linux-kernel - Re: [PATCH 2/2] ppc64le live patch: get rid of mini stack frame

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Message-ID: <1457657442.22078.1.camel@ellerman.id.au>
Date:	Fri, 11 Mar 2016 11:50:42 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Torsten Duwe <duwe@....de>, Petr Mladek <pmladek@...e.com>
Cc:	jeyu@...hat.com, jkosina@...e.cz, jikos@...nel.org,
	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	kamalesh@...ux.vnet.ibm.com, linuxppc-dev@...abs.org,
	live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH 2/2] ppc64le live patch: get rid of mini stack frame

On Thu, 2016-03-10 at 14:04 +0100, Torsten Duwe wrote:
> On Thu, Mar 10, 2016 at 01:51:16PM +0100, Petr Mladek wrote:
> > On Thu 2016-03-10 13:25:08, Petr Mladek wrote:
> > > On Wed 2016-03-09 18:30:17, Torsten Duwe wrote:
> > > > After the mini stack frame is no longer required for TOC storage, it can
> > > > be eliminated iff the functionality of klp_return_helper, which required
> > > > a stack frame for the extra return address previously, is carried out
> > > > by the replacement function now. This requires _every_ live patch replacement
> > > > function to execute the following (or similar) sequence of machine instructions
> > > > just before every return to the original caller:
> > > 
> > > I have thought about it and it is a nono from my point of view.
> > > It is too error prone, especially that there are functions that
> > > call return on several locations.
> 
> Yes, that's what I think as well when I look at it.

> > BTW: How is this solved in kretprobes? Or is it easier there?
> 
> Without any look at the code I assume it uses solution 3. Once
> you have a probing framework in place, you can remember the real
> return addresses in a data structure. As I wrote, the function
> graph tracer does it this way so it would be abvious.

Yeah it has a linked list of struct kretprobe_instance's, each of which stores
the real return address.

I have some ideas for how to fix livepatch, but this week is a bit busy with
merge window prep.

cheers