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: Sat, 30 Mar 2024 15:37:22 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: 梦龙董 <dongmenglong.8@...edance.com>
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
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 <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, John Fastabend
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 <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>,
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Subject: Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add
 support to record and check the accessed args

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?

> 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.

-- Steve