Re: [PATCH 0/3] livepatch: add shadow variable API

[Miroslav Benes]

On Tue, 13 Jun 2017, Joe Lawrence wrote:

> On 06/13/2017 06:19 AM, Miroslav Benes wrote:
> >> If you are referring to stacking livepatches ... to be honest I hadn't
> >> thought of that scenario.  In that case, we might be able to get away
> >> with pushing something like this into the hash:
> >>
> >>  klp #1:  klp_shadow_attach(ptr, "shadow_var", ...)
> >>  klp #2:  klp_shadow_attach(ptr, "shadow_var_v2", ...)
> > 
> > I thought this was the reason to have a string there. Otherwise, a 
> > pointer to original data would be enough, wouldn't it?
> 
> Well, one could attach multiple shadow variables to the same data
> structure, ie, one for each new data element.  In the stacking case, you
> might add a spinlock in patch 1, then a linked-list in patch 2.  Patched
> codepaths would then use klp_shadow_get(obj, "spinlock") or
> klp_shadow_get(obj, "list") as needed.

Ok, I mixed two different things into one. Yes, this is a valid use case.

> Versioning shadow variables would be a bit more involved.  You'd have to
> figure out if you A) convert existing shadow variables to the new format
> on livepatch module load, or B) convert on the fly, or C) handle none,
> v1, and v2 instances of the shadow variables.  /head spins

I'm gonna pretend I didn't read this.

> To be honest, I don't think we've never needed anything beyond basic
> shadow variables in kpatch, so I'm only speculating about their
> potential (ab)uses :)  That said, since this patchset is introducing the
> API, it would be good to be reasonably flexible.

I'd worry about that later. If we ever come upon that.

Thanks,
Miroslav