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:   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
Message-ID: <20230513001757.75ae0d1b@rorschach.local.home>
In-Reply-To: <ee28e791-b3ab-3dfd-161b-4e7ec055c6ff@meta.com>
References: <20230510122045.2259-1-zegao@tencent.com>
        <6308b8e0-8a54-e574-a312-0a97cfbf810c@meta.com>
        <ZFvUH+p0ebcgnwEg@krava>
        <CAD8CoPC_=d+Aocp8pnSi9cbU6HWBNc697bKUS1UydtB-4DFzrA@mail.gmail.com>
        <ee28e791-b3ab-3dfd-161b-4e7ec055c6ff@meta.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

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