linux-kernel - Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support to record and check the accessed args

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Date: Mon, 1 Apr 2024 10:28:17 +0800
From: 梦龙董 <dongmenglong.8@...edance.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Jiri Olsa <jolsa@...nel.org>, 
	Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...ux.dev>, Eddy Z <eddyz87@...il.com>, 
	Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, 
	John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller" <davem@...emloft.net>, 
	David Ahern <dsahern@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	X86 ML <x86@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Quentin Monnet <quentin@...valent.com>, bpf <bpf@...r.kernel.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-riscv <linux-riscv@...ts.infradead.org>, linux-s390 <linux-s390@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, linux-trace-kernel@...r.kernel.org, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, linux-stm32@...md-mailman.stormreply.com
Subject: Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support
 to record and check the accessed args

On Sun, Mar 31, 2024 at 3:34 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Sat, 30 Mar 2024 11:18:29 +0800
> 梦龙董 <dongmenglong.8@...edance.com> wrote:
>
> > > If you really want to have thousands of functions, why not just register it
> > > with ftrace itself. It will give you the arguments via the ftrace_regs
> > > structure. Can't you just register a program as the callback?
> > >
> >
> > Ennn...I don't understand. The main purpose for
> > me to use TRACING is:
> >
> > 1. we can directly access the memory, which is more
> >    efficient.
>
> I'm not sure what you mean by the above. Access what memory?
>

We need to use the helper of bpf_probe_read_kernel
when we read "skb->sk" in kprobe, and the "skb" is the
1st arg in ip_rcv(). And we can directly read "skb->sk"
in tracing, which is more efficient. Isn't it?

> > 2. we can obtain the function args in FEXIT, which
> >     kretprobe can't do it. And this is the main reason.
>
> I didn't mention kretprobe. If you need access to the exit of the function,
> you can use Masami's fgraph update.
>
>  fentry -> ftrace_trampoline -> your_code
>
> For fgraph:
>
>  fentry -> ftrace_trampoline -> fgraph [sets up return call] -> your_entry_code
>
>  function ret -> fgraph_ret_handler -> your_exit_code
>
> And you will be able to pass data from the entry to the exit code,
> including parameters.

Yeah, the fgraph sounds like a nice solution to my problem.
I'll have a try on it.

Thanks!
Menglong Dong

>
> -- Steve
>
>