Subject: Re: [PATCH/RFC] ppc64 livepatch: frameless klp_return_helper using odd TOC

From: Torsten Duwe <[email-masked]>
Date: Thu, 24 Mar 2016 11:27:57 +0100

On Thu, Mar 24, 2016 at 03:44:55PM +0530, Kamalesh Babulal wrote:
> * Torsten Duwe <[email-masked]> [2016-03-23 16:58:58]:
> 
> > 
> > Since nobody liked the extra stack frame nor its workarounds, here is
> > the next attempt. Assumptions:
> > 
> > 1. Heuristics are bad. The better they are, the more subtly the
> >    way they might fail.
> > 
> > 2. The TOC pointer is usually dividable by 4, if not by 8. An odd
> >    value never occurs.
> > 
> > Conclusively, this patch unambiguously creates an odd TOC value when
> > an ftraced function's global entry point is used. Ftrace_caller will
> > then immediately fix it, and alongside gather the information whether
> > the made call was local or global.
> > 
> > In case of live patching this information is furthermore used to decide
> > whether a klp_return_helper needs to be inserted or not.
> > CAVEAT: any frameless klp_return_helper does not play well with
> > sibling calls! There's an emergency exit that might work, at worst
> > it will cause an oops, but it surely avoids a lockup.
> > At least the live patching modules on ppc64le will need to be compiled
> > using the -fno-optimize-sibling-calls compiler flag!
> > 
> > Thanks go to Michael Matz and Richard Biener for reassurance about
> > heuristics and pointers to the compiler flag.
> > 
> > Signed-off-by: Torsten Duwe <[email-masked]>
> 
> Hi Torsten,
> 
> Should this patch be applied over Petr Mladek's v4 ?

Yes. Just omit the changes it makes to entry_64.S and use this instead.

	Torsten