On Fri, 18 Oct 2019, Jessica Yu wrote: > +++ Miroslav Benes [16/10/19 15:29 +0200]: > >On Wed, 16 Oct 2019, Miroslav Benes wrote: > > > >> On Wed, 16 Oct 2019, Peter Zijlstra wrote: > >> > >> > On Tue, Oct 15, 2019 at 06:27:05PM -0400, Steven Rostedt wrote: > >> > > >> > > (7) Seventh session, titled "klp-convert and livepatch relocations", > >> > > was led > >> > > by Joe Lawrence. > >> > > > >> > > Joe started the session with problem statement: accessing non exported > >> > > / static > >> > > symbols from inside the patch module. One possible workardound is > >> > > manually via > >> > > kallsyms. Second workaround is klp-convert, which actually creates > >> > > proper > >> > > relocations inside the livepatch module from the symbol database during > >> > > the > >> > > final .ko link. > >> > > Currently module loader looks for special livepatch relocations and > >> > > resolves > >> > > those during runtime; kernel support for these relocations have so far > >> > > been > >> > > added for x86 only. Special livepatch relocations are supported and > >> > > processed > >> > > also on other architectures. Special quirks/sections are not yet > >> > > supported. > >> > > Plus klp-convert would still be needed even with late module patching > >> > > update. > >> > > vmlinux or modules could have ambiguous static symbols. > >> > > > >> > > It turns out that the features / bugs below have to be resolved before > >> > > we > >> > > can claim the klp-convert support for relocation complete: > >> > > - handle all the corner cases (jump labels, static keys, ...) > >> > > properly and > >> > > have a good regression tests in place > >> > > >> > I suppose all the patches in this series-of-series here will make life > >> > harder for KLP, static_call() and 2 byte jumps etc.. > >> > >> Yes, I think so. We'll have to deal with that once it lands. That is why > >> we want to get rid of all this arch-specific code in livepatch and > >> reinvent the late module patching. So it is perhaps better to start > >> working on it sooner than later. Adding Petr, who hesitantly signed up for > >> the task... > > > >Thinking about it more... crazy idea. I think we could leverage these new > >ELF .text per vmlinux/module sections for the reinvention I was talking > >about. If we teach module loader to relocate (and apply alternatives and > >so on, everything in arch-specific module_finalize()) not the whole module > >in case of live patch modules, but separate ELF .text sections, it could > >solve the issue with late module patching we have. It is a variation on > >Steven's idea. When live patch module is loaded, only its section for > >present modules would be processed. Then whenever a to-be-patched module > >is loaded, its .text section in all present patch module would be > >processed. > > > >The upside is that almost no work would be required on patch modules > >creation side. The downside is that klp_modinfo must stay. Module loader > >needs to be hacked a lot in both cases. So it remains to be seen which > >idea is easier to implement. > > > >Jessica, do you think it would be feasible? > > I think that does sound feasible. I'm trying to visualize how that > would look. I guess there would need to be various livepatching hooks > called during the different stages (apply_relocate_add(), > module_finalize(), module_enable_ro/x()). > > So maybe something like the following? > > When a livepatch module loads: > apply_relocate_add() > klp hook: apply .klp.rela.$objname relocations *only* for > already loaded modules > module_finalize() > klp hook: apply .klp.arch.$objname changes for already loaded modules > module_enable_ro() > klp hook: only enable ro/x for .klp.text.$objname for already > loaded modules > > When a to-be-patched module loads: > apply_relocate_add() > klp hook: for each patch module that patches the coming > module, apply .klp.rela.$objname relocations for this object > module_finalize() > klp hook: for each patch module that patches the coming > module, apply .klp.arch.$objname changes for this object > module_enable_ro() > klp hook: for each patch module, apply ro/x permissions for > .klp.text.$objname for this object > > Then, in klp_module_coming, we only need to do the callbacks and > enable the patch, and get rid of the module_disable_ro->apply > relocs->module_enable_ro block. > > Does that sound like what you had in mind or am I totally off? Sort of. What I had in mind was that we could get rid of all special .klp ELF section if module loader guarantees that only sections for loaded modules are processed. Then .klp.rela.$objname is not needed and proper .rela.text.$objname (or whatever its text section is named) should be sufficient. The same for the rest (.klp.arch). Only then it would be useful. Miroslav
