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Date:   Sun, 9 Apr 2023 13:25:44 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Yafang Shao <laoar.shao@gmail.com>
Cc:     mhiramat@kernel.org, alexei.starovoitov@gmail.com,
        linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Jiri Olsa <olsajiri@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] tracing: Refuse fprobe if RCU is not watching
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On Sun, 9 Apr 2023 20:45:39 +0800
Yafang Shao <laoar.shao@gmail.com> wrote:
> 
> I didn't notice preempt_{disable,enable}_notrace before :(
> Seems that is a better solution. I have verified that it really works.

Great.

> 
> BTW, why can we attach fentry to preempt_count_sub(), but can't attach
> it to migrate_disable() ?
> migrate_disable() isn't hidden from ftrace either...

You can't?

 # trace-cmd -p function -l migrate_disable
 # trace-cmd show
          <idle>-0       [000] ..s2. 153664.937829: migrate_disable <-bpf_prog_run_clear_cb
          <idle>-0       [000] ..s2. 153664.937834: migrate_disable <-bpf_prog_run_clear_cb
          <idle>-0       [000] ..s2. 153664.937835: migrate_disable <-bpf_prog_run_clear_cb
          <idle>-0       [000] ..s2. 153665.769555: migrate_disable <-bpf_prog_run_clear_cb
          <idle>-0       [000] ..s2. 153665.772109: migrate_disable <-bpf_prog_run_clear_cb

I think bpf prevents it. Perhaps that's another solution:

See 35e3815fa8102 ("bpf: Add deny list of btf ids check for tracing programs")

-- Steve