Kexec

2024-01-02T14:10:10Z

Bro: fix wiki link

{{DISPLAYTITLE:kexec}}
[https://en.wikipedia.org/wiki/Kexec kexec] is a system call that enables loading and booting into another kernel. This is useful for faster reboots that skip the firmware initialisation process and the bootloader.

==Installing kexec-tools==
The userspace tools required to use it can be installed via {{cmd|apk add {{pkg|kexec-tools}} {{pkg|kexec-tools-doc}}}} The tools are not available on all flavors of Alpine, additionally not all kernels are compiled with the kexec syscall enabled. You will most likely want to check your {{path|/boot/config-*}} file for {{codeline|CONFIG_KEXEC=y}}

==Usage==
===Manually===

{{note|Currently multiple initrd (e.g. for loading CPU microcode) is not supported}}
On a typical Alpine setup, it can be used via:

{{Cmd|# kexec -l /boot/vmlinuz-edge --initrd \
/boot/initramfs-edge --reuse-cmdline \
&& openrc shutdown
# kexec -e}}

Current Alpine kernels have been [https://gitlab.alpinelinux.org/alpine/aports/-/commit/909d020b81c09bf0df649f8aa6b7da10377a0667 hardened] and return
<code>kexec_load failed: Operation not permitted</code>
if kexec is called without unlocking it with the kernel boot parameter

<code>kexec_load_disabled=0</code>.

Without it the [https://linux.die.net/man/8/sysctl sysctl setting] <code>kernel.kexec_load_disabled</code> defaults to 1 and it can't be turned off in the running kernel, so you need to add the parameter to your [[Bootloaders|bootloader]] configuration and reboot.

There are no other Alpine-specific considerations for Kexec. Please review the man page and existing references below for more details. This page is deliberately kept short in order to avoid duplicating existing documentation.

===Automatically on every reboot/shutdown===
kexec can be set to run automatically for faster rebooting. This is very useful on servers.

First create two openrc services and edit the <code>BOOTPART</code>, <code>KERNEL</code>, and <code>INITRD</code> variables if not using the defaults:
{{cat|/etc/init.d/kexec-load|<nowiki>#!/sbin/openrc-run
description="kexec for faster reboot"
# Define defaults
: "${BOOTPART:=/boot}"
: "${KERNEL:=vmlinuz-edge}"
: "${INITRD:=initramfs-edge}"

depend() {
need localmount
}

start() {
:
}

stop() {
if ! yesno ${RC_GOINGDOWN}; then
einfo "kexec-load: Not rebooting or powering off; not loading kernel"
exit
fi
ebegin "kexec-load: loading kernel for faster reboot"
kexec -l "${BOOTPART}/${KERNEL}" \
--initrd "${BOOTPART}/${INITRD}" \
--reuse-cmdline
ewend $? Failed.
}</nowiki>}}
{{cat|/etc/init.d/kexec-exec|<nowiki>#! /sbin/openrc-run
description="kexec for faster reboot"
depend() {
after killprocs savecache mount-ro
}
start() {
ebegin "kexec-exec: Using kexec for faster reboot"
kexec -e
ewend $? "kexec-exec No kernel loaded."
return 0
}</nowiki>}}
Now give these services execute permission and assign them to the appropriate runlevels:
{{cmd|chmod a+x /etc/init.d/kexec-load
chmod a+x /etc/init.d/kexec-exec
rc-update add kexec-load default
rc-update add kexec-exec shutdown
rc-service kexec-load start
}}

kexec will run on your next <code>reboot</code> or <code>poweroff</code> enjoy!

{{Note|With both of the above service enabled, the system will reboot via kexec even if you are attempting to <code>poweroff</code>. To temporarily restore default <code>poweroff</code> or <code>reboot</code> behavior, simply run <code>rc-service kexec-load stop</code> beforehand.}}

== See also ==

* [https://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git/log/ kexec-tools changelog]
* [https://wiki.gentoo.org/wiki/Kexec Gentoo Wiki: Kexec]
* [https://wiki.archlinux.org/title/Kexec ArchWiki: Kexec]
[[Category: Booting]]
[[Category: Kernel]]

Kexec

2024-01-02T14:03:38Z

Bro: Explain the boot parameter kexec_load_disabled=0 which unlocks kexec in current (hardened) Alpine kernels.

{{DISPLAYTITLE:kexec}}
[https://en.wikipedia.org/wiki/Kexec kexec] is a system call that enables loading and booting into another kernel. This is useful for faster reboots that skip the firmware initialisation process and the bootloader.

==Installing kexec-tools==
The userspace tools required to use it can be installed via {{cmd|apk add {{pkg|kexec-tools}} {{pkg|kexec-tools-doc}}}} The tools are not available on all flavors of Alpine, additionally not all kernels are compiled with the kexec syscall enabled. You will most likely want to check your {{path|/boot/config-*}} file for {{codeline|CONFIG_KEXEC=y}}

==Usage==
===Manually===

{{note|Currently multiple initrd (e.g. for loading CPU microcode) is not supported}}
On a typical Alpine setup, it can be used via:

{{Cmd|# kexec -l /boot/vmlinuz-edge --initrd \
/boot/initramfs-edge --reuse-cmdline \
&& openrc shutdown
# kexec -e}}

Current Alpine kernels have been [https://gitlab.alpinelinux.org/alpine/aports/-/commit/909d020b81c09bf0df649f8aa6b7da10377a0667 hardened] and return
<code>kexec_load failed: Operation not permitted</code>
if kexec is called without unlocking it with the kernel boot parameter

<code>kexec_load_disabled=0</code>.

Without it the [https://linux.die.net/man/8/sysctl sysctl setting] <code>kernel.kexec_load_disabled</code> defaults to 1 and it can't be turned off in the running kernel, so you need to add the parameter to your [[#Bootloaders|bootloader]] configuration and reboot.

There are no other Alpine-specific considerations for Kexec. Please review the man page and existing references below for more details. This page is deliberately kept short in order to avoid duplicating existing documentation.

===Automatically on every reboot/shutdown===
kexec can be set to run automatically for faster rebooting. This is very useful on servers.

First create two openrc services and edit the <code>BOOTPART</code>, <code>KERNEL</code>, and <code>INITRD</code> variables if not using the defaults:
{{cat|/etc/init.d/kexec-load|<nowiki>#!/sbin/openrc-run
description="kexec for faster reboot"
# Define defaults
: "${BOOTPART:=/boot}"
: "${KERNEL:=vmlinuz-edge}"
: "${INITRD:=initramfs-edge}"

depend() {
need localmount
}

start() {
:
}

stop() {
if ! yesno ${RC_GOINGDOWN}; then
einfo "kexec-load: Not rebooting or powering off; not loading kernel"
exit
fi
ebegin "kexec-load: loading kernel for faster reboot"
kexec -l "${BOOTPART}/${KERNEL}" \
--initrd "${BOOTPART}/${INITRD}" \
--reuse-cmdline
ewend $? Failed.
}</nowiki>}}
{{cat|/etc/init.d/kexec-exec|<nowiki>#! /sbin/openrc-run
description="kexec for faster reboot"
depend() {
after killprocs savecache mount-ro
}
start() {
ebegin "kexec-exec: Using kexec for faster reboot"
kexec -e
ewend $? "kexec-exec No kernel loaded."
return 0
}</nowiki>}}
Now give these services execute permission and assign them to the appropriate runlevels:
{{cmd|chmod a+x /etc/init.d/kexec-load
chmod a+x /etc/init.d/kexec-exec
rc-update add kexec-load default
rc-update add kexec-exec shutdown
rc-service kexec-load start
}}

kexec will run on your next <code>reboot</code> or <code>poweroff</code> enjoy!

{{Note|With both of the above service enabled, the system will reboot via kexec even if you are attempting to <code>poweroff</code>. To temporarily restore default <code>poweroff</code> or <code>reboot</code> behavior, simply run <code>rc-service kexec-load stop</code> beforehand.}}

== See also ==

* [https://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git/log/ kexec-tools changelog]
* [https://wiki.gentoo.org/wiki/Kexec Gentoo Wiki: Kexec]
* [https://wiki.archlinux.org/title/Kexec ArchWiki: Kexec]
[[Category: Booting]]
[[Category: Kernel]]

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Kexec

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