Re: [PATCH] tracing: Refuse fprobe if RCU is not watching

Masami Hiramatsu
2023-03-21T14:12:36Z

On Tue, 21 Mar 2023 02:01:03 +0000
Yafang Shao <laoar.shao@gmail.com> wrote:

> It hits below warning on my test machine when running
> selftests/bpf/test_progs,
>
> [ 702.223611] ------------[ cut here ]------------
> [ 702.224168] RCU not on for: preempt_count_sub+0x0/0xa0
> [ 702.224770] WARNING: CPU: 14 PID: 5267 at include/linux/trace_recursion.h:162 fprobe_handler.part.0+0x1b8/0x1c0
> [ 702.231740] CPU: 14 PID: 5267 Comm: main_amd64 Kdump: loaded Tainted: G O 6.2.0+ #584
> [ 702.233169] RIP: 0010:fprobe_handler.part.0+0x1b8/0x1c0
> [ 702.241388] Call Trace:
> [ 702.241615] <TASK>
> [ 702.241811] fprobe_handler+0x22/0x30
> [ 702.242129] 0xffffffffc04710f7
> [ 702.242417] RIP: 0010:preempt_count_sub+0x5/0xa0
> [ 702.242809] Code: c8 50 68 94 42 0e b5 48 cf e9 f9 fd ff ff 0f 1f 80 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 e8 4b cd 38 0b <55> 8b 0d 9c d0 cf 02 48 89 e5 85 c9 75 1b 65 8b 05 be 78 f4 4a 89
> [ 702.244752] RSP: 0018:ffffaf6187d27f10 EFLAGS: 00000082 ORIG_RAX: 0000000000000000
> [ 702.245801] RAX: 000000000000000e RBX: 0000000001b6ab72 RCX: 0000000000000000
> [ 702.246804] RDX: 0000000000000000 RSI: ffffffffb627967d RDI: 0000000000000001
> [ 702.247801] RBP: ffffaf6187d27f30 R08: 0000000000000000 R09: 0000000000000000
> [ 702.248786] R10: 0000000000000000 R11: 0000000000000000 R12: 00000000000000ca
> [ 702.249782] R13: ffffaf6187d27f58 R14: 0000000000000000 R15: 0000000000000000
> [ 702.250785] ? preempt_count_sub+0x5/0xa0
> [ 702.251540] ? syscall_enter_from_user_mode+0x96/0xc0
> [ 702.252368] ? preempt_count_sub+0x5/0xa0
> [ 702.253104] ? syscall_enter_from_user_mode+0x96/0xc0
> [ 702.253918] do_syscall_64+0x16/0x90
> [ 702.254613] entry_SYSCALL_64_after_hwframe+0x72/0xdc
> [ 702.255422] RIP: 0033:0x46b793
>
> This issue happens under CONFIG_CONTEXT_TRACKING_USER=y. When a task
> enters from user mode to kernel mode, or enters from user mode to irq,
> it excutes preempt_count_sub before RCU begins watching, and thus this
> warning is triggered.
>
> We should not handle fprobe if RCU is not watching.

Good catch!

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

Thank you!

>
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> Cc: Jiri Olsa <olsajiri@gmail.com>
> ---
> kernel/trace/fprobe.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index e8143e3..fe4b248 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -27,6 +27,9 @@ static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
> struct fprobe *fp;
> int bit;
>
> + if (!rcu_is_watching())
> + return;
> +
> fp = container_of(ops, struct fprobe, ops);
> if (fprobe_disabled(fp))
> return;
> --
> 1.8.3.1
>

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