Building and manipulating images for your devices¶
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.
Basics of building an image¶
kernel - LAVA typically works with Linux but there’s nothing to say that other kernels can’t be used - just don’t expect LAVA (or LAVA developers) to be able to have direct knowledge of any issues with kernels other than Linux
rootfs - generally a simple, minimal tarball of a filesystem created by any of the many tools available to bootstrap the operating system. Debian based distributions commonly use
debootstrapor similar tools.
Changes to the rootfs to make it bootable - raw bootstraps are rarely bootable directly, various init changes are needed and some of these are board specific (e.g. which device to use for the serial console).
Obtaining a kernel¶
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.
Obtaining 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.
Inspecting existing images¶
Tools to install and get to know¶
parted - there are lots of sites with information on
parted, the simplest way to get used to it is to use it on empty block devices - an example is at the end of this section.
dd - a utility to copy a file which can take input from devices like
/dev/zero, commonly used to create empty files of a known size and to copy images directly from one block device to another.
qemu - a wide variety of support for booting images, including images for architectures other than the host architecture.
mount - already installed but there are options which will become second-nature after working with boot images.
gzip - images are typically compressed for download. There are other compression algorithms but most images contain a lot of empty space (for later tests to take) so
gzipis usually enough to get suitable compression. Compressed images will need
gunzipbefore being mountable.
losetup - this is the tool used to configure loop device support in the Linux kernel.
chroot - change root into a directory containing a new rootfs, if using
qemu, this rootfs could even run binaries for a different architecture.
chrootputs you into a new shell inside the rootfs where you can modify files and execute programs without affecting the external system. (There are limitations to how much a chroot can protect the external system, but these are unlikely to affect building a boot image.)
Concepts behind boot images¶
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.
offsets - once decompressed, many boot images contain multiple partitions, so a simple
mountoperation, even using the
loopoption, will fail. An offset tells
mountwhere to look in the image to find the start of the partition to be mounted. Offsets are determined by the original setup of the image and can be determined using tools like
loop devices - the Linux
loopkernel module can allow an image to be mounted as a block device. Such mount operations need to be performed as
sudo. Loop devices can be limited but see Increasing the number of loop devices.
boot partitions - some bootloaders require that files required to boot the device are stored using a particular filesystem, often
FAT. To allow the rootfs to use a different filesystem like ext2, ext3 or ext4, the boot files are stored on a separate partition.
serial console - a device to which the device can write messages during boot and provide a login prompt (which can be automated for a LAVA test job).
root password - one thing that many people forget when creating a rootfs using their favorite Linux distribution is that the root password is typically created by an installer and not by the bootstrap tool. Depending on the security of the OS, you may need to
chrootinto the new rootfs before finishing the image and set a usable root password with the
Find the offset¶
First, decompress your image. These examples will assume that the resulting file is called
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
ext4filesystem in partition 3 starting at an offset of 58720256 bytes.
Other tasks using
partedwill need root access or
Mounting partitions using loop and offset¶
To mount the boot partition, pass the
$ sudo mkdir -p /mnt/boot $ sudo mount -o loop,offset=4194304 test.img /mnt/boot
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
Remember to check the output of
mountand avoid mounting the same partition more than once or moving the image without using
Creating new images¶
QEMU has easy support for creating empty images:
$ qemu-image create test.img
ddto create an empty file which can be used to host partitions and form the basis of a new boot image.
/dev/zerois recommended for this; it is the fastest data source, and will also help give good compression as the empty space in the image file will all be full of zero bytes.
ddcan 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
parteddocumentation for how to create the partition layout you want and experiment with your empty test image file.
partedhas 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
partedhas 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
$ 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
Making a bootstrap rootfs usable¶
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
systemdshould automatically start a getty on the kernel’s default console device, but older images using
sysvinitwill 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
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
# 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
chrootinto the rootfs to be able to execute the
passwdutility. Manual changes to
/etc/passwdcan 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
Other steps which may be required¶
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
Increasing the number of loop devices¶
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
options loop max_loop=64