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 -- 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
