On Thu 2019-10-24 15:16:34, Peter Zijlstra wrote:
> On Wed, Oct 23, 2019 at 12:00:25PM -0500, Josh Poimboeuf wrote:
>
> > > This then raises a number of questions:
> > >
> > > 1) why is that RELA (that obviously does not depend on any module)
> > > applied so late?
> >
> > Good question. The 'pv_ops' symbol is exported by the core kernel, so I
> > can't see any reason why we'd need to apply that rela late. In theory,
> > kpatch-build isn't supposed to convert that to a klp rela. Maybe
> > something went wrong in the patch creation code.
> >
> > I'm also questioning why we even need to apply the parainstructions
> > section late. Maybe we can remove that apply_paravirt() call
> > altogether, along with .klp.arch.parainstruction sections.
Hmm, the original bug report against livepatching was actually about
paravirt ops, see below.
> > I'll need to look into it...
>
> Right, that really should be able to run early. Esp. after commit
>
> 11e86dc7f274 ("x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call()")
>
> paravirt patching is unconditional. We _never_ run with the indirect
> call except very early boot, but modules should have them patched way
> before their init section runs.
>
> We rely on this for spectre-v2 and friends.
Livepatching has the same requirement. The module code has to be fully
livepatched before the module gets actually used. It means before
mod->init() is called and before the module is moved into
MODULE_STATE_LIVE state.
> > > 3) Is there ever a possible module-dependent RELA to a paravirt /
> > > alternative site?
> >
> > Good question...
>
> > > Then for 3) we only have alternatives left, and I _think_ it unlikely to
> > > be the case, but I'll have to have a hard look at that.
> >
> > I'm not sure about alternatives, but maybe we can enforce such
> > limitations with tooling and/or kernel checks.
>
> Right, so on IRC you implied you might have some additional details on
> how alternatives were affected; did you manage to dig that up?
I am not sure what Josh had in mind. But the problem with livepatches,
paravort ops, and alternatives was described in the related patchset, see
https://lkml.kernel.org/r/1471481911-5003-1-git-send-email-jeyu@xxxxxxxxxx
The original bug report is
https://lkml.kernel.org/r/20160329120518.GA21252@xxxxxxxxxxxxx
Best Regards,
Petr
