In computing, kexec (abbreviated from kernel execution, and derived from the Unix/Linux kernel call exec) is a mechanism of the Linux kernel that allows "live" booting of a new kernel "over" the currently running kernel. Essentially, kexec skips the bootloader stage (hardware initialization phase by the system firmware, BIOS or UEFI) and directly loads the new kernel into memory, where it starts executing 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 physical devices to be in a well-defined state, as they are after a system reboot, when the BIOS, UEFI or 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, 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.
- kdump (Linux) – Linux kernel's crash dump mechanism, which internally uses kexec
- kGraft – Linux kernel live patching technology developed by SUSE
- kpatch – Linux kernel live patching technology developed by Red Hat
- Ksplice – Linux kernel live patching technology developed by Ksplice, Inc. and later bought by Oracle
- Hariprasad Nellitheertha (May 4, 2004). "Reboot Linux faster using kexec". IBM. Archived from the original on January 21, 2013. Retrieved December 5, 2013.
- David Pendell (August 16, 2008). "Reboot like a racecar with kexec". linux.com. Retrieved December 5, 2013.
- "Linux kernel 3.17, Section 1.10. Signed kexec kernels". kernelnewbies.org. October 5, 2014. Retrieved November 3, 2014.
- Jake Edge (June 25, 2014). "Reworking kexec for signatures". LWN.net. Retrieved August 9, 2014.
- Matthew Garrett (December 3, 2013). "Subverting security with kexec". dreamwidth.org. Retrieved December 5, 2013.
- Kees Cook (December 10, 2013). "Live patching the kernel". outflux.net. Retrieved December 12, 2013.
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