Essentially, kexec skips the
bootloader stage and
hardware initialization phase performed by the system
firmware (
BIOS or
UEFI), and directly loads the new kernel into
main memory and starts executing it immediately. This avoids the long times associated with a full reboot, and can help systems to meet high-availability requirements by minimizing
downtime. While feasible, implementing a mechanism such as kexec raises two major challenges: • Memory of the currently running kernel is overwritten by the new kernel, while the old one is still executing. • The new kernel will usually expect all hardware devices to be in a well defined state, in which they are after a system reboot because the system firmware resets them to a "sane" state. Bypassing a real reboot may leave devices in an unknown state, and the new kernel will have to recover from that. Support for allowing only
signed kernels to be booted through kexec was merged into version 3.17 of the
Linux kernel mainline, which was released on October 5, 2014. This disallows a
root user to load arbitrary code via kexec and execute it, complementing the
UEFI secure boot and in-kernel security mechanisms for ensuring that only signed
Linux kernel modules can be inserted into the running kernel. Kexec is used by
LinuxBoot to boot the main kernel from the Linux kernel located in the firmware. == See also ==