On 09/03/16 20:19, 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. I don't doubt that. I added it to the changelog to keep the history. I've been working with the constraints we have to get a solution that does not put the burden on the patch writer. That is why this is marked experimental as it needs a lot of testing. I think we should mark livepatching on PPC as experimental to begin with >> @@ -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. > Yes and I think -pg without -mprofile-kernel does a good job of doing it. In my patch I try to detect a call via stub and one without. The one with the stub will do the right thing (global calls). For local calls I have the store in CR+4 hook. > 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 We thought of a shadow stack as well, but the copying can be expensive. I;ve not looked at trace_functions_graph.c in detail, will look > 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. It could break, but as with any code, the code is only as good as the test cases it passes :) We can document our design in detail > > 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... Sure, look forward to it. I am keen on getting live-patching working. I think v4 with the documented limitation is fine - see Michael's email as well > Torsten > Thanks for looking into this, Balbir Singh. -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
