Re: Patching kthread functions

[Evgenii Shatokhin <eshatokhin@virtuozzo.com>]

On 01.10.2020 14:13, Miroslav Benes wrote:
> On Wed, 30 Sep 2020, Evgenii Shatokhin wrote:
> 
>> Hi,
>>
>> I wonder, can livepatch from the current mainline kernel patch the main
>> functions of kthreads, which are running or sleeping constantly? Are there any
>> best practices here?
> 
> No. It is a "known" limitation, "" because we discussed it a couple of
> times (at least with Petr), but it is not documented :(
> 
> I wonder if it is really an issue practically. I haven't met a case
> yet when we wanted to patch such thing. But yes, you're correct, it is not
> possible.

Well, I have recently encountered such case, with kaio_fsync_thread() 
function from our custom kernel, the code at the URL below. Our 
customers were interested in a particular bug fix there: there was a 
race, potentially leading to data corruption.

We still use the old-style kpatch.ko-based patches for that kernel 
version, so we definitely cannot deliver the fix via a live kernel 
patch, only a regular kernel update will do. That made me wonder if the 
modern livepatch could handle it.

>   
>> I mean, suppose we have a function which runs in a kthread (passed to
>> kthread_create()) and is organized like this:
>>
>> while (!kthread_should_stop()) {
>>    ...
>>    DEFINE_WAIT(_wait);
>>    for (;;) {
>>      prepare_to_wait(waitq, &_wait, TASK_INTERRUPTIBLE);
>>      if (we_have_requests_to_process || kthread_should_stop())
>>        break;
>>      schedule();
>>    }
>>    finish_wait(waitq, &_wait);
>>    ...
>>    if (we_have_requests_to_process)
>>      process_one_request();
>>    ...
>> }
>>
>> (The question appeared when I was looking at the following code:
>> https://src.openvz.org/projects/OVZ/repos/vzkernel/browse/drivers/block/ploop/io_kaio.c?at=refs%2Ftags%2Frh7-3.10.0-1127.8.2.vz7.151.14#478)
>>
>> The kthread is always running and never exits the kernel.
>>
>> I could rewrite the function to add klp_update_patch_state() somewhere, but
>> would it help?
> 
> In fact, we used exactly this approach in, now obsolete, kGraft. All
> kthreads had to be manually annotated somewhere safe, where safe meant
> that the thread could be switched to a new universe without the problem
> wrt to calling old/new functions in the loop...
> 
>> No locks are held right before and after "schedule()", and the thread is not
>> processing any requests at that point.
> 
> ... like this.
> 
>> But even if I place
>> klp_update_patch_state(), say, just before schedule(), it would just switch
>> task->patch_state for that kthread.
> 
> Correct.
> 
>> The old function will continue running, right?
> 
> Correct. It will, however, call new functions.

Ah, I see.

So, I guess, our best bet would be to rewrite the thread function so 
that it contains just the event loop and calls other non-inline 
functions to actually process the requests. And, perhaps, - place 
klp_update_patch_state() before schedule().

This will not help with this particular kernel version but could make it 
possible to live-patch the request-processing functions in the future 
kernel versions. The main thread function will remain unpatchable but it 
will call the patched functions once we switch the patch_state for the 
thread.

> 
>> Looks like we can only switch to the patched code of the function at the
>> beginning, via Ftrace hook. So, if the function is constantly running or
>> sleeping, it seems, it cannot be live-patched.
> 
> Yes and no. Normally, a task cannot run indefinitely and if it sleeps, we
> can deal with that (thanks to stack checking or signaling/kicking the
> task), but kthreads' main loops are special.
> 
>> Is that so? Are there any workarounds?
> 
> Petr, do you remember the crazy workarounds we talked about? My head is
> empty now. And I am sure, Nicolai could come up with something.

Interesting. I am all ears.

Thanks!
Evgenii

> 
> Thanks
> Miroslav
> .
>