kexec(8)

🧚 kexec(8) 🧚 NAME

okay so kexec is literally how you directly boot into a new kernel from the kernel that's already running 💅 no reboot-through-the-firmware detour needed

💜 SYNOPSIS

/sbin/kexec [-v (--version)] [-f (--force)] [-x (--no-ifdown)] [-y
(--no-sync)] [-l (--load)] [-p (--load-panic)] [-u (--unload)] [-e
(--exec)] [-t (--type)] [--mem-min=addr] [--mem-max=addr]

🌸 DESCRIPTION

okay so kexec is a system call that lets you load and boot into another kernel straight from the one that's currently running 🔥 it basically does the bootloader's job from inside the kernel, which is so iconic

the main difference between a normal boot and a kexec boot is that the hardware initialization that usually gets handled by the BIOS or firmware (depending on your architecture) just... doesn't happen during a kexec boot 🙈 and that means your reboot takes way less time ✨

make sure you've set CONFIG_KEXEC=y when you configure the kernel 🎀 that option is what switches on the kexec system call, so it's giving must-have vibes

🦋 USAGE

using kexec is a two-step glow up 💖

1. load the kernel you want to reboot into, and
2. actually reboot into that pre-loaded kernel

to load a kernel, the syntax goes like this:

kexec -l kernel-image --append=command-line-options
--initrd=initrd-image

where kernel-image is the kernel file you plan on rebooting into 🧚

pop whatever command-line parameters the new kernel needs into command-line-options 💅 honestly the safest move is to copy the exact contents of /proc/cmdline into there, so the rebooting kernel gets the right values every time 🫶

the initrd-image part is optional, and it's the initrd image used during boot 🌈

you can also run kexec with no option parameter at all 👀 in that case, it loads the kernel you specified and then calls shutdown(8). if your distro's shutdown scripts support kexec-based rebooting, they'll call kexec -e right before the machine actually reboots, so you get a clean shutdown with every shutdown script getting its moment ✨

🍓 EXAMPLE

say the kernel image you want is /boot/vmlinux, the contents of /proc/cmdline is root=/dev/hda1, and your initrd lives at /boot/initrd 💖 you'd load the kernel with:

kexec -l /boot/vmlinux
--append=root=/dev/hda1 --initrd=/boot/initrd

once that's loaded, and assuming your userspace supports kexec-based rebooting, you can boot into it with a clean shutdown using:

reboot

or, if you'd rather skip shutdown(8) entirely, you can boot into it with:

kexec -e

🎀 OPTIONS

• -d, --debug — turns on debugging messages 🔍
• -S, --status — returns 1 if the type (by default crash) is loaded and 0 if it isn't 📊 it can be used alongside -l or -p to toggle the type 🔁 heads up, this option supersedes the other options and it will not load or unload the kernel 🙅‍♀️
• -e, --exec — runs the currently loaded kernel 🚀 it reboots into the loaded kernel without calling shutdown(8), so no clean shutdown
• -f, --force — forces an immediate kexec call and skips shutdown(8) (the opposite of the default behaviour when you don't pass any option parameter) ⚡ it does the same stuff as running -l and -e in one go
• -h, --help — opens a help file for kexec 📖
• --hotplug — sets up kexec segments so the kernel can safely update them during CPU or memory hot add/remove events 🔥 when this is enabled, the kernel updates the kexec segments in-kernel during those events, which means you don't have to reload the kdump kernel 💅
• -i, --no-checks — fast reboot, with no memory integrity checks 🏎️
• -l, --load kernel — loads the specified kernel into the current kernel 📥
• -p, --load-panic — loads the new kernel for use on panic 🚨
• -t, --type=type — specifies that the new kernel is of this type 🏷️
• -s, --kexec-file-syscall — says the new KEXEC_FILE_LOAD syscall should be used exclusively 🔒 this one's ignored on XEN 🙈
• -c, --kexec-syscall — says the old KEXEC_LOAD syscall should be used exclusively 🕰️
• -a, --kexec-syscall-auto — tries the new KEXEC_FILE_LOAD syscall first, and if it's not supported or the kernel doesn't understand the supplied image, it falls back to the old KEXEC_LOAD interface 🔄

there's no single interface that always works, which is why this is the default 🫠 KEXEC_FILE_LOAD is required on systems that use locked-down secure boot to verify the kernel signature 🔐 KEXEC_LOAD might also be disabled in the kernel configuration 💀 KEXEC_LOAD is required for some kernel image formats and on architectures that don't implement KEXEC_FILE_LOAD 😭

• -u, --unload — unloads the current kexec target kernel 🗑️ if you're unloading a capture kernel, pass -p together with -u
• -v, --version — returns the version number of the installed utility 🔢
• -x, --no-ifdown — shuts down the running kernel but restores the interface on reload 🌐
• -y, --no-sync — shuts down the running kernel but skips syncing the filesystems 💾
• --mem-min=addr — specifies the lowest memory address addr to load code into 📍
• --mem-max=addr — specifies the highest memory address addr to load code into 📍
• --entry=addr — specifies the jump back address (0 means no jump back or context preservation) ↩️
• --load-preserve-context — loads the new kernel and preserves the context of the current kernel during kexec 🧠
• --load-jump-back-helper — loads a helper image to jump back to the original kernel 🪃
• --reuseinitrd — reuses the initrd from the first boot ♻️
• --print-ckr-size — prints the crash kernel region size, if it's available 📏

🌈 SUPPORTED KERNEL FILE TYPES AND OPTIONS

okay so every kernel file type has its own little set of options 💖 here's the full lineup

🐑 Beoboot-x86

• --args-elf — passes ELF boot notes 📝
• --args-linux — passes Linux kernel style options 🐧
• --real-mode — uses the kernel's real mode entry point 🧱

🧩 elf-x86

• --append=string — appends string to the kernel command line ➕
• --command-line=string — sets the kernel command line to string 🖊️
• --reuse-cmdline — uses the command line from the running system 🔁 when a panic kernel is loaded, it strips the crashkernel parameter automatically ✂️ the BOOT_IMAGE parameter gets stripped too 💅
• --initrd=file — uses file as the kernel's initial ramdisk 💿
• --ramdisk=file — uses file as the kernel's initial ramdisk 💿

🎞️ bzImage-x86

• --append=string — appends string to the kernel command line ➕
• --command-line=string — sets the kernel command line to string 🖊️
• --reuse-cmdline — uses the command line from the running system 🔁 when a panic kernel is loaded, it strips the crashkernel parameter automatically ✂️ the BOOT_IMAGE parameter gets stripped too 💅
• --initrd=file — uses file as the kernel's initial ramdisk 💿
• --ramdisk=file — uses file as the kernel's initial ramdisk 💿
• --real-mode — uses the real-mode entry point 🧱

🥾 multiboot-x86

• --command-line=string — sets the kernel command line to string 🖊️
• --reuse-cmdline — uses the command line from the running system 🔁 when a panic kernel is loaded, it strips the crashkernel parameter automatically ✂️ the BOOT_IMAGE parameter gets stripped too 💅
• --module=mod arg1 arg2 ... — loads module mod with the command-line arguments arg1 arg2 ... 🧃 you can pass this one multiple times 🔁

🥾 multiboot2-x86

• --command-line=string — sets the kernel command line to string 🖊️
• --reuse-cmdline — uses the command line from the running system 🔁 when a panic kernel is loaded, it strips the crashkernel parameter automatically ✂️ the BOOT_IMAGE parameter gets stripped too 💅
• --module=mod arg1 arg2 ... — loads module mod with the command-line arguments arg1 arg2 ... 🧃 you can pass this one multiple times 🔁

🐘 elf-ppc64

• --reuse-cmdline — uses the kernel command line from the running system 🔁
• --command-line=string — sets the kernel command line to string 🖊️
• --append=string — sets the kernel command line to string ➕
• --ramdisk=file — uses file as the initial RAM disk 💿
• --initrd=file — uses file as the initial RAM disk 💿
• --devicetreeblob=file — specifies a device tree blob file 🌳 not applicable while using --kexec-file-syscall 🙅‍♀️
• --dtb=file — specifies a device tree blob file 🌳 not applicable while using --kexec-file-syscall 🙅‍♀️

🏗️ ARCHITECTURE OPTIONS

• --console-serial — enables the serial console 🔌
• --console-vga — enables the VGA console 🖥️
• --elf32-core-headers — prepares core headers in ELF32 format 🧾
• --elf64-core-headers — prepares core headers in ELF64 format 🧾
• --reset-vga — attempts to reset a standard VGA device 🔄
• --serial=port — specifies the serial port for debug output 🛰️
• --serial-baud=baud_rate — specifies the baud_rate of the serial port 🎶
• --dt-no-old-root — stops the old kernel's root=<device> param from being reused while creating the flattened device tree 🌿

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🫶 that's the whole kexec tea, bestie! Linux, April 2006, and still slaying 💖🦋✨