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

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Message-ID: <alpine.LSU.2.21.1910231057270.4266@pobox.suse.cz>
Date:   Wed, 23 Oct 2019 11:04:04 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Jessica Yu <jeyu@...nel.org>,
        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, live-patching@...r.kernel.org,
        pmladek@...e.com
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

On Tue, 22 Oct 2019, Josh Poimboeuf wrote:

> On Tue, Oct 22, 2019 at 10:27:49AM +0200, Miroslav Benes wrote:
> > > 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).
> 
> If I understand correctly, using kvm as an example to-be-patched module,
> we'd have:
> 
>   .text.kvm
>   .rela.text.kvm
>   .altinstructions.kvm
>   .rela.altinstructions.kvm
>   __jump_table.kvm
>   .rela__jump_table.kvm
> 
> etc.  i.e. any "special" sections would need to be renamed.
> 
> Is that right?

Yes.
 
> But also I think *any* sections which need relocations would need to be
> renamed, for example:
> 
>   .rodata.kvm
>   .rela.rodata.kvm
>   .orc_unwind_ip.kvm
>   .rela.orc_unwind_ip.kvm

Correct.
 
> It's an interesting idea.
> 
> We'd have to be careful about ordering issues.  For example, there are
> module-specific jump labels stored in mod->jump_entries.  Right now
> that's just a pointer to the module's __jump_table section.  With late
> module patching, when kvm is loaded we'd have to insert the klp module's
> __jump_table.kvm entries into kvm's mod->jump_entries list somehow.

Yes.
 
> Presumably we'd also have that issue for other sections.  Handling that
> _might_ be as simple as just hacking up find_module_sections() to
> re-allocate sections and append "patched sections" to them.
>
> But then you still have to worry about when to apply the relocations.
> If you apply them before patching the sections, then relative
> relocations would have the wrong values.  If you apply them after, then
> you have to figure out where the appended relocations are.

Ah, right. That is a valid remark.
 
> And if we allow unpatching then we'd presumably have to be able to
> remove entries from the module specific section lists.

Correct.

> So I get the feeling a lot of complexity would creep in.  Even just
> thinking about it requires more mental gymnastics than the
> one-patch-per-module idea, so I view that as a bad sign.

Yes, the devil is in the details. It would be better if the approach 
helped even someone/something else in the kernel. Without it, it is 
probably better to stick to Steven's proposal and handle the complexity 
elsewhere.

Thanks
Miroslav