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Date:   Sat, 13 May 2023 00:17:57 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Yonghong Song <yhs@meta.com>
Cc:     Ze Gao <zegao2021@gmail.com>, Jiri Olsa <olsajiri@gmail.com>,
        Song Liu <song@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ze Gao <zegao@tencent.com>, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH] bpf: reject blacklisted symbols in kprobe_multi to
 avoid recursive trap
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On Fri, 12 May 2023 07:29:02 -0700
Yonghong Song <yhs@meta.com> wrote:

> A fprobe_blacklist might make sense indeed as fprobe and kprobe are 
> quite different... Thanks for working on this.

Hmm, I think I see the problem:

fprobe_kprobe_handler() {
   kprobe_busy_begin() {
      preempt_disable() {
         preempt_count_add() {  <-- trace
            fprobe_kprobe_handler() {
		[ wash, rinse, repeat, CRASH!!! ]

Either the kprobe_busy_begin() needs to use preempt_disable_notrace()
versions, or fprobe_kprobe_handle() needs a
ftrace_test_recursion_trylock() call.

-- Steve