Custom Gentoo Linux kernel

Upgrading kernel

Install new kernel version and update config:

USE="symlink" emerge -av gentoo-sources
cd /usr/src
cp -a linux-OLD/.config linux/
make oldconfig

Configure setup options in /etc/genkernel.conf. Most important are:

• INSTALL – install kernel to /boot directory, equivalent to:

  genkernel --install kernel

• SAVE_CONFIG – store kernel config in /etc/kernels/,
• LVM, MDADM, MDADM_CONFIG – add initramfs support for booting from LVM/mdadm device , equivalent to one of:

  genkernel --lvm initramfs
  genkernel --mdadm --mdadm-config=/etc/mdadm.conf initramfs

• FIRMWARE, ALLFIRMWARE, FIRMWARE_DIR – include all firmware files from /lib/firmware in initramfs. This assumes sys-kernel/linux-firmware is installed with +savedconfig and /etc/portage/savedconfig/sys-kernel/linux-firmware contains list of selected firmware files. Equivalent to:

  genkernel --all-firmware initramfs

• MODULEREBUILD – re-emerge packages containing kernel modules after new kernel has been built, equivalent to:

  emerge @module-rebuild

• COMPRESS_INITRD_TYPE – compression algorithm has to be enabled in kernel, e.g. xz setting requires RD_XZ kernel option,
• TMPDIR – /var/tmp/portage/genkernel

then run:

genkernel all

Noteworthy kernel options

Option names and configuration symbols are based on Linux Kernel v6.12.

Bluetooth

Sending files over interface using e.g. blueman:

• RFCOMM protocol support (BT_RFCOMM)

Security hardening

Control flow Enforcement Technology, used when +cet USE flag is enabled for build toolchain:

• Indirect Branch Tracking (X86_KERNEL_IBT)
• X86 userspace shadow stack (X86_USER_SHADOW_STACK)

Wireguard

• IP: policy routing (IP_MULTIPLE_TABLES)

###. TODO: merge wiki kernel install