26 Nov 2023, 15:08

Building a NetBSD ramdisk kernel

When I used OpenBSD, I was a big fan of bsd.rd: a kernel that includes a root file system with an installer and a few tools. When I invariably did something bad to my root file system, I could use that to repair things. bsd.rd is also helpful for OS updates. And there is only a single file involved.

On NetBSD however, there is usually no netbsd.rd kernel installed, or even available by default. The facility is there, it’s just not standard. To be fair, there are a number of architectures that use kernels with a ramdisk for installation.

Recently, I have been toying with NetBSD on an Orange Pi 5. This is a 64-bit ARM board, using the evbarm-aarch64 architecture. I am booting from an SD card (details in a followup post) but once booted, the kernel does not see the card any more, only the NVMe SSD. So my thoughts went back to bsd.rd and I decided that I want one!

It turns out that there are two ways of getting a kernel with a ramdisk:

  1. Building the ramdisk image into the kernel itself, bsd.rd-style. This is called an “instkernel” in NetBSD terminology.
  2. A loadable kernel module, miniroot.kmod. The modules.tar.xz set contains an “empty” miniroot module, to which you can apparently add your own image.

I was unable to make #2 work, so I will only be talking about #1 here.

How to create a NetBSD ramdisk kernel

For the ramdisk image, you can use a pre-built “daily” image, which is available at http://nycdn.netbsd.org/pub/NetBSD-daily/HEAD/latest/evbarm-aarch64/installation/ramdisk/ as ramdisk.fs. Its size is 3072 KiB. Keep that number in mind for later.

For the next steps, you need a NetBSD source tree. The default location is /usr/src but any other location works just as well.

The Orange Pi 5 uses the GENERIC64 kernel configuration but there is no “install” variant. So you need to add a new configuration file, sys/arch/evbarm/conf/GENERIC64_INSTALL, with these contents:

include "arch/evbarm/conf/GENERIC64"


This disables the dynamic ramdisk size and use a fixed size of 6144 sectors x 512 bytes (= 3072 KiB) instead. It also sets the ramdisk as the root filesystem and defaults to single-user mode.

Now let’s build the kernel. I did this on a Mac but any OS would do:

$ cd /usr/src
$ ./build.sh -O /Volumes/obj/ -m evbarm64 -N1 -j 7 -U tools
$ ./build.sh -O /Volumes/obj/ -m evbarm64 -N1 -j 7 -U -u kernel=GENERIC64_INSTALL

Side note: the obj directory needs to be on a case-sensitive filesystem. On a Mac, you can create a case-sensitive APFS dataset named obj, which is mounted under /Volumes/obj.

Now the magic bit, adding the image into the kernel:

$ /Volumes/obj/tools/mdsetimage/mdsetimage -v /Volumes/obj/sys/arch/evbarm/compile/GENERIC64_INSTALL/netbsd ~/Downloads/ramdisk.fs
mapped /Volumes/obj/sys/arch/evbarm/compile/GENERIC64_INSTALL/netbsd
got symbols from /Volumes/obj/sys/arch/evbarm/compile/GENERIC64_INSTALL/netbsd
root @ 0xc6e328/3145728
copying image /Users/bsiegert/Downloads/ramdisk.fs into /Volumes/obj/sys/arch/evbarm/compile/GENERIC64_INSTALL/netbsd (3145728 bytes)
done copying image

Now you can copy the sys/arch/evbarm/compile/GENERIC64_INSTALL/netbsd file to the SD card and boot from it. This boots into the NetBSD installer, which also has a (limited) shell available.

In my case, I was able to install the system to the SSD. Success! 🎉