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
