linux-kernel - Re: [RFC bpf-next 4/4] selftests/bpf: Add attach bench test

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Date:   Thu, 28 Apr 2022 20:09:45 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [RFC bpf-next 4/4] selftests/bpf: Add attach bench test

On Thu, 28 Apr 2022 19:53:03 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> > But that __fentry__ call is not part of __bpf_tramp_exit, actually.
> > Whether to call it a bug or limitation is secondary. It marks
> > __bpf_tramp_exit as attachable through kprobe/ftrace while it really
> > isn't.  
> 
> I'm confused by what you mean by "marks __bpf_tramp_exit as attachable"?
> What does? Where does it get that information? Does it read
> available_filter_functions?

OK, I think I see the issue you have. Because the functions shown in
available_filter_functions which uses the simple "%ps" to show the function
name:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/trace/ftrace.c#n3692

And the code that does the actual matching uses kallsyms_lookup()

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/trace/ftrace.c#n4017

Which appears not to match the function for the address, you can't pass in
__bpf_tramp_exit because it wont match the symbol returned by
kallsyms_lookup.

This does indeed look like a bug in %ps.

But in the mean time, I could open code %ps and see if that fixes it. I'll
give it try.

-- Steve