Re: [PATCH 3/3] x86/ftrace: Use text_poke()

[Alexei Starovoitov]

On Tue, Oct 22, 2019 at 05:04:30PM -0400, Steven Rostedt wrote:
> 
> I gave a solution for this. And that is to add another flag to allow
> for just the minimum to change the ip. And we can even add another flag
> to allow for changing the stack if needed (to emulate a call with the
> same parameters).

your solution is to reduce the overhead.
my solution is to remove it competely. See the difference?

> By doing this work, live kernel patching will also benefit. Because it
> is also dealing with the unnecessary overhead of saving regs.
> 
> And we could possibly even have kprobes benefit from this if a kprobe
> doesn't need full regs.

Neither of two statements are true. The per-function generated trampoline
I'm talking about is bpf specific. For a function with two arguments it's just:
push rbp 
mov rbp, rsp
push rdi
push rsi
lea  rdi,[rbp-0x10]
call jited_bpf_prog
pop rsi
pop rdi
leave
ret

fentry's nop is replaced with call to the above.
That's it.
kprobe and live patching has no use out of it.

> But you said that you can't have this and trace the functions at the
> same time. Which also means you can't do live kernel patching on these
> functions either.

I don't think it's a real use case, but to avoid further arguing
I'll add one nop to the front of generated bpf trampoline so that
ftrace and livepatch can use it.