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

[Jessica Yu]

+++ 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().