Linux Kernel Module Cheat

8. initrd

The kernel can boot from an CPIO file, which is a directory serialization format much like tar: https://superuser.com/questions/343915/tar-vs-cpio-what-is-the-difference

The bootloader, which for us is provided by QEMU itself, is then configured to put that CPIO into memory, and tell the kernel that it is there.

This is very similar to the kernel image itself, which already gets put into memory by the QEMU -kernel option.

With this setup, you don’t even need to give a root filesystem to the kernel: it just does everything in memory in a ramfs.

To enable initrd instead of the default ext2 disk image, do:

./build-buildroot --initrd
./run --initrd

By looking at the QEMU run command generated, you can see that we didn’t give the -drive option at all:

cat "$(./getvar run_dir)/run.sh"

Instead, we used the QEMU -initrd option to point to the .cpio filesystem that Buildroot generated for us.

Try removing that -initrd option to watch the kernel panic without rootfs at the end of boot.

When using .cpio, there can be no filesystem persistency across boots, since all file operations happen in memory in a tmpfs:

date >f
poweroff
cat f
# can't open 'f': No such file or directory

which can be good for automated tests, as it ensures that you are using a pristine unmodified system image every time.

Not however that we already disable disk persistency by default on ext2 filesystems even without --initrd: Section 23.3, “Disk persistency”.

One downside of this method is that it has to put the entire filesystem into memory, and could lead to a panic:

end Kernel panic - not syncing: Out of memory and no killable processes...

This can be solved by increasing the memory as explained at Memory size:

./run --initrd --memory 256M

The main ingredients to get initrd working are:

BR2_TARGET_ROOTFS_CPIO=y: make Buildroot generate images/rootfs.cpio in addition to the other images.

It is also possible to compress that image with other options.
qemu -initrd: make QEMU put the image into memory and tell the kernel about it.
CONFIG_BLK_DEV_INITRD=y: Compile the kernel with initrd support, see also: https://unix.stackexchange.com/questions/67462/linux-kernel-is-not-finding-the-initrd-correctly/424496#424496

Buildroot forces that option when BR2_TARGET_ROOTFS_CPIO=y is given