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Date:   Tue, 16 May 2023 13:31:53 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Yonghong Song <yhs@meta.com>, 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 Sat, 13 May 2023 00:17:57 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> 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.

Oops, I got it. Is preempt_count_add() tracable? If so, kprobe_busy_begin()
should be updated.

Thanks,

> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>