Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 28 Apr 2022 20:09:45 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>
Subject: Re: [RFC bpf-next 4/4] selftests/bpf: Add attach bench test
Message-ID: <20220428200945.5f6a5ba2@gandalf.local.home>
In-Reply-To: <20220428195303.6295e90b@gandalf.local.home>
References: <20220407125224.310255-1-jolsa@kernel.org>
        <20220407125224.310255-5-jolsa@kernel.org>
        <CAEf4BzbE1n3Lie+tWTzN69RQUWgjxePorxRr9J8CuiQVUfy-kA@mail.gmail.com>
        <20220412094923.0abe90955e5db486b7bca279@kernel.org>
        <CAEf4BzaQRcZGMqq5wqHo3wSHZAAVvY6AhizDk_dV_GtnwHuxLQ@mail.gmail.com>
        <20220416232103.c0b241c2ec7f2b3b985a2f99@kernel.org>
        <20220428095803.66c17c32@gandalf.local.home>
        <CAADnVQKi+4oBt2C__qz7QoHqTtXYLUjaqwTNFoSE=up9c9k4cA@mail.gmail.com>
        <20220428160519.04cc40c0@gandalf.local.home>
        <CAEf4Bzbu3zuDcPj3ue8D6VCdMTw2PEREJBU42CbR1Pe=5qOrTQ@mail.gmail.com>
        <20220428195303.6295e90b@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Thu, 28 Apr 2022 19:53:03 -0400
Steven Rostedt <rostedt@goodmis.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