linux-kernel - Re: [PATCH][v4] livepatch/ppc: Enable livepatching on powerpc

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Message-ID: <20160304181657.GA30434@lst.de>
Date:	Fri, 4 Mar 2016 19:16:57 +0100
From:	Torsten Duwe <duwe@....de>
To:	Petr Mladek <pmladek@...e.com>
Cc:	linuxppc-dev@...abs.org, Balbir Singh <bsingharora@...il.com>,
	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	kamalesh@...ux.vnet.ibm.com, jeyu@...hat.com, jkosina@...e.cz,
	live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH][v4] livepatch/ppc: Enable livepatching on powerpc

On Fri, Mar 04, 2016 at 02:01:37PM +0100, Petr Mladek wrote:
> 
> Do I understand it correctly that we could not patch functions that
> pass arguments on the stack with this implementation? If yes, how hard
> would be to get it working, please? At least, it would be great to
> catch this problem and handle it with grace. Otherwise, it might
> be hard to debug.

No, those functions only require special attention.
I needed _any_ location to store the caller's TOC;
and the stack is thread-safe and recursion-safe.
The current caller's frame is already full so I had
to create a new one.

A patch function could e.g. grab that TOC value in a
prologue and then pop that stack frame. Or it could
add those 32 bytes to the assumed arguments' stack offsets.

> 
> > Where shall we put this warning?
> 
> Sadly, we do not have any Documentation/livepatch/ yet/.
> I still hope that we could handle it somehow in the code.

I really think some documentation would be good, a live patch
howto for a start...

	Torsten