netdev - [PATCH bpf v2 0/2] rethook: Reject getting a rethook if RCU is not watching

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Date:   Wed,  8 Jun 2022 01:10:52 +0900
From:   "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: [PATCH bpf v2 0/2] rethook: Reject getting a rethook if RCU is not watching

Hi,

Here is the 2nd version of the patches to reject rethook if RCU is
not watching. The 1st version is here;

https://lore.kernel.org/all/165189881197.175864.14757002789194211860.stgit@devnote2/

This is actually related to the idle function tracing issue
reported by Jiri on LKML (*)

(*) https://lore.kernel.org/bpf/20220515203653.4039075-1-jolsa@kernel.org/

Jiri reported that fprobe (and rethook) based kprobe-multi bpf
trace kicks "suspicious RCU usage" warning. This is because the
RCU operation is used in the kprobe-multi handler. However, I
also found that the similar issue exists in the rethook because
the rethook uses RCU operation.

I added a new patch [1/2] to test this issue by fprobe_example.ko.
(with this patch, it can avoid using printk() which also involves
the RCU operation.)

 ------
 # insmod fprobe_example.ko symbol=arch_cpu_idle use_trace=1 stackdump=0 
 fprobe_init: Planted fprobe at arch_cpu_idle
 # rmmod fprobe_example.ko 

 =============================
 WARNING: suspicious RCU usage
 5.18.0-rc5-00019-gcae4ec21e87a-dirty #30 Not tainted
 -----------------------------
 include/trace/events/lock.h:37 suspicious rcu_dereference_check() usage!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 1

 RCU used illegally from extended quiescent state!
 no locks held by swapper/0/0.

 stack backtrace:
 CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.18.0-rc5-00019-gcae4ec21e87a-dirty #30
 ------

After applying [2/2] fix (which avoid initializing rethook on
function entry if !rcu_watching()), this warning was gone.

 ------
 # insmod fprobe_example.ko symbol=arch_cpu_idle use_trace=1 stackdump=0
 fprobe_init: Planted fprobe at arch_cpu_idle
 # rmmod fprobe_example.ko 
 fprobe_exit: fprobe at arch_cpu_idle unregistered. 225 times hit, 230 times missed
 ------

Note that you can test this program until the arch_cpu_idle()
is marked as noinstr. After that, the function can not be
traced.

Thank you,

---

Masami Hiramatsu (Google) (2):
      fprobe: samples: Add use_trace option and show hit/missed counter
      rethook: Reject getting a rethook if RCU is not watching

 kernel/trace/rethook.c          |    9 +++++++++
 samples/fprobe/fprobe_example.c |   21 +++++++++++++++++----
 2 files changed, 26 insertions(+), 4 deletions(-)

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