[kpatch] livepatch vs kpatch hooks and stack check

[Evgenii Shatokhin]

Hi,

On 26.04.2019 16:32, Joe Lawrence wrote:
> On 4/26/19 3:51 AM, Mario Smarduch wrote:
>> Hello,
>>    I noticed that with livepatch the old and new functions can
>> run in parallel, but with kpatch the kpatch module load fails on
>> "activeness safety check".
> 
> Hi Mario,
> 
> I think you are referring to kpatch's use of stop_machine() to quiesce 
> the system before patching vs. livepatch's consistency model[1] and its 
> convergence on a patched state?
> 
>> Am I missing something with livepatch? Is there another way to
>> build and run it?
> 
> AFAIK there is no way around the consistency model with livepatch.  What 
> you might be able to do is hack the kpatch-build to utilize the kpatch 
> support module instead of the livepatch API.  (Look for the 
> $KPATCH_MODULE code in the kpatch-build script.)  I don't think this use 
> case is very well tested and in fact, the kpatch.ko will be deprecated 
> eventually.

We have been using the old-style (i.e. relying on kpatch.ko, 
non-Livepatch) binary patches in Virtuozzo for more than 2 years now. 
They still work pretty well for us. I guess, we will continue using this 
until Livepatch matures a bit more.

If you are going to try this approach too, here is the branch with the 
patched kpatch and kpatch-build, might be helpful:

https://github.com/euspectre/kpatch/commits/vz-test

2 patches are relevant here:

* 
https://github.com/euspectre/kpatch/commit/84b9ba7392d8b79dc4c38ec244331ba61e93bf0c 
- this adds an option to kpatch-build which allows making the old-style 
patches

* 
https://github.com/euspectre/kpatch/commit/5b54fe4b443573d7c7416ed861e99970ec1ce814 
- this instructs kpatch script to look for the old-style patches first. 
Useful if the kernel has Livepatch enabled as well, which is the case 
for us.

Hope this helps.

> 
> Do you have a particular case in mind which the livepatch consistency 
> model does not support?
> 
> [1] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/livepatch/livepatch.txt?h=v5.1-rc6#n63 
> 
> 
> Regards,
> 
> -- Joe
> 

Regards,
Evgenii