LinuxWorld.com

Ksplice: kernel patches without reboots

By Jonathan Corbet, LinuxWorld.com, 05/07/08

The kernel developers are generally quite good about responding to security problems. Once a vulnerability in the kernel has been found, a patch comes out in short order; system administrators can then apply the patch (or get a patched kernel from their distributor), reboot the system, and get on with life knowing that the vulnerability has been fixed. It is a system which works pretty well.

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One little problem remains, though: rebooting the system is a pain. At a minimum, it requires a few minutes of down time. In many situations, that down time cannot be tolerated. Reboots also disrupt any ongoing work, break existing network connections, and can cause the loss of results from long-running processes. And, most importantly of all, reboots prove traumatic for a certain subset of Linux administrators who prize a long uptime above almost all other things. Administrators currently have to choose between multi-year uptimes and security fixes; anything which frees them from a dilemma of this magnitude can only be welcome.

That "anything" might just be a recently-announced project called ksplice. With ksplice, system administrators can have the best of both worlds: security fixes without unsightly reboots.

An in-depth explanation of how ksplice works can be found in this document [PDF]. In short, ksplice requires as input the source tree for the running kernel and the security patch. It will then build two kernels, one with the patch and one without; the kernels are built with a special set of options which makes it easy to figure out which functions change as a result of the patch. The two kernels will be compared, with the purpose of finding those functions. Changes can propagate further than one might expect, especially if, for example, an inline function is modified.

Once a list of changed functions has been made, the updated code for those functions is packaged into a kernel module and loaded into the system. Then comes the tricky part: getting the running kernel to start using the new code. That requires patching the running code, which is a risky thing to do. Ksplice starts with a call to stop_machine_run(), which dumps a high-priority thread onto each processor, thus taking control of all processors in the system. It then examines all threads in the system to ensure that none of them are running in the functions to be replaced; if so, trampoline jumps are patched into the beginning of each replaced function (they "bounce" the call to the old code into the replacement code) and life continues. Otherwise ksplice will back off and try again later.

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