Re: [PATCH v4 13/15] livepatch: change to a per-task consistency model

Miroslav Benes
2017-02-07T08:21:54Z

> > And finally, the section "Limitations" has this text under the first
> > bullet:
> >
> > + The patch must not change the semantic of the patched functions.
> >
> > The current implementation guarantees only that either the old
> > or the new function is called. The functions are patched one
> > by one. It means that the patch must _not_ change the semantic
> > of the function.
> >
> > I think it is confusing. The consistency model allows us to change the
> > semantic of a function. To certain degree. Of course, there are cases that
> > cannot be patched, or have to be patched carefully. For example if a
> > function takes a lock by calling foo_lock(), foo_lock() is not on a stack
> > afterwards. Then the locking semantics may be changed with a livepatch.
> > One has to make sure to patch also the caller foo_lock() to enforce the
> > consistency. And so on... But I do not consider a limitation of livepatch.
> > It is a feature of the consistency model, which is weaker than kGraft's or
> > kpatch's (or stronger. It depends on your point of view.)
> >
> > So, I propose to remove this text and better describe the properties of
> > the consistency model above in the section 3. Maybe a quote from an old
> > mail thread (Nov 2014) would be sufficient. I don't remember what was
> > mentioned and what not.
> >
> > What do you think?
>
> I'll remove the above limitation.
>
> I'm not sure how to improve the consistency model section. It already
> has at least some mentions of changed function semantics and locking
> semantics. I'll leave it alone for now, unless you have a specific
> suggestion.

Fair enough. Let's see if I can come up with something.

> > > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> > > index 6602b34..ed90ad1 100644
> > > --- a/include/linux/livepatch.h
> > > +++ b/include/linux/livepatch.h
> > > @@ -68,7 +92,7 @@ struct klp_func {
> > > * @funcs: function entries for functions to be patched in the object
> > > * @kobj: kobject for sysfs resources
> > > * @mod: kernel module associated with the patched object
> > > - * (NULL for vmlinux)
> > > + * (NULL for vmlinux)
> >
> > This looks superfluous.
>
> This is a minor whitespace fix -- remove a space before tab. I figured
> I'd go ahead and fix it since I'm already changing some of the
> surrounding code.

Ok, no problem.

Thanks,
Miroslav