Date: Wed, 9 Mar 2016 10:44:23 +0100
From: Petr Mladek <pmladek@suse.com>
To: Torsten Duwe <duwe@lst.de>
Cc: Balbir Singh <bsingharora@gmail.com>, linuxppc-dev@ozlabs.org,
        linux-kernel@vger.kernel.org, rostedt@goodmis.org,
        kamalesh@linux.vnet.ibm.com, jeyu@redhat.com, jkosina@suse.cz,
        live-patching@vger.kernel.org, mbenes@suse.cz
Subject: Re: [PATCH][v6][RFC] livepatch/ppc: Enable livepatching on powerpc
Message-ID: <20160309094423.GH10940@pathway.suse.cz>
References: <1457506780-19556-1-git-send-email-bsingharora@gmail.com>
 <20160309091904.GA23039@lst.de>
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On Wed 2016-03-09 10:19:04, Torsten Duwe wrote:
> On Wed, Mar 09, 2016 at 05:59:40PM +1100, Balbir Singh wrote:
> > 
> > The previous revision was nacked by Torsten, but compared to the alternatives
> 
> I nacked it because I was confident it couldn't work. Same goes
> for this one, sorry. My good intention was to save us all some work.
> 
> > @@ -1265,6 +1271,51 @@ ftrace_call:
> >  	ld	r0, LRSAVE(r1)
> >  	mtlr	r0
> >  
> > +#ifdef CONFIG_LIVEPATCH
> > +	beq+	4f		/* likely(old_NIP == new_NIP) */
> > +	/*
> > +	 * For a local call, restore this TOC after calling the patch function.
> 
> This is the key issue.
> 
> Ftrace_caller can gather and save the current r2 contents, no problem;
> but the point is, it needs to be restored *after* the replacement function.
> I see 3 ways to accomplish this:
> 
> 1st: make _every_ replacement function aware of this, and make it restore
>      the TOC manually just before each return statement.
> 
> 2nd: provide a global hook to do the job, and use a stack frame to execute it.
> 
> 3rd: have a global hook like solution 2, but let it have its own data
>      structure, I'd call it a "shadow stack", for the real return addresses.
>      See struct fgraph_cpu_data in kernel/trace/trace_functions_graph.c
> 
> Using heuristics to determine whether the call was local or global
> makes me feel highly uncomfortable; one day it will break and
> nobody will remember why.
> 
> Balbir, the problem with your patch is that it goes only half the way from
> my solution 2 towards solution 1. When you call a helper function on return,
> you need a place to store the real return address.
> 
> I'll try to demonstrate a solution 1 as well, but you'll probably won't like
> that either...

To be honest, I still do not have a good picture about all the
problems in my head. Anyway, I would really appreciate if we could
find a solution that would work transparently. I mean that adding
an extra hacks into selected functions in the patch might be quite
error prone and problems hard to debug. I think that we all want this
but I wanted to be sure :-)

BTW: I am getting close to send a patch with some basic livepatch
documentation. It might be used to document "temporary" limitations.

Best Regards,
Petr