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Date:   Tue, 16 May 2023 14:10:28 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>, 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>, 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 Tue, 16 May 2023 13:31:53 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> 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.

OK, preempt_count_add() is NOKPROBE_SYMBOL() so kprobe_busy_begin() should
be safe. The problem is in fprobe_kprobe_handler() then.

Thanks!

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


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