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

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Message-ID: <20191022215841.2qsmhd6vxi4mwade@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 22 Oct 2019 14:58:43 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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, 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.