Message-ID: <20191015155107.GA21110@linux-8ccs>
Date:   Tue, 15 Oct 2019 17:51:07 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, 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
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

+++ Peter Zijlstra [15/10/19 15:56 +0200]:
>On Tue, Oct 15, 2019 at 03:07:40PM +0200, Jessica Yu wrote:
>> I'm having trouble visualizing what changes you're planning on making.
>
>I want all the text poking (jump_label, ftrace, klp whatever) to happen
>_before_ we do the protection changes.
>
>I also want to avoid flipping the protection state around unnecessarily,
>because that clearly is just wasted work.

OK, that makes sense, thanks for clarifying. 

>> I get that you want to squash ftrace_module_enable() into
>> ftrace_module_init(), before module_enable_ro(). I'm fine with that as
>> long as the races Steven described are addressed for affected arches.
>
>Right, the problem is set_all_modules_text_*(), that relies on COMING
>having made the protection changes.
>
>After the x86 changes, there's only 2 more architectures that use that
>function. I'll work on getting those converted and then we can delete
>that function and worry no more about it.
>
>> And livepatch should be OK as long as klp_module_coming() is always
>> called *after* ftrace_module_enable(). But are you moving
>> klp_module_coming() before the module_enable_* calls as well?  And if
>> so, why?
>
>I wanted to move the COMING notifier callback before the protection
>changes, because that is the easiest way to convert jump_label and
>static_call and AFAICT nothing really cared aside from ftrace.

I think I would be fine with this as long as no notifiers/users rely
on the assumption that COMING == module enabled protections already.
I've yet to audit all the module notifiers (but I trust you've done
this already), and I think ftrace was the only user that relied on
this. For livepatch it's probably not immediately fixable without some
serious overhaul.

>The alternative is introducing additional module states, which just adds
>complexity we don't really need if we can just flip things around a
>little.

Yeah, I would prefer not adding more states if possible :-)

>> > The fact that it is executable; also the fact that you do it right after
>> > we mark it ro. Flipping the memory protections back and forth is just
>> > really poor everything.
>> >
>> > Once this ftrace thing is sorted, we'll change x86 to _refuse_ to make
>> > executable (kernel) memory writable.
>>
>> Not sure if relevant, but just thought I'd clarify: IIRC,
>> klp_module_coming() is not poking the coming module, but it calls
>> module_enable_ro() on itself (the livepatch module) so it can apply
>> relocations and such on the new code, which lives inside the livepatch
>> module, and it needs to possibly do this numerous times over the
>> lifetime of the patch module for any coming module it is responsible
>> for patching (i.e., call module_enable_ro() on the patch module, not
>> necessarily the coming module). So I am not be sure why
>> klp_module_coming() should be moved before complete_formation(). I
>> hope I'm remembering the details correctly, livepatch folks feel free
>> to chime in if I'm incorrect here.
>
>You mean it does module_disable_ro() ? That would be broken and it needs
>to be fixed. Can some livepatch person explain what it does and why?

Gah, sorry, yes I meant module_disable_ro().

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