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

From: Kamalesh Babulal
Date: Thu Mar 24 2016 - 06:16:15 EST

* Torsten Duwe <duwe@xxxxxx> [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 <duwe@xxxxxxx>

Hi Torsten,

Should this patch be applied over Petr Mladek's v4 ?