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

[Miroslav Benes <mbenes@suse.cz>]

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