Date: Wed, 9 Mar 2016 10:19:04 +0100
From: Torsten Duwe <duwe@lst.de>
To: Balbir Singh <bsingharora@gmail.com>
Cc: linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org, rostedt@goodmis.org,
        kamalesh@linux.vnet.ibm.com, pmladek@suse.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
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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...

	Torsten