linux-kernel - Re: [PATCH/RFC] ppc64 livepatch: frameless klp_return

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Message-ID: <20160324101453.GA1445@linux.vnet.ibm.com>
Date:	Thu, 24 Mar 2016 15:44:55 +0530
From:	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:	Torsten Duwe <duwe@....de>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	Petr Mladek <pmladek@...e.com>, jeyu@...hat.com,
	jkosina@...e.cz, jikos@...nel.org, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, linuxppc-dev@...abs.org,
	live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH/RFC] ppc64 livepatch: frameless klp_return_helper using
 odd TOC

* Torsten Duwe <duwe@....de> [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@...e.de>

Hi Torsten,

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

Thanks,
Kamalesh.