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kgraft to Patch Ubuntu Kernel without Reboot


It is simply how to allow kernel swap without needing reboot.

Article by: arges

New live kernel patching projects have hit LKML recently [1][2], and I’ve taken the opportunity to test drive kGraft with the Ubuntu kernel. This post documents how to get a sample patch working.

A Simple Example First, I had to take the patches from [3] and apply them against the ubuntu-utopic kernel, which is based on 3.15-rc8 as of this post. They cherry-picked cleanly and the branch I’m using is stored here [4]. In addition to applying the patches I had to also enable CONFIG_KGRAFT. A pre-built test kernel can be downloaded here [5].

Next, I created a test VM and installed the test kernel, headers, and build dependencies into that VM and rebooted. Now after a successful reboot, we need to produce an actual patch to test. I’ve created a github project [6] with the sample patch; to make it easy to clone and get started.

sudo apt-get install git build-essential
git clone https://github.com/arges/kgraft-examples.git
cd kgraft-examples

The code in kgraft_patcher.c is the example found in samples/kgraft [7]. Now we can build it easily using the Makefile I have in my project by typing make.

Next, the module needs to be inserted using the following:

sudo insmod ./kgraft_patcher.ko

Run the following to see if the module loaded properly:

lsmod | grep kgraft

You’ll notice some messages printed with the following:

[ 211.762563] kgraft_patcher: module verification failed: signature and/or required key missing - tainting kernel [ 216.800080] kgr failed after timeout (30), still in degraded mode [ 246.880146] kgr failed after timeout (30), still in degraded mode [ 276.960211] kgr failed after timeout (30), still in degraded mode

This means that not all processes have entered the kernel and may not have a “new universe” flag set. Run the following to see which processes still needs to be updated.

cat /proc/*/kgr_in_progress

In order to get all processes to enter the kernel sometimes a signal needs to be sent to get the process to enter the kernel.

An example of this is found in the kgraft-examples [6] called ‘gethurry.sh’:

for p in $(ls /proc/ | grep '^[0-9]'); do
  if [[ -e /proc/$p/kgr_in_progress ]]; then
    if [[ `sudo cat /proc/$p/kgr_in_progress` -eq 1 ]]; then
     echo $p;
     sudo kill -SIGCONT $p

Here is checks for all processes that have ‘kgr_in_progress’ set and sends a SIGCONT signal to that process.

I’ve noticed that I had to also send a SIGSTOP followed by a SIGCONT to finally get everything synced up.

Eventually you’ll see:

[ 1600.480233] kgr succeeded

Now your kernel is running the new patch without rebooting!