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

Peter Zijlstra
2023-03-21T15:51:42Z

On Tue, Mar 21, 2023 at 10:17:11AM -0400, Steven Rostedt wrote:
> 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.
> >
> > 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;
>
> Hmm, at least on 6.3, this should not be an issue anymore. I believe that
> all locations that have ftrace callbacks should now have rcu watching?
>
> I think we *want* a warn on when this happens.
>
> Peter?

You always want a wanring, because silently dropping stuff is very poor
form. But yes, we must not enter tracing then RCU isn't watching, that's
a fundamental fail and should be fixed.