linux-kernel - Re: [PATCH v3 5/6] x86/ftrace: Use text

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Date:   Mon, 21 Oct 2019 16:14:20 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        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
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

+++ Petr Mladek [18/10/19 15:40 +0200]:
>On Fri 2019-10-18 15:03:42, Jessica Yu wrote:
>> +++ Miroslav Benes [16/10/19 15:29 +0200]:
>> > On Wed, 16 Oct 2019, Miroslav Benes wrote:
>> > 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.$objname for already
>>        loaded modules
>
>Just for record. We should also set ro for the not-yet used
>.klp.text.$objname at this stage so that it can't be modified
>easily "by accident".

If we also set ro protection already for .klp.text.$objname for
not-yet loaded modules, I think this would unfortunately mean we would
still have to do the protection flipping for late module patching that
Peter was trying to avoid, right?

That is, we *still* end up having to do the whole module_disable_ro()
-> apply_relocate_add() -> module_finalize() -> module_enable_ro()
thing for late module patching, except now we've moved that work to
the module loader instead of in klp_module_coming.. It sounds just as
complicated as the current way :/

However, I think this complaint would not apply if livepatch switches
to the one patch module per module model..