View source for Custom Kernel

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This process of building a '''custom configured kernel''' assumes you are running on Alpine Linux utilizing abuild & aports.

== But why? ==

You want to build a custom kernel to enable experimental hardware or features or outdated hardware, to reduce bloat further, to tune the kernel to the hardware.

The lts kernel for most Alpine ARCHs uses defaults to balance throughput at the expense of some responsiveness, and support for many devices.  You can tweak the kernel for desktop use and low latency and responsiveness.

You should disable modules to increase security.  By default, Alpine will install modules but not disable most of them.  Disabling modules will reduce an DMA attack but not eliminate it completely.  If you have a newer processor with VT-d, you can mitigate as long as you:

Leave <code>CONFIG_INTEL_IOMMU_DEFAULT_ON=y</code> or pass <code>intel_iommu=on</code> as a kernel parameter and disable kernel logging so the attacker doesn't gain DMAR address information through dmesg.[https://blog.frizk.net/2016/11/disable-virtualization-based-security.html]  Also remove references to the kernel version to calculate the IOMMU addresses.[https://link.springer.com/content/pdf/10.1186/s13173-017-0066-7.pdf]

You may also want to harden your kernel by adding at least some of the config changes recommended by <code>kernel-hardening-checker</code> [https://github.com/a13xp0p0v/kernel-hardening-checker/]

To increase the security of the boot process, if you have a TPM, you could set <code>CONFIG_INTEL_TXT=y</code> (Enable Intel(R) Trusted Execution Technology (Intel(R) TXT)) (which is not enabled in the hardened kernel by default), then you would need the SINIT module (provided only by Intel)[https://software.intel.com/en-us/articles/intel-trusted-execution-technology], a possibly compiled TrustedGrub2[https://github.com/Rohde-Schwarz-Cybersecurity/TrustedGRUB2], trousers[https://sourceforge.net/projects/trousers/?source=navbar], tboot[https://sourceforge.net/projects/tboot/].  These packages are not in aports and it is unknown if these tools work on musl.  It's not recommended for Edge.  Also, there would be trigger packages to generate hashes for the kernel and the mkinitfs updates.

== Setting up the Alpine Build System ==

First, you need to follow the steps in [[Creating_an_Alpine_package#Setup_your_system_and_account|Setup your system and account for building packages]].  You also need to configure your {{path|/etc/apk/repositories}} so that they search locally for your apks.  See [[Creating_an_Alpine_package#Testing_the_package_locally|Testing the package locally]] for details.

After setting up accounts and repos, change your shell's current working directory to '''aports''' that you just cloned.

{{cmd|$ cd aports}}

== Working with aports ==

We will try using an existing lts kernel just tweaking the {{path|lts.ARCH.config}} file.  

=== Switching to the proper release version ===

You need to switch to the proper branch that matches the release so that the kernel compiles against the dependencies properly.

{| cellpadding="5" border="1" class="wikitable"
|- 
! Alpine version
! Remote branch
|-
| Edge
| master
|-
| 3.22.0
| 3.22-stable
|- 
|}

The following is required to get access to the {{path|APKBUILD}} released for that version of Alpine and which you will create a commit for.

If you are on 3.22 do:

{{cmd|$ git checkout -b 3.22-stable origin/3.22-stable}}

If you are on Edge do:

{{cmd|$ git checkout master}}

=== Creating your config ===

You can use {{pkg|linux-lts}} but what you should do is create a local branch by doing:

For Alpine 3.22 (linux-lts - 6.12):

{{cmd|$ git checkout -b my-custom-kernel origin/3.22-stable}}

For Alpine Edge:

{{cmd|$ git checkout -b my-custom-kernel}}

Doing it this way, you do less work in maintaining.  All you need to do is keep ''master'' or ''3.22-stable'' in sync[https://help.github.com/articles/syncing-a-fork/][https://help.github.com/articles/configuring-a-remote-for-a-fork/] and merge any conflicts.  

First switch to the branch by doing 
<code>git checkout my-custom-kernel</code>.  
Then, you need to navigate to the {{path|main/linux-lts}} folder where you should see a APKBUILD and some config- files.

You may want to change APKBUILD to change the name of your kernel and package (custom flavor or revision) and comment out or remove lines with config files that are not for your cpu architectures or virt* configs that are for virtualized environments.(They provide optimized kernel configurations for running Alpine as a guest in virtual machines.)
Setting custom name/flavor will prevent overriding your default ''linux-lts'' kernel modules and commenting out unused configs will speed up the process of applying kernel patches by applying it for your architecture only. (Obviously ARCH in the following example is whatever architecture (x86, x86_64, ...) you use.)

When you are done with your edits to the APKBUILD file copy it {{path|lts.ARCH.config}} to {{path|.config}}.  You can then move the {{path|.config}} back overriding the {{path|lts.ARCH.config}} generated by <code>make localmodconfig</code> or <code>make menuconfig</code> (discussed below in the ''Configuring kernel'' section).  After generating your config, you need to run <code>abuild checksum</code>.
Then add your changes, eg <code>git add APKBUILD lts.ARCH.config</code>. 
And commit <code>git commit APKBUILD config-NAME.ARCH -m "Enabled these options ...."</code> for your customization the '''ARCH'''itecture of your system.  You do this so that git can keep your code separate from Alpine's and so your changes float forward between kernel updates.

== Adding custom patches ==

Custom patches should be added to ''sources=''.

After you added the URL, you need to produce a checksum by doing <code>abuild checksum</code>.

The custom patches may not be autopatched, due to being distributed as an archive or different patch level, so you need to define what to do with it in the prepare().

== Configuring kernel ==

Attempt to create a config for your currently running system and modules first.

To create a kernel config based on your currently running PC, use:
<code>make localmodconfig</code>
This command reads your current kernel's configuration from /proc/config.gz (if available) or /boot/config-* and creates a .config file that includes only the modules currently loaded on your system. This results in a minimal configuration tailored to your hardware.
Alternatively, you can use:
<code>make localyesconfig</code>
This is similar to ''localmodconfig'', but instead of building features as modules (m), it builds them directly into the kernel (y), which can result in a larger kernel image but eliminates the need for separate module files.
Note: Before running either command, make sure you have all the hardware you want to support actively in use (USB devices plugged in, network cards active, etc.) so their modules are loaded and included in the configuration.

The alternative is to use the kernel configuration menu in the build-NAME folder, but before you do that you need to <code>sudo apk add {{pkg|ncurses-dev}}</code>

After you are done using the menu in the build-NAME folder by doing <code>make menuconfig</code>, you want to remove <code>ncurses-dev</code>.  When you are done, it will be stored in ''.config'' which you need to again override the {{path|lts.ARCH.config}} file.  When you are done updating the {{path|config-NAME.ARCH}}, you need to do <code>abuild checksum</code>.

The options in the kernel config are typically defaults.  If your device is old, it may be set to n by default.

=== Vanilla targets and tuning ===

{|cellpadding="5" border="1" class="wikitable"
!ARCH
!Processor Type / CPU Selection / System Type
!Code Generation / Instruction Extensions
!Timer Frequency
!Preemption Model
!Bitness
|-
|s390x
|IBM zEnterprise 114 and 196
|IBM zBC12 and zEC12 (<code>-march=zEC12 -mtune=zEC12</code>)
|1000 Hz
|No Forced Preemption (Server)
|64
|-
|ppc64le
|Server processors
|POWER8 (<code>-mcpu=power8</code>), AltiVec (<code>-Wa,-maltivec</code> to assembler or <code>-maltivec -mabi=altivec</code>), VSX
|1000 HZ
|No Forced Preemption (Server)
|64
|-
|ppc
|
512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx
* Apple PowerMac based machines
|AltiVec (<code>-Wa,-maltivec</code> to assembler or <code>-maltivec -mabi=altivec</code>) on >=74xx
|1000 HZ
|No Forced Preemption (Server)
|32
|-
|x86_64
|Generic-x86-64
|(-mtune=generic ; SIMD assembly modules enabled based on simple compile test and/or presence of CPU flag)
|1000 HZ
|Voluntary Kernel Preemption (Desktop)
|64
|-
|x86
|586/K5/5x86/6x86/6x86MX
|(-mtune=generic ; SIMD assembly modules enabled based on simple compile test and/or presence of CPU flag)
|1000 HZ
|Voluntary Kernel Preemption (Desktop)
|32
|-
|armv7
|
* ARMv7 based platforms (Cortex-A, PJ4, Scorpion, Krait)
* Freescale i.MX family -- Cortex A (i.MX51, i.MX53, i.MX6 Quad/DualLite, i.MX6 SoloLite, i.MX6 SoloX, i.MX6 UltraLite, i.MX7 Dual)
* Qualcomm -- (MSM8X60, MSM8960, MSM8974)
* Allwinner SoCs -- (A10 (sun4i), A10s / A13 (sun5i), A31 (sun6i), A20 (sun7i), sun8i Family, (sun9i))
* ARM Ldt Versatile Express family -- 
|Either <code>-march=armv7-a</code> or <code>-march=armv5t -Wa,-march=armv7-a</code> based on a compile test. <code>-mfpu=vfp</code>
|1000 Hz
|Voluntary Kernel Preemption (Desktop)
|32
|-
|aarch64
|
* Allwinner sunxi 64-bit SoC Family
* Broadcom BCM2835 family
* Marvell Berlin SoC Family
* ARMv8 based Samsung Exynos SoC family
* ARMv8 based Freescale Layerscape SoC family
* Hisilicon SoC Family
* Mediatek MT65xx & MT81xx ARMv8 SoC
* Marvell EBU SoC Family
* Qualcomm Platforms
* Rockchip Platforms
* AMD Seattle SoC Family
* Altera's Stratix 10 SoCFPGA Family
* NVIDIA Tegra SoC Family
* Spreadtrum SoC platform
* Cavium Inc. Thunder SoC Family
* ARMv8 software model (Versatile Express)
* AppliedMicro X-Gene SOC Family
* Xilinx ZynqMP Family
|
|1000 HZ
|Voluntary Kernel Preemption (Desktop)
|64
|-
|}
If you do desktop multitasking, you may want to switch to Voluntary Kernel Preemption (Desktop) or Preemptible Kernel (Low-Latency Desktop) and up the Timer Frequency.  If you run a dedicated render farm node or a dedicated bitcoin miner use No Forced Preemption (Server) and decrease the Timer Frequency.

Optimized modules (most are already compiled as modules):
* raid6 -- altivec, avx512, ssse3, avx2, mmx, sse, sse2, neon
* some operations of raid5 -- mmx (32 bit), sse (64 bit), avx
For Kernel API:
* 32-bit memcpy -- 3dnow
* 32-bit memory page clearing and copying -- sse (Athlon/K7 only), mmx
From x86/crypto, arm/crypto, powerpc/crypto:
* CAMELLIA -- avx2, avx, aes-ni
* CHACHA20 -- avx2, neon
* CAST5 -- avx
* CAST6 -- avx
* TWOFISH -- avx
* SERPENT -- avx2, avx, sse2
* SHA1 -- avx2, ssse3, neon, spe
* SHA2 -- avx2
* SHA256 -- ssse3, neon, spe
* SHA512 -- avx2, ssse3, neon
* POLY1305 -- avx2
* GHASH -- pclmulqdq (part of aes-ni), vmx (power8)
* AES -- aes-ni, neon, vmx (power8), spe
* CRC32 -- pclmulqdq, sse, neon, vmx (power8)
* CRCT10DIF -- pclmulqdq, sse, neon, vmx (power8)

=== Fast reboots with kexec ===
{{main|kexec}}

If you want to reboot the kernel fast avoiding the POST test, you need {{ic|doas apk add {{pkg|kexec-tools}}}} and enable kexec in the kernel:

  Processor type and features
    [*] kexec system call

=== Hibernation to prevent data loss ===

  Power management and ACPI options
    [*] Hibernation (aka 'suspend to disk')

Hibernation should be used if you have a laptop.  You don't want the laptop to suddenly shut off resulting in data loss, you want it to save your work based on a percentage of battery life (this requires special script).  When hibernation resumes, should lock and ask for credentials.  Depending on your needs, the hibernated image can be encrypted/decrypted which again requires additional customization to scripts.

Hibernation with an unsanitized swap file is generally insecure because data and unlocked memory pages are swapped out in plaintext.  To increase the security either disable swap or use an encrypted swap.  The swap file/partition is typically used as the hibernation resume image.

== Building ==

Before building, make sure you have ccache installed.  This should reduce compile time on multiple builds.
You may also want to read up on compile flags - perhaps you want your build to be optimized for speed (or size which is the default).
Review <code>/etc/abuild.conf</code> and set:
<code>
PACKAGER_PRIVKEY="/home/MY_USER_HERE/.abuild/my-email@mydomain.com-ID.rsa"
USE_CCACHE=1
</code>

If you have your config ready, first try building with <code>abuild -rK</code> which install most of the dependencies and keep buildtime temp dirs and files (srcdir/pkgdir/deps).

If it complains about a dependency like {{pkg|elfutils-dev}} use <code>-rKd</code>.
Then, when it prompts for values for new found config options just hold enter till it starts compiling the kernel.
Unless you removed or commented out <code>virt.*.config</code> configs
there should be two sets one for -lts and the other for the -virt.
Just {{Key|Ctrl}}+{{Key|C}} out of the compilation process after 
the second set so you can further customize the config.
Then you go into the {{path|src/linux-VER}} and edit the config file.
Copy the {{path|.config}} file overriding the {{path|lts.ARCH.config}} in the srcdir.

== Installing ==

To install it you do a {{ic|doas apk add linux-NAME}} where NAME is your custom kernel name.

== Testing ==

Before you test, you should install the lts kernel too, using <code>apk add {{pkg|linux-lts}}</code>.  You may be missing a module and can't boot, so you use the other kernel as the fallback boot kernel.

Normally during the installation using with ''apk'' there are tools that will update initramfs and a boot loader automatically for you. For easier debugging you may want to change boot loader config and remove <code>quiet</code> from linux command line in <code>/etc/default/grub</code> or <code>/etc/update-extlinux.conf</code>.
Or perhaps you want to change other kernel options in <code>/etc/mkinitfs/mkinitfs.conf</code> - please remember to generate initramfs and update your bootloader manually, eg:
<code>
# mkinitfs
# grub-mkconfig -o /boot/grub/grub.cfg
</code>

To test, first you should make a bootable Alpine USB image.  Then, when you have your rescue USB done, you <code>reboot</code> the computer.

To test it, you basically do trial and error.  Sometimes your config is missing something if you want to have a bare minimum setting.

If you are curious about correctness testing, some kernel modules or components do preform self tests at the beginning of the boot process.  The tools may have test suites that you run with the make command.

== See Also ==
* [https://wiki.archlinux.org/title/Kernel Archwiki Kernels]
* [https://wiki.gentoo.org/wiki/Kernel Gentoo Wiki Kernel]
* [https://wiki.gentoo.org/wiki/Kernel/Configuration Gentoo Wiki Kernel Configuration]
* [[How to build the Alpine Linux kernel]]

[[Category:Kernel]]