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

From: Joe Lawrence
Date: Wed Sep 04 2019 - 12:26:55 EST

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

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

Hi Miroslav,

I was tinkering with the "blue-sky" ideas that I mentioned to Josh the other

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.

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