Re: [PATCH 0/8] livepatch: klp-convert tool

[Josh Poimboeuf]

On Fri, Oct 20, 2017 at 02:44:32PM +0200, Torsten Duwe wrote:
> On Thu, Oct 19, 2017 at 06:00:54PM +0200, Miroslav Benes wrote:
> > On Thu, 19 Oct 2017, Josh Poimboeuf wrote:
> > > 
> > > Sounds nice, though I wonder what the obstacles are?
> > 
> > Those GCC optimizations you mentioned below and which I didn't connect to 
> > klp-convert itself.
> 
> I have a bad feeling about the IPA stuff in general. An obj-based approach
> is cool in a way that it still works, and is sure to work, if the IPA
> assumptions that led to the optimisations still hold, but as soon as they
> break, you're screwed big time.

Huh?  The obj-based approach (e.g., kpatch, bin-diff) inherently detects
such changes.  Or am I misunderstanding?  If so, please elaborate.

> For -fpatchable-function-entries I switched
> off IPA-RA, as especially on RISC there's _nothing_ you can do between
> functions without at least one scratch reg. But for live patching, I'd like
> the kernel to be compiled in the first place with 100% ABI adherence, IOW
> all IPA optimisations turned off. Does anyone have numbers on the performance
> impact?

I agree that would be the best option.

I don't think anyone has measured it because we don't know how to get
the compiler to do that :-)  My guess is that it will be something close
to negligible.

> > Nothing serious aside from that, I hope. Nicolai is currently implementing 
> > C parser for kernel sources.
> >  
> > > > You could verify the result and its correctness.
> > > 
> > > Does that mean it's easier to do code review?  Or something else?
> > 
> > Yes, the code review.
> > 
> > > > It could also be beneficial if we'd like to pursue automatic
> > > > verification in the future.
> > > 
> > > What do you mean by automatic verification?
> > 
> > Formal verification. Theoretically we could have a formal specification of 
> > our consistency model and we could prove/disprove whether a livepatch and 
> > its implementation are correct with respect to it. It is a vague idea 
> > though and I personally haven't got sufficient knowledge to do anything 
> > about it.
> 
> For example, if the patched functions and the fixes meet its criteria, you
> could use CMBC (http://www.cprover.org/cbmc/) to _prove_ that the live patch
> changes exactly what you claim to, and nothing else.

Can it also prove that the patch is applied in a safe manner?

-- 
Josh