This section looks into how to manipulate, inspect and create boot images. The details of preparing a suitable kernel or configuring a particular bootloader is beyond the scope of this documentation. Instead, we’ll concentrate on how to look inside available images, what needs to be done to use a different operating system as the rootfs and how to mount, modify or create boot images.
This documentation relies on support present in the Linux kernel. Other kernels can be put inside boot images but using such kernels at runtime to create boot images is beyond the scope of this page.
This often requires specialist knowledge of the particular board and you may be dependent on a landing team or other third party for a kernel configuration and patches. Some sources only provide a binary image, sometimes already combined with a bootloader.
Similar to a kernel, you may have little choice over which bootloader to use, although it is entirely reasonable to use a more limited bootloader provided by someone else to chainload a more capable bootloader which has more functionality. Note - the Linux kernel can be used as a secondary bootloader using kexec. The details of how to do this will vary according to the board, available bootloader and boot requirements.
From here on, this page works on how to get a kernel and bootloader into an image to boot on the device.
LAVA V2 uses GuestFS to remove the need for loop devices, offsets or losetup during LAVA operations. The tools remain useful for times when the image needs to be modified outside LAVA.
First, decompress your image. These examples will assume that the resulting file is called test.img
Print the partition offsets:
$ parted test.img -s unit b print
Model: (file)
Disk /home/linaro/documents/arndale-vmgroup/test.img: 1073741824B
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 512B 4194303B 4193792B primary
2 4194304B 58720255B 54525952B primary fat32 boot, lba
3 58720256B 1073741823B 1015021568B primary ext4
In this example, there is an unused partition starting at an offset of 512 bytes, followed by a VFAT-formatted boot partition starting at an offset of 4194304 bytes, then the main rootfs is an ext4 filesystem in partition 3 starting at an offset of 58720256 bytes.
Other tasks using parted will need root access or sudo.
To mount the boot partition, pass the loop and offset options to mount:
$ sudo mkdir -p /mnt/boot
$ sudo mount -o loop,offset=4194304 test.img /mnt/boot
Note
Failures from mount complaining about a bad superblock can arise from a wrong offset.
When you are finished with the filesystem, make sure you unmount it:
$ sudo umount /mnt/boot
Warning
Remember to check the output of mount and avoid mounting the same partition more than once or moving the image without using umount.
QEMU has easy support for creating empty images:
$ qemu-image create test.img
Use dd to create an empty file which can be used to host partitions and form the basis of a new boot image.
dd can create a file of any size, subject to the free space on your machine. Specify the size of each block to write and the number of blocks. To create an image of 1 GB (1024 MB) use:
$ sudo dd if=/dev/zero of=test.img bs=1M count=1024
Create a partition table. While it is possible to use images without partition tables if all files are in a single filesystem, some devices or bootloaders may refuse to boot from such images:
$ sudo losetup /dev/loop0 test.img
$ sudo parted /dev/sda -s unit mb mktable msdos
If you are copying the layout of a known-working image you can use parted to replicate the partitions. If you just need a boot partition, then allow space for modification. It is very likely that you or someone using your image will want to change the kernel image or test a second kernel. Always try to leave enough space in your boot partition to have a second kernel image. Remember that kernel images may increase in size as more functionality is supported.
Refer to the parted documentation for how to create the partition layout you want and experiment with your empty test image file. parted has an interactive mode which can be used to get used to the tool and the options:
$ sudo parted test.img
One example setup could be:
$ sudo parted /dev/loop0 -s unit mb mkpart primary 1 10
$ sudo parted /dev/loop0 -s unit mb mkpart primary 11 110
$ sudo parted /dev/loop0 -s unit mb mkpart primary 111 1024
parted /dev/loop0 unit B -s print
Model: (file)
Disk /dev/loop0: 1073741824B
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 1048576B 10485759B 9437184B primary
2 10485760B 110100479B 99614720B primary
3 110100480B 1024458751B 914358272B primary
Create a filesystem for each partition. After parted has created the partitions, the loop devices need to be configured to use the offsets declared by parted:
$ sudo losetup -o 10485760 /dev/loop1 /dev/loop0
$ sudo mkfs.vfat /dev/loop1
$ sudo losetup -o 110100480 /dev/loop2 /dev/loop0
$ sudo mkfs.ext3 /dev/loop2
Copy your files onto the new filesystems:
$ sudo mount -o loop,offset=10485760 test.img /mnt/boot/
$ pushd /mnt/boot/
$ sudo tar -xzf /tmp/boot.tar.gz
$ popd
$ sudo umount /mnt/boot/
Clean up your losetup operations:
$ sudo losetup -d /dev/loop2
$ sudo losetup -d /dev/loop1
$ sudo losetup -d /dev/loop0
Ensure that there are no loopback mounts remaining:
$ sudo losetup -a
set the serial console - Each device tends to have a different device used for the serial console, and you may need to configure a serial console login (getty) in your image too. Recent Linux images using systemd should automatically start a getty on the kernel’s default console device, but older images using sysvinit will need some explicit configuration.
For Debian, this would need to be done in /etc/inittab. This example is from an iMX.53 image:
# echo T0:23:respawn:/sbin/getty -L ttymxc0 115200 vt102 >> ./etc/inittab
https://linux.codehelp.co.uk/?p=49
The bootloader settings for the board usually indicate which device is to be used as the serial console.
set default networking - Depending on your bootstrap tool, there may well be no network interfaces defined. For Debian, this can be implemented using a file in /etc/network/interfaces.d/, e.g.:
# echo auto lo eth0 > ./etc/network/interfaces.d/base
# echo iface lo inet loopback >> ./etc/network/interfaces.d/base
# echo iface eth0 inet dhcp >> ./etc/network/interfaces.d/base
set a root password - This is surprisingly easy to forget until after the image has booted. Depending on the distribution, this step can involve using qemu to chroot into the rootfs to be able to execute the passwd utility. Manual changes to /etc/passwd can be ignored, depending on the shadow / authentication precautions implemented by the distribution:
$ sudo cp /usr/bin/qemu-armhf-static ./usr/bin/
$ sudo chroot .
# passwd
# exit
enable the serial console in securetty - e.g. the arndale board has a serial console in a device which does not generally appear in /etc/securetty, so this needs to be added:
# echo ttySAC2 >> ./etc/securetty
set a useful hostname - choose your board hostname and your local domain (so that a fully qualified hostname can be supported):
# echo board > ./etc/hostname
# echo 127.0.0.1 board board.domain >> ./etc/hosts
It can be useful to increase the number of available loopback devices from the default of 8. This can be done by adding a file in /etc/modprobe.d/:
options loop max_loop=64