Re:

Ze Gao
2023-05-21T14:20:49Z

On Sun, May 21, 2023 at 6:09 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> On Sun, 21 May 2023 10:08:46 +0200
> Jiri Olsa <olsajiri@gmail.com> wrote:
>
> > On Sat, May 20, 2023 at 05:47:24PM +0800, Ze Gao wrote:
> > >
> > > Hi Jiri,
> > >
> > > Would you like to consider to add rcu_is_watching check in
> > > to solve this from the viewpoint of kprobe_multi_link_prog_run
> >
> > I think this was discussed in here:
> > https://lore.kernel.org/bpf/20230321020103.13494-1-laoar.shao@gmail.com/
> >
> > and was considered a bug, there's fix mentioned later in the thread
> >
> > there's also this recent patchset:
> > https://lore.kernel.org/bpf/20230517034510.15639-3-zegao@tencent.com/
> >
> > that solves related problems
>
> I think this rcu_is_watching() is a bit different issue. This rcu_is_watching()
> check is required if the kprobe_multi_link_prog_run() uses any RCU API.
> E.g. rethook_try_get() is also checks rcu_is_watching() because it uses
> call_rcu().

Yes, that's my point!

Regards,
Ze

>
> >
> > > itself? And accounting of missed runs can be added as well
> > > to imporve observability.
> >
> > right, we count fprobe->nmissed but it's not exposed, we should allow
> > to get 'missed' stats from both fprobe and kprobe_multi later, which
> > is missing now, will check
> >
> > thanks,
> > jirka
> >
> > >
> > > Regards,
> > > Ze
> > >
> > >
> > > -----------------
> > > From 29fd3cd713e65461325c2703cf5246a6fae5d4fe Mon Sep 17 00:00:00 2001
> > > From: Ze Gao <zegao@tencent.com>
> > > Date: Sat, 20 May 2023 17:32:05 +0800
> > > Subject: [PATCH] bpf: kprobe_multi runs bpf progs only when rcu_is_watching
> > >
> > > From the perspective of kprobe_multi_link_prog_run, any traceable
> > > functions can be attached while bpf progs need specical care and
> > > ought to be under rcu protection. To solve the likely rcu lockdep
> > > warns once for good, when (future) functions in idle path were
> > > attached accidentally, we better paying some cost to check at least
> > > in kernel-side, and return when rcu is not watching, which helps
> > > to avoid any unpredictable results.
> > >
> > > Signed-off-by: Ze Gao <zegao@tencent.com>
> > > ---
> > > kernel/trace/bpf_trace.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > index 9a050e36dc6c..3e6ea7274765 100644
> > > --- a/kernel/trace/bpf_trace.c
> > > +++ b/kernel/trace/bpf_trace.c
> > > @@ -2622,7 +2622,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
> > > struct bpf_run_ctx *old_run_ctx;
> > > int err;
> > >
> > > - if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> > > + if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1 || !rcu_is_watching())) {
> > > err = 0;
> > > goto out;
> > > }
> > > --
> > > 2.40.1
> > >
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@kernel.org>