Re: [PATCH] Revert "kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled"

[Miroslav Benes <mbenes@suse.cz>]

On Sun, 19 Jul 2020, Joe Lawrence wrote:

> On 7/17/20 2:29 PM, Josh Poimboeuf wrote:
> > Use of the new -flive-patching flag was introduced with the following
> > commit:
> > 
> >    43bd3a95c98e ("kbuild: use -flive-patching when CONFIG_LIVEPATCH is
> >    enabled")
> > 
> > This flag has several drawbacks:
> > 
> > [ ... snip ... ]
> > 
> > - While there *is* a distro which relies on this flag for their distro
> >    livepatch module builds, there's not a publicly documented way to
> >    create safe livepatch modules with it.  Its use seems to be based on
> >    tribal knowledge.  It serves no benefit to those who don't know how to
> >    use it.
> > 
> >    (In fact, I believe the current livepatch documentation and samples
> >    are misleading and dangerous, and should be corrected.  Or at least
> >    amended with a disclaimer.  But I don't feel qualified to make such
> >    changes.)
> 
> FWIW, I'm not exactly qualified to document source-based creation either,
> however I have written a few of the samples and obviously the kselftest
> modules.
> 
> The samples should certainly include a disclaimer (ie, they are only for API
> demonstration purposes!) and eventually it would be great if the kselftest
> modules could guarantee their safety as well.  I don't know quite yet how we
> can automate that, but perhaps some kind of post-build sanity check could
> verify that they are in fact patching what they intend to patch.

That's a good idea. We should have something like that. I don't know how 
to make it nice. Just horrible post-build hacks that would check that 
modules were compiled as expected
 
> As for a more general, long-form warning about optimizations, I grabbed
> Miroslav's LPC slides from a few years back and poked around at some
> IPA-optimized disassembly... Here are my notes that attempt to capture some
> common cases:
> 
> http://file.bos.redhat.com/~jolawren/klp-compiler-notes/livepatch/compiler-considerations.html
> 
> It's not complete and I lost steam about 80% of the way through today. 
> :)  But if it looks useful enough to add to Documentation/livepatch, we 
> can work on it on-list and try to steer folks into using the automated
> kpatch-build, objtool (eventually) or a source-based safety checklist.

It looks really useful. Could you prepare a patch and submit it, please? 
We could discuss it there.

> The
> source-based steps have been posted on-list a few times, but I think it only
> needs to be formalized in a doc.

Yes, I think they were. We discussed it with Nicolai to (better) document 
our workflow. It is currently based on klp-ccp 
(https://github.com/SUSE/klp-ccp), but we need a proper documentation how 
to prepare a live patch starting with an ordinary patch.

Thanks
Miroslav