linux-kernel - Re: [PATCH] bpf: reject blacklisted symbols in kprobe

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Date:   Sat, 13 May 2023 00:17:57 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yonghong Song <yhs@...a.com>
Cc:     Ze Gao <zegao2021@...il.com>, Jiri Olsa <olsajiri@...il.com>,
        Song Liu <song@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ze Gao <zegao@...cent.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: reject blacklisted symbols in kprobe_multi to
 avoid recursive trap

On Fri, 12 May 2023 07:29:02 -0700
Yonghong Song <yhs@...a.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