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

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