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Patch the Linux Kernel Without Reboots
evanbro writes "ZDNet is reporting on ksplice, a system for applying patches to the Linux kernel without rebooting. ksplice requires no kernel modifications, just the source, the config files, and a patch. Author Jeff Arnold discusses the system in a technical overview paper (PDF). Ted Ts'o comments, 'Users in the carrier grade linux space have been clamoring for this for a while. If you are a carrier in telephony and don't want downtime, this stuff is pure gold.'"
Update: 04/24 10:04 GMT by KD : Tomasz Chmielewsk writes on LKML that the idea seems to be patented by Microsoft.
There was a kernel exploit recently where someone submitted a patch that modified the running kernel using this technology. It didn't work for me, so I had to resort to patching the .c that was affected - but a lot of people reported that it worked.
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He basically compiles a patched and unpatched kernel with the same compiler, compares the ELF output, and uses that to generate a binary file that corresponds to the change. That gets wrapped in a generic module for use, another module installs it along with JMPs to bypass the old code and use the new, and he performs the checks needed to make sure he can safely install the redirects.
He also has to differentiate real changes from incidental ones (the example given is changing the address of a function - all references to it will change, but they don't really need to be included in the binary diff).
The only human work required is to check whether a patch makes semantic changes to a data structure... whether eg. an unsigned integer variable that was being used as a number is now a packed set of flags - the data declaration is the same, but it's being used differently.
Interesting paper. Also a useful new set of capabilities for any Linux user who can't handle downtime for quarterly patching... worth its weight in gold in some businesses.
Erik
A company that I once had dealings with was quite proud of their five nines. The motivation? It cost them $18,000 per second they were down. 30 seconds isn't just 30 seconds sometimes.
The very fact that there is load balancing means that every server is likely to have active connections going through it. If you currently have connections going through a specific server, you don't want to drop those connections in order to reboot that particular machine. This allows updates to a live machine.
...) so that if a hardware failure occurs the remaining hardware can keep the service up. Single points of failure are avoided like the plague in datacenters that require 100% uptime. Part of that is to deal with hardware failures but part is also to provide an ability to perform software/firmware upgrades when necessary. Once again, you migrate all traffic off the system you're upgrading then apply the upgrades offline. Upgrading a kernel, especially, in an online environment, is something virtually any sysadmin would want to avoid if at all possible.
If you have a load balanced environment then you have the ability to redirect new connections away from a given server. Then it's just a matter of waiting for the active connections to terminate before the machine ends up in an idle state where you can safely apply patches offline. I've worked in a number of telephony environments and this was always the way we would patch systems. Stop accepting new connections, wait for existing ones to end, then perform the patch, reboot, verify, and start accepting connections again.
Second, this is telephony, meaning it is the infrastructure on which the internet is based. There's no dns tricks or tcp/ip you can use to send people to a different "server" if that server is the switch connected to your fiber backbone. Basically, there are points in the infrastructure where there are by necessity a single chokepoint.
Any mission critical hardware, switches, routers, servers, etc. should be set up in redundant pairs (or triplets,
Redundancy is key, and any commercial datacenter will offer it all the way from their connections to the outside world to the connections they provide their customers. Every datacenter used by every company I ever worked for (about 10) offered redundant power and redundant network drops (using HSRP, VRRP, etc) for our equipment. If the datacenter needed to upgrade a router they'd move all traffic off one router so they could upgrade and test it, then move traffic off the other and repeat the process. Similarly if we needed to upgrade our firewalls, switches, etc. we'd fail over to the second redundant device first. In some cases we had bonded interfaces right on the end servers so as long as one path remained active we could power down an entire switch, router, firewall, etc. In other cases we relied on load balancing across servers that were alternately connected to one or another switch.
If you change something in a configuration that requires a change to the startup script, then you also change the startup script.
A patch to the kernel almost never requires changes to startup scripts. They're not talking about adding new functionality with user-space-addressable interfaces with this tool. They're talking about being able to install about 84% of security hotfixes in a hurry outside your scheduled reboots then rebooting on your regular maintenance schedule.
There's a difference between what YOU as an end user consider to be an open connection, and what the telecom equipment consider as a connection.
For all you know, your apparent always-on connection was actually a virtual connection being frequently switched & reswitched over many different real physical connections. That would be a fairly standard architecture for having a network infrastructure which can have components being worked on while data is still flowing through the network.
When the telecom provider is "waiting for active connections to go away" on a particular device only means that all of the virtual connections that are momentarily being switched that device have been successfully switched to another device. It doesn't mean that any of those virtual connections have to be terminated.