Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

[Jessica Yu]

+++ 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?

Thanks!

Jessica