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

From: Masami Hiramatsu (Google)
Date: Tue Jun 07 2022 - 12:11:04 EST


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@xxxxxxxxxx/

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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