On Thu, Feb 09, 2017 at 01:14:52PM -0500, Steven Rostedt wrote: > > [ sending again with Masami Cc'd ] > > On Thu, 9 Feb 2017 13:14:14 -0500 > Steven Rostedt <rost...@goodmis.org> wrote: > > > On Thu, 9 Feb 2017 18:06:44 +0000 > > Russell King - ARM Linux <li...@armlinux.org.uk> wrote: > > > > > On Thu, Feb 09, 2017 at 12:13:22PM -0500, Steven Rostedt wrote: > > > > Then came along live kernel patching, which I believe this series is > > > > trying to support. What is needed by pt_regs is a way to "hijack" the > > > > function being called to instead call the patched function. That is, > > > > ftrace is not being used for tracing, but in reality, being used to > > > > modify the running kernel. It is being used to change what function > > > > gets called. ftrace is just a hook for that mechanism. > > > > > > So, would I be correct to assume that the only parts of pt_regs that > > > would be touched are those which contain arguments to the function, > > > and the register which would contain the return value? > > > > > > > For live kernel patching, perhaps. > > > > But for kprobes, I think they can touch anything. Matters what the > > creater of the kprobe wanted to do. > > Thing is, by saving all of them is the easiest way to ensure that the whole context is the same when the replacing function gets called, as I said before.
We can't be sure that while __ftrace_ops_list_func is executing, any of the regs will have the value they had when the function-to-be-replaced was called. That's the reason I say we need to save them all. > > -- Steve >