linux-kernel - Re: [PATCH 3/3] x86/ftrace: Use text

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Date:   Tue, 22 Oct 2019 19:21:32 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Song Liu <songliubraving@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH 3/3] x86/ftrace: Use text_poke()

On Tue, 22 Oct 2019 15:45:26 -0700
Andy Lutomirski <luto@...capital.net> wrote:

> >> On Oct 22, 2019, at 2:58 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> >> 
> >> 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  
> 
> Why are you saving rsi?  You said upthread that you’re saving the
> args, but rsi is already available in rsi.

The above is for two parameters, and is being called before the
function with those two parameters. The jited_bpf_prog will be called
with those two parameters as well, but it may also clobber them. Then
we need to restore those two parameters before calling the original
function.

> 
> Just how custom is this bpf program?  It seems to clobber no regs
> (except args), and it doesn’t return anything. Is it entirely
> specific to the probed function?  If so, why not just call it
> directly?

It's injecting the jited_bpf_prog to be called when the probed function
is called, with the same parameters as the probed function.

my_probed_function
  call trampoline


trampoline
  save parameters
  call jited_bpf_prog (with same parameters)
  restore paremeters
  ret

Jumps back to the my_probed_function, where my_probed_function is
clueless it was just interrupted.

No need to save any clobbered registers but the parameters, as the
jited_bpf_prog needs to save any registers that it clobbers just like
the my_probed_function saves its.

-- Steve