Chapter 34. Compiling the kernel

Table of Contents

34.1. Requirements and procedure
34.2. Installing the kernel sources
34.3. Creating the kernel configuration file
34.4. Building the kernel manually
34.4.1. Configuring the kernel manually
34.4.2. Generating dependencies and recompiling manually
34.5. Building the kernel using
34.6. Installing the new kernel
34.7. If something went wrong

There are several reasons you might want to compile a NetBSD kernel:

34.1. Requirements and procedure

To recompile the kernel you must have installed the compiler set (comp.tgz).

The basic steps to an updated or customised kernel then are:

  1. Install or update the kernel sources

  2. Create or modify the kernel configuration file

  3. Building the kernel from the configuration file, either manually or using

  4. Install the kernel

34.2. Installing the kernel sources

If you chose to use AnonCVS to fetch the entire source tree, be patient, the operation can last many minutes, because the repository contains thousands of files.

If you have a source tarball, you can extract it as root:

# cd /
# tar zxf /path/to/syssrc.tgz

Even if you used the tarball from the release, you may wish to use AnonCVS to update the sources with changes that have been applied since the release. This might be especially relevant if you are updating the kernel to include the fix for a specific bug, including a vulnerability described in a NetBSD Security Advisory. You might want to get the latest sources on the relevant release or critical updates branch for your version, or Security Advisories will usually contain information on the dates or revisions of the files containing the specific fixes concerned. See Section 32.4, “Fetching by CVS” for more details on the CVS commands used to update sources from these branches.

Once you have the sources available, you can create a custom kernel: this is not as difficult as you might think. In fact, a new kernel can be created in a few steps which will be described in the following sections.

34.3. Creating the kernel configuration file


The directories described in this section are amd64 specific. Users of other architectures must substitute the appropriate directories, see the subdirectories of src/sys/arch for a list.

The kernel configuration file defines the type, the number and the characteristics of the devices supported by the kernel as well as several kernel configuration options. For the amd64 port, kernel configuration files are located in the /usr/src/sys/arch/amd64/conf directory.

Please note that the names of the kernel configuration files are historically in all uppercase, so they are easy to distinguish from other files in that directory:

$ cd /usr/src/sys/arch/amd64/conf/
$ ls
ALL                 INSTALL             XEN3_DOM0           kern.ldscript.kaslr
CVS                 INSTALL_XEN3_DOMU   XEN3_DOMU           majors.amd64
GENERIC             MODULAR             files.amd64         std.amd64
GENERIC_KASLR       Makefile.amd64      kern.ldscript       std.xen
GENERIC_USERMODE    NOCOMPAT            kern.ldscript.Xen

The easiest way to create a new file is to copy an existing one and modify it. Usually the best choice on most platforms is the GENERIC configuration, as it contains most drivers and options. In the configuration file there are comments describing the options; a more detailed description is found in the options(4) man page. So, the usual procedure is:


The modification of a kernel configuration file basically involves three operations:

  1. support for hardware devices is included/excluded in the kernel (for example, SCSI support can be removed if it is not needed.)

  2. support for kernel features is enabled/disabled (for example, enable NFS client support, enable Linux compatibility, ...)

  3. tuning kernel parameters.

Lines beginning with # are comments; lines are disabled by commenting them and enabled by removing the comment character. It is better to comment lines instead of deleting them; it is always possible uncomment them later.

The output of the dmesg(8) command can be used to determine which lines can be disabled. For each line of the type:


both XXX and YYY must be active in the kernel configuration file. You'll probably have to experiment a bit before achieving a minimal configuration but on a desktop system without SCSI and PCMCIA you can halve the kernel size.

You could also examine the options in the configuration file and disable the ones that you don't need. Each option has a short comment describing it, which is normally sufficient to understand what the option does. Many options have a longer and more detailed description in the options(4) man page.

34.4. Building the kernel manually

Based on your kernel configuration file, either one of the standard configurations or your customised configuration, a new kernel must be built.

These steps can either be performed manually, or using the command that was introduced in section Chapter 33, Crosscompiling NetBSD with This section will give instructions on how to build a native kernel using manual steps, the following section Section 34.5, “Building the kernel using describes how to use to do the same.

  • Configure the kernel

  • Generate dependencies

  • Compile the kernel

34.4.1. Configuring the kernel manually

When you've finished modifying the kernel configuration file (which we'll call MYKERNEL), you should issue the following command:

$ config MYKERNEL

If MYKERNEL contains no errors, the config(1) program will create the necessary files for the compilation of the kernel, otherwise it will be necessary to correct the errors before running config(1) again.

Notes for cross-compiling

As the config(1) program used to create header files and Makefile for a kernel build is platform specific, it is necessary to use the nbconfig program that's part of a newly created toolchain (created for example with

/usr/src/ -m sparc64 tools

). That aside, the procedure is just as like compiling a "native" NetBSD kernel. The command is for example:

% /usr/src/tooldir.NetBSD-9.0-amd64/bin/nbconfig MYKERNEL

This command has created a directory ../compile/MYKERNEL with a number of header files defining information about devices to compile into the kernel, a Makefile that is setup to build all the needed files for the kernel, and link them together.

34.4.2. Generating dependencies and recompiling manually

Dependencies generation and kernel compilation is performed by the following commands:

$ cd ../compile/MYKERNEL
$ make depend
$ make

It can happen that the compilation stops with errors; there can be a variety of reasons but the most common cause is an error in the configuration file which didn't get caught by config(1). Sometimes the failure is caused by a hardware problem (often faulty RAM chips): the compilation puts a higher stress on the system than most applications do. Another typical error is the following: option B, active, requires option A which is not active. A full compilation of the kernel can last from some minutes to several hours, depending on the hardware.

The result of a successful make command is the netbsd file in the compile directory, ready to be installed.

Notes for cross-compiling

For crosscompiling a sparc64 kernel, it is necessary to use the crosscompiler toolchain's nbmake-sparc64 shell wrapper, which calls make(1) with all the necessary settings for crosscompiling for a sparc64 platform:

% cd ../compile/MYKERNEL/
% /usr/src/tooldir.NetBSD-9.0-amd64/bin/nbmake-sparc64 depend
% /usr/src/tooldir.NetBSD-9.0-amd64/bin/nbmake-sparc64

This will churn away a bit, then spit out a kernel:

text    data     bss     dec     hex filename
5016899  163728  628752 5809379  58a4e3 netbsd
% ls -l netbsd
-rwxr-xr-x  1 feyrer  666  5874663 Dec  2 23:17 netbsd
% file netbsd
netbsd: ELF 64-bit MSB executable, SPARC V9, version 1 (SYSV), statically linked, not stripped 

Now the kernel in the file netbsd can either be transferred to an UltraSPARC machine (via NFS, FTP, scp, etc.) and booted from a possible harddisk, or directly from the cross-development machine using NFS.

34.5. Building the kernel using

After creating and possibly editing the kernel config file, the manual steps of configuring the kernel, generating dependencies and recompiling can also be done using the src/ script, all in one go:

$ cd /usr/src
$ ./ kernel=MYKERNEL 

This will perform the same steps as above, with one small difference: before compiling, all old object files will be removed, to start with a fresh build. This is usually overkill, and it's fine to keep the old file and only rebuild the ones whose dependencies have changed. To do this, add the -u option to

$ cd /usr/src
$ ./ -u kernel=MYKERNEL 

At the end of its job, will print out the location where the new compiled kernel can be found. It can then be installed.

34.6. Installing the new kernel

Whichever method was used to produce the new kernel file, it must now be installed. The new kernel file should be copied to the root directory, after saving the previous version.

# mv /netbsd /netbsd.old
# mv netbsd /

Customization can considerably reduce the kernel's size. In the following example netbsd.old is the install kernel and netbsd is the new kernel.

-rwxr-xr-x  3 root  wheel  3523098 Dec 10 00:13 /netbsd
-rwxr-xr-x  3 root  wheel  7566271 Dec 10 00:13 /netbsd.old

The new kernel is activated after rebooting:

# shutdown -r now

34.7. If something went wrong

When the computer is restarted it can happen that the new kernel doesn't work as expected or even doesn't boot at all. Don't worry: if this happens, just reboot with the previously saved kernel and remove the new one (it is better to reboot single user):

  • Reboot the machine

  • Press the space bar at the boot prompt during the 5 seconds countdown

  • Type

    > boot netbsd.old -s
  • Now issue the following commands to restore the previous version of the kernel:

    # fsck /
    # mount /
    # mv netbsd.old netbsd
    # reboot

This will give you back the working system you started with, and you can revise your custom kernel config file to resolve the problem. In general, it's wise to start with a GENERIC kernel first, and then make gradual changes.