1
0004a0 00 48 00 00 00 00 00 00 ff 87 00 00 00 00 00 00
0004b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
7ffbe00 28 73 2a c1 1f f8 d2 11 ba 4b 00 a0 c9 3e c9 3b
7ffbe10 c7 31 a0 8e f8 6e 00 43 84 c5 da 09 21 61 21 bf
7ffbe20 00 08 00 00 00 00 00 00 ff 47 00 00 00 00 00 00
7ffbe30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
7ffbe80 af 3d c6 0f 83 84 72 47 8e 79 3d 69 d8 47 7d e4
7ffbe90 4b 50 63 6e 86 f8 85 40 bf a1 87 b5 21 79 cb a1
7ffbea0 00 48 00 00 00 00 00 00 ff 87 00 00 00 00 00 00
7ffbeb0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
7fffe00 45 46 49 20 50 41 52 54 00 00 01 00 5c 00 00 00
7fffe10 91 cc 7f dd 00 00 00 00 ff ff 03 00 00 00 00 00
7fffe20 01 00 00 00 00 00 00 00 00 08 00 00 00 00 00 00
7fffe30 de ff 03 00 00 00 00 00 ef 38 9c 97 d2 03 11 4c
7fffe40 80 12 43 fa 71 52 7d f3 df ff 03 00 00 00 00 00
7fffe50 80 00 00 00 80 00 00 00 f3 71 67 54 00 00 00 00
7fffe60 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
8000000

sgdisk -c 2:"Linux Boot" hd.img
sgdisk -A 2:set:2 hd.img
sgdisk -i2 hd.img
Partition GUID code: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 (Linux filesystem)
Partition unique GUID: 6E63504B-F886-4085-BFA1-87B52179CBA1
First sector: 18432 (at 9.0 MiB)
Last sector: 34815 (at 17.0 MiB)
Partition size: 16384 sectors (8.0 MiB)
Attribute flags: 0000000000000004
Partition name: 'Linux Boot'

UEFI

Unified Extensible Firmware Interface (UEFI) replaces BIOS for PC booting. UEFI can be used with both MBR and GPT formatted disks (i.e there is no GPT requirement!).

QEMU

By default QEMU starts with a BIOS. This must be replaced with Open Virtual Machine Firmware UEFI (OVMF) firmware.

#sudo apt install ovmf
cp /usr/share/OVMF/OVMF_CODE_4M.fd /usr/share/OVMF/OVMF_VARS_4M.fd .
qemu ... \
  -drive if=pflash,format=raw,readonly=on,file=OVMF_CODE_4M.fd \
  -drive if=pflash,format=raw,file=OVMF_VARS_4M.fd

NOTE: In many places the --bios option is proposed. I haven't got that to work. "pflash" works fine though.

EDK2

OVMF is a part of EDK2 which has a build system that is... un-intuitive (to put it nicely). Some good hints can be found here. If you must build it, try:

export edk2d=/your/path/to/edk2
git clone --depth 1 https://github.com/tianocore/edk2.git $edk2d
cd $edk2d
git submodule update --init --depth 1
cd -
./admin.sh edk2_build

The EFI partition

An EFI partition is created with a U type to sfdisk. It should be formatted with a FAT file system. The recommendation is FAT32, but OVMF has no such requirement, but it has this:

NOTE: If FAT32 is used on the EFI partition it must be larger than minimum size for FAT32. 34MiB seems about minimum partition size.

Kernel

Power management and ACPI options >
  [*] ACPI (Advanced Configuration and Power Interface) Support
Processor type and features >
  [*] EFI runtime service support
  [*]   EFI stub support

CONFIG_EFI_STUB - This kernel feature allows a bzImage to be loaded directly by EFI firmware without the use of a bootloader.

Syslinux

Syslinux is maintained (sort of) in kernel.org. Syslinux hasn't been updated since 2014 and can't be built with modern tools (i.e. 1, 2, 3), so a patch is applied when built. Pre-built binaries are included in the archive, so building locally may not be needed.

Syslinux actually contains several bootloaders:

syslinux - For FAT file systems
extlinux - For ext (Linux) file systems
pxelinux - For network boot
isolinux - For bootable CD-rom

In the examples syslinux is used for boot from non-partitioned media and extlinux for MBR partitioned media.

NOTE: The doc's say that syslinux can be used for partitioned media, but I can't get that to work.

Dependencies:

sudo apt install nasm
sudo apt install libgnutls28-dev

U-boot

The U-boot bootloader is the main choice for embedded systems or SoC (System on Chip). There are some really great posts on u-boot in Mike's homepage. Since SoC's often use ARM, --arch=aarch64 should be set.

export __arch=aarch64
./admin.sh setup
./admin.sh qemu
# Rebuild:
./admin.sh uboot_build --menuconfig
./admin.sh uboot-image
./admin.sh qemu

We use GPT partitioned media with an EFI partition. U-boot may work with other configurations, but this seem to be the "normal" way.

Boot script

U-boot will load and execute the boot.scr if it exist on the EFI partition. You can try your own with:

./admin.sh uboot-image --bootscr=/path/to/your/script
./admin.sh qemu

Grub

(work in progress)

GRUB has grown to become huge and complicated. Since I am more interrested in booting SoC (System on Chip) I will focus on U-boot. Maybe I get back to GRUB later.

Build locally:

./admin.sh grub_build --arch=x86_64
./admin.sh grub_build --arch=aarch64

Development and contributions

Issues and PR's are welcome. Please note that the license is CC0-1.0, meaning that everything you contribute will become public domain.

I use Ubuntu Linux, 24.04.2 LTS at the moment. Other Linux distros should work, but are not tested.

Here are some dependencies:

sudo apt install -y efitools ovmf fdisk gcc-aarch64-linux-gnu qemu-system-arm

I am sure there are plenty more. PR's are welcome for updates here.

By default everything is stored under /tmp/tmp/$USER because I mount a tmpfs (ramdisk) on /tmp/tmp for experiments. You may change that by setting the $TEMP environment variable.

Source archives must be downloaded (by you).

./admin.sh versions
linux-6.15.4         (/home/uablrek/archive/linux-6.15.4.tar.xz)
busybox-1.36.1       (/home/uablrek/archive/busybox-1.36.1.tar.bz2)
syslinux-6.03        (/home/uablrek/Downloads/syslinux-6.03.tar.xz)
u-boot-2025.07       (/home/uablrek/Downloads/u-boot-2025.07.tar.gz)
grub-2.12            (/home/uablrek/Downloads/grub-2.12.tar.xz)

They are searched for in $HOME/Downloads:$HOME/archive by default.

The kernel source will be unpacked in $KERNELDIR if necessary, which defaults to $HOME/tmp/linux. The kernel is not built in this directory, so you may write-protect it if you like.

uablrek / linux-boot