On Wed, Sep 04, 2019 at 10:49:32AM +0200, 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: > > + 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. Yes, and I like what I see. Especially the removal of the .klp.arch nastiness! > + 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? I think the .klp.arch sections are the big ones: .klp.arch.altinstructions .klp.arch.parainstructions .klp.arch.jump_labels (doesn't exist yet) And that's just x86... And then of course there's the klp coming/going notifiers which have also been an additional source of complexity. > + Do we use them in livepatches? How often? I don't have a number, but it's very common to patch a function which uses jump labels or alternatives. > + How often new problematic features appear? I'm not exactly sure what you mean, but it seems that anytime we add a new feature, we have to try to wrap our heads around how it interacts with the weirdness of late module patching. > + 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? Perhaps, though I assume this would be some out-of-band testing thing. > + Would be possible to reset the livepatch code in memory > when the related module is unloaded and safe us half > of the troubles? Maybe, but I think that would solve a much lower percentage of our troubles than half :-/ > + 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. I think we've already done that for the existing proposals. Maybe Miroslav can summarize them at the LPC session. > 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. I'd like to hear more specific negatives about Joe's recent patches, which IMO, are the best option we've discussed so far. -- Josh
