Live Patching Now Available For Linux

New submitter cyranix writes "You may never have to reboot your Linux machine ever again, even for kernel patching," and excerpts from the long (and nicely human-readable) description of newly merged kernel code that does what Ksplice has for quite a while (namely, offer live updating for Linux systems, no downtime required), but without Oracle's control. It provides a basic infrastructure for function "live patching" (i.e. code redirection), including API for kernel modules containing the actual patches, and API/ABI for userspace to be able to operate on the patches (look up what patches are applied, enable/disable them, etc). It's relatively simple and minimalistic, as it's making use of existing kernel infrastructure (namely ftrace) as much as possible. It's also self-contained, in a sense that it doesn't hook itself in any other kernel subsystem (it doesn't even touch any other code). It's now implemented for x86 only as a reference architecture, but support for powerpc, s390 and arm is already in the works (adding arch-specific support basically boils down to teaching ftrace about regs-saving).

Comments

Unintended consequences?

[marciot]

Maybe I’m old school, but this sort of bothers me. One of the nice things about rebooting is that it clears out old crud and gives you a reassurance that the system can bring itself up by its bootstraps. I can imagine live patching giving rise to a scenario where you have a machine that hasn’t been rebooted for years and when a power glitch finally brings it down, you find that what is on disk is different than what was in RAM and your kernel is corrupt or not bootable.

I think live patching would make sense if we had non-volatile system RAM (i.e. universal memory), but until then, it seems like rebooting is a pretty good sanity check that things are alright.

Ironicly, some older OSes are easy to "patch live"

[davidwr]

The OSes that ran on 8086-era computers and on very early Macs, as well as most consumer 8-bit OSes could in principle be patched or even completely overwritten without a reboot.

I vaguely remember an early Mac implementation of Lisp which basically "took over" the machine and gave you a command-line environment (look Ma! No menus!). You "ran" it by running a standard Mac application which basically took over the machine.

I seem to remember some DOS (if you can call that an OS) programs that worked basically the same way: They loaded themselves into memory, kicked the OS out, then when they quit, they asked you to insert a DOS disk and re-loaded DOS from disk without doing a hardware/BIOS-level reboot (or they knew how to read the hard disk boot tracks and loaded it from there).

With the advent of chips that provided real privilege levels and OSes that actually took advantage of them, such "takeovers" without the cooperation of the already-loaded OS became impossible by design (but still possible using exploits of course).