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

From: Steven Rostedt <rostedt@goodmis.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Nadav Amit <nadav.amit@gmail.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Song Liu <songliubraving@fb.com>,
	Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH 3/3] x86/ftrace: Use text_poke()
Date: Tue, 22 Oct 2019 18:17:40 -0400	[thread overview]
Message-ID: <20191022181740.5a2893a6@gandalf.local.home> (raw)
In-Reply-To: <20191022215841.2qsmhd6vxi4mwade@ast-mbp.dhcp.thefacebook.com>

On Tue, 22 Oct 2019 14:58:43 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.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?

You're just trimming it down. I'm curious to what overhead you save by
not saving all parameter registers, and doing a case by case basis?

> 
> > 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

What exactly does the jited_bpf_prog do? Does it modify context?
or is it for monitoring only.

Do only GPL BPF programs get this access?

> 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.

And how does this nop get accounted for? It needs to update the ftrace
dyn_ftrace array that stores all the function locations.

-- Steve