Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal

[Joe Lawrence <joe.lawrence@redhat.com>]

On 9/4/19 4:49 AM, Petr Mladek wrote:
> On Tue 2019-09-03 15:02:34, Miroslav Benes wrote:
>> On Mon, 2 Sep 2019, Joe Lawrence wrote:
>>
>>> On 9/2/19 12:13 PM, Miroslav Benes wrote:
>>>>> I can easily foresee more problems like those in the future.  Going
>>>>> forward we have to always keep track of which special sections are
>>>>> needed for which architectures.  Those special sections can change over
>>>>> time, or can simply be overlooked for a given architecture.  It's
>>>>> fragile.
>>>>
>>>> Indeed. It bothers me a lot. Even x86 "port" is not feature complete in
>>>> this regard (jump labels, alternatives,...) and who knows what lurks in
>>>> the corners of the other architectures we support.
>>>>
>>>> So it is in itself reason enough to do something about late module
>>>> patching.
>>>>
>>>
>>> Hi Miroslav,
>>>
>>> I was tinkering with the "blue-sky" ideas that I mentioned to Josh the other
>>> day.
>>
>>> I dunno if you had a chance to look at what removing that code looks
>>> like, but I can continue to flesh out that idea if it looks interesting:
>>
>> Unfortunately no and I don't think I'll come up with something useful
>> before LPC, so anything is really welcome.
>>
>>>
>>>    https://github.com/joe-lawrence/linux/tree/blue-sky
>>>
>>> A full demo would require packaging up replacement .ko's with a livepatch, as
>>> well as "blacklisting" those deprecated .kos, etc.  But that's all I had time
>>> to cook up last week before our holiday weekend here.
>>
>> Frankly, I'm not sure about this approach. I'm kind of torn. The current
>> solution is far from ideal, but I'm not excited about the other options
>> either. It seems like the choice is basically between "general but
>> technically complicated fragile solution with nontrivial maintenance
>> burden", or "something safer and maybe cleaner, but limiting for
>> users/distros". Of course it depends on whether the limitation is even
>> real and how big it is. Unfortunately we cannot quantify it much and that
>> is probably why our opinions (in the email thread) differ.
> 
> I wonder what is necessary for a productive discussion on Plumbers:
> 

Pre-planning this part of the miniconf is a great idea.

>    + Josh would like to see what code can get removed when late
>      handling of modules gets removed. I think that it might be
>      partially visible from Joe's blue-sky patches.
> 
> 
>    + I would like to better understand the scope of the current
>      problems. It is about modifying code in the livepatch that
>      depends on position of the related code:
> 
>        + relocations are rather clear; we will need them anyway
> 	to access non-public (static) API from the original code.
> 
>        + What are the other changes?
> 
>        + Do we use them in livepatches? How often?
> 
>        + How often new problematic features appear?
> 
>        + Would be possible to detect potential problems, for example
> 	by comparing the code in the binary and in memory when
> 	the module is loaded the normal way?
> 
>        + Would be possible to reset the livepatch code in memory
> 	when the related module is unloaded and safe us half
> 	of the troubles?
> 
> 
>      + It might be useful to prepare overview of the existing proposals
>        and agree on the positives and negatives. I am afraid that some
>        of them might depend on the customer base and
>        use cases. Sometimes we might not have enough information.
>        But it might be good to get on the same page where possible.
> 
>        Anyway, it might rule out some variants so that we could better
>        concentrate on the acceptable ones. Or come with yet another
>        proposal that would avoid the real blockers.
> 
> 
> Any other ideas?

I'll just add to your list that late module patching introduces 
complexity for klp-convert / livepatch style relocation support. 
Without worrying about unloaded modules, I *think* klp-convert might 
already be able to handle relocations in special sections (altinsts, 
parainst, etc.).

I've put the current klp-convert patchset on top of the blue-sky branch 
to see if this indeed the case, but I'm not sure if I'll get through 
that experiment before LPC.

> 
> Would it be better to discuss this in a separate room with
> a whiteboard or paperboard?
> 

Whiteboard would probably be ideal, but paper would work and be more 
transportable than the former.

-- Joe