On Wed, 28 Oct 2020 11:29:16 -0400 Steven Rostedt <[email protected]> wrote:
> Hi Masami, > > Talking with Peter and Thomas on IRC, where they really don't like > passing a partial pt_regs around, got me thinking of redoing the REGS > parameter of ftrace. Kprobes is the only user that requires the full > registers being saved, and that's only because some kprobe user might > want them. Yes, kprobes can be used for debugging use case. I think we can skip coller-saved registers (because ftrace is embedded by compiler and it may save such registers correctly), but we still need a pt_regs on memory to access it. > > On Wed, 28 Oct 2020 10:25:02 -0400 > Steven Rostedt <[email protected]> wrote: > > > > typedef void (*ftrace_func_t)(unsigned long ip, unsigned long parent_ip, > > struct ftrace_ops *op, struct pt_regs *regs); > > > > > Most registers of pt_regs at a start of a function is rather useless. > What if we got rid of FL_SAVE_REGS all together and had a "ftrace_regs" > structure passed in that would have only access to all the argument > registers, the stack pointer and the instruction pointer? Yeah, that's OK to me. If someone wants to investigate suspicious compiler bug, they should not put a kprobe on the fentry call site. (in most cases symbol+5 will put a breakpoint right after fentry nop) > Then kprobes could just create its own pt_regs, fill in all the data > from ftrace_regs and then fill the rest with zeros or possibly whatever > the values currently are (does it really matter what those registers > are?), including flags. That sounds good to me. > > Not only would this simplify the code, it would probably allow moving > more of the kprobe code from the arch specific to the generic code, and > remove a lot of duplication. Ah, right. > > This would also help speed up the processing of live kernel patching. > > And best of all, it would give everything access to the arguments of a > function and a stack pointer with out (ab)using pt_regs. > > Do you think this would be feasible? Yes, I agreed. Thank you, > > -- Steve -- Masami Hiramatsu <[email protected]>
