Date:   Fri, 18 Oct 2019 15:03:42 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Lawrence <joe.lawrence@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com,
        live-patching@...r.kernel.org, pmladek@...e.com
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

+++ 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

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