Re: [PATCH][v6][RFC] livepatch/ppc: Enable livepatching on powerpc

From: Torsten Duwe
Date: Wed Mar 09 2016 - 04:19:21 EST

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
> + 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...