Building a Kernel from Source
Setting up a proper kernel yourself - lean, mean and tailored to your hardware, is the challenge by which a linux user can graduate to becoming a Funtoo knight ;-)
Even though many of us are using enterprise-ready kernels in datacenters, there is almost nobody who hasn't at least considered building a kernel for his laptop / PC. We are showing here how an intermediate Linux user can use an alternative to the standard beginners "genkernel" approach, to compile a custom kernel, in a relatively speedy and easy set up.
- Understand the command line
- Know where the kernel files are located
You start from an installed Funtoo system on the disk, or at least, you are on stage3 in a chrooted environment from a live cd, following somehow the Funto Installation Tutorial.
Less advanced version
Emerging the kernel sources
To begin, we have to figure out which kernel sources we will use. If you are unsure about which sources are available and what their benefits and drawbacks are, check out the Kernels page.
After you have made a decsion as to which kernel you want to install, emerge it:
root # emerge -1 sys-kernel/[kernel-name]
-1 oneshot emerges the kernel to prevent the kernel from updating automatically when running emerge world updates.
Portage will now go about installing the sources to /usr/src. However, it is preferable to also create a symlink with eselect as such:
root # eselect kernel set 1
You may use
root # eselect kernel list
to display a list of kernels currently installed.
Using the above command is useful, because it allows you to always be able to go to the directory "/usr/src/linux" and find your current working kernel and build files. The "/usr/src/linux" directory is simply a symlink to the kernel you selected with eselect, in the "/usr/src" directory.
Note that portage can also set this up for you automatically, by setting the "symlink" USE flag for your kernel.
Configuring the kernel
Now that the kernel sources are on your system, you should configure them. To do this, change your directory to /usr/src/linux
root # cd /usr/src/linux
We are now in the kernel sources directory, next sanitize the sources:
this will delete your .config
root # make clean && make distclean && make mrproper
Copy configurations if you have previous configurations to copy:
root # cp /boot/config-3.13.7 .config
If the configuration you copy is for a kernel older than the one you are building you must
root # make oldconfig
We can now run a script that allows us to modify the configuration. Run:
root # make menuconfig
root # make nconfig
here is where you insert kernel configs, or press / to search for kernel modules by name
While you edit the sources, keep the following in mind:
- To build something into your kernel, press y when you have it selected.
- To exclude something from your kernel, press n when you have it selected.
- To build something as a module, press m.
Things that you may need to include in your kernel:
- Wireless/LAN drivers
- Support for your graphics card
- Support for your audio card
- Support for USB devices
as built in:
- scsi sata & or ata controllers
- file system used
Many pages on the wiki will tell you the kernel requirements for the application that they are about. Keep your eyes open for the blue background, white text sections of pages. Like on this one: uvesafb
Building and installing the kernel sources
If you're using LZO compression in kernel settings, emerge lzop
root # emerge lzop
After you finish configuring your kernel sources, you will need to build them. To build your sources, run the following:
root # make
You can add -j<number of processing cores + 1> after make to build the kernel more quickly.
When the kernel and its modules finish building, install them:
root # make modules_install && make install
Now that you have installed your kernel and modules, it is a good idea to install an Initramfs.
- If your system has a separate /usr partition, is encrypted, or uses some other non-standard configuration, it will probably not boot without an initramfs.
- If your system is unencrypted, with file system, and hard drive controllers compiled in rather than as modules, it will not need an initramfs.
Getting ready to start
In this case we are building a kernel that is booting root in LVM over encrypted LUKS container. If you don't have this setup, don't worry, you just don't need all the modules, but everything else is similar.
First, there is the decision which linux kernel sources we need. There are plenty of them in the repositories around, often it is not easy to distinguish between them.
I would always trust my distribution of choice and take what is has to offer - and funtoo has a lot to offer!
I really do recommend (especially if it is your first time) to build a debian-sourced genkernel like described in chapter 5 "Using Debian-Sources with Genkernel" in the Funtoo Kernels Tutorial.
From there you should have a running system booting nicely from your own build (just little bit bloated) kernel. This is more than you can expect from any other ready to go distribution.
We are using Red Hat's dracut in order to build a nice initramfs (containing all the necessary tools and extra drivers our kernel might need to start the system). Although dracut is the way to go, more sophisticated and not as buggy as gentoo's genkernel approach, more and more funtoo geeks start using slashbeast's better-initramfs, which we will cover at the end of this howto! So after having set up a genkernel from debian or gentoo sources we are going to build a kernel with either (or both) dracut or/and better-initramfs. So gentoo sources with genkernel is always my backup if anything is not working correctly on my system. For the slightly more geeky approach with my own initram I am using pf-sources, ck-sources or any other more or less heavily patched sources.
The source you use on your system is up to you. For a laptop or desktop system, the following are recommended:
- sys-kernel/sysrescue-std-sources -- confirm
If you are unsure of which sources you would like to use, emerge
gentoo-sources. That's always a safe bet for a general system. For more information on available kernels, check out: Funtoo Linux Kernels
Preparing the kernel
We go now to the sources directory and enter the following commands to update the kernel's .config file:
root # cd /usr/src/linux/ root # make clean CLEAN . CLEAN arch/x86/kernel/acpi/realmode CLEAN arch/x86/kernel/cpu CLEAN arch/x86/kernel CLEAN arch/x86/vdso CLEAN arch/x86/lib CLEAN drivers/gpu/drm/radeon CLEAN drivers/net/wan CLEAN drivers/scsi/aic7xxx CLEAN drivers/tty/vt CLEAN drivers/video/logo CLEAN firmware CLEAN kernel CLEAN lib/raid6 CLEAN lib CLEAN security/apparmor CLEAN security/selinux CLEAN usr CLEAN arch/x86/boot/compressed CLEAN arch/x86/boot CLEAN .tmp_versions CLEAN vmlinux System.map .tmp_kallsyms2.S .tmp_kallsyms1.o .tmp_kallsyms2.o .tmp_kallsyms1.S .tmp_vmlinux1 .tmp_vmlinux2 .tmp_System.map root # zcat /proc/config.gz > /usr/src/linux/.config
Next, we run
make localmodconfig. You will get some questions which you can answer mostly with either M (compiled as a module) or Y (compiled directly into the kernel). If you are not sure what to choose, press enter, and the default option will be selected.
root # make localmodconfig Enable different security models (SECURITY) [Y/n/?] y Enable the securityfs filesystem (SECURITYFS) [Y/?] y Socket and Networking Security Hooks (SECURITY_NETWORK) [Y/?] y Security hooks for pathname based access control (SECURITY_PATH) [Y/?] y Low address space for LSM to protect from user allocation (LSM_MMAP_MIN_ADDR)  65536 NSA SELinux Support (SECURITY_SELINUX) [Y/n/?] y NSA SELinux boot parameter (SECURITY_SELINUX_BOOTPARAM) [N/y/?] n NSA SELinux runtime disable (SECURITY_SELINUX_DISABLE) [N/y/?] n NSA SELinux Development Support (SECURITY_SELINUX_DEVELOP) [Y/n/?] y NSA SELinux AVC Statistics (SECURITY_SELINUX_AVC_STATS) [Y/n/?] y NSA SELinux checkreqprot default value (SECURITY_SELINUX_CHECKREQPROT_VALUE)  1 NSA SELinux maximum supported policy format version (SECURITY_SELINUX_POLICYDB_VERSION_MAX) [Y/n/?] y NSA SELinux maximum supported policy format version value (SECURITY_SELINUX_POLICYDB_VERSION_MAX_VALUE)  19 TOMOYO Linux Support (SECURITY_TOMOYO) [Y/n/?] y Default maximal count for learning mode (SECURITY_TOMOYO_MAX_ACCEPT_ENTRY)  2048 Default maximal count for audit log (SECURITY_TOMOYO_MAX_AUDIT_LOG)  1024 Activate without calling userspace policy loader. (SECURITY_TOMOYO_OMIT_USERSPACE_LOADER) [Y/n/?] y AppArmor support (SECURITY_APPARMOR) [Y/n/?] y AppArmor boot parameter default value (SECURITY_APPARMOR_BOOTPARAM_VALUE)  1 Integrity Measurement Architecture(IMA) (IMA) [Y/n/?] y EVM support (EVM) [N/y/?] (NEW) Default security module 1. SELinux (DEFAULT_SECURITY_SELINUX) 2. TOMOYO (DEFAULT_SECURITY_TOMOYO) 3. AppArmor (DEFAULT_SECURITY_APPARMOR) > 4. Unix Discretionary Access Controls (DEFAULT_SECURITY_DAC) choice[1-4?]: 4 warning: (ACPI_HOTPLUG_CPU) selects ACPI_CONTAINER which has unmet direct dependencies (ACPI && EXPERIMENTAL) warning: (MEDIA_TUNER) selects MEDIA_TUNER_TEA5761 which has unmet direct dependencies (MEDIA_SUPPORT && VIDEO_MEDIA && I2C && EXPERIMENTAL) root # root # configuration written to .config root # warning: (GFS2_FS) selects DLM which has unmet direct dependencies (EXPERIMENTAL && INET && SYSFS && CONFIGFS_FS && (IPV6
Now comes the most adventurous part!
Building the Kernel
root # make -j8 bzImage root # make -j8 modules root # make modules_install root # make install
Make sure that you have built and installed your kernel sources / modules before building an initramfs.
To get your initramfs up and running, check out the Initramfs page. After following all the directions on the page to get your initramfs set up, continue following the ones here.
grub.cfg with boot update, then reboot and see how it works!
root # boot-update -v root # reboot