Re: [RFC] kretprobe: Prevent triggering kretprobe from within kprobe_flush_task

[Jiri Olsa]

On Thu, Apr 09, 2020 at 09:38:06PM +0900, Masami Hiramatsu wrote:
> Hi Jiri,
> 
> On Wed,  8 Apr 2020 18:46:41 +0200
> Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> 
> > hi,
> > Ziqian reported lockup when adding retprobe on _raw_spin_lock_irqsave.
> 
> Hmm, kprobe is lockless, but kretprobe involves spinlock.
> Thus, eventually, I will blacklist the _raw_spin_lock_irqsave()
> for kretprobe.

I thought of blacklisting, but we were using that kretprobe
in a bcc script.. it's not overloaded by using bpf trampolines,
but still might be useful

SNIP

> > The code within the kretprobe handler checks for probe reentrancy,
> > so we won't trigger any _raw_spin_lock_irqsave probe in there.
> > 
> > The problem is in outside kprobe_flush_task, where we call:
> > 
> >   kprobe_flush_task
> >     kretprobe_table_lock
> >       raw_spin_lock_irqsave
> >         _raw_spin_lock_irqsave
> > 
> > where _raw_spin_lock_irqsave triggers the kretprobe and installs
> > kretprobe_trampoline handler on _raw_spin_lock_irqsave return.
> 
> Hmm, OK. In this case, I think we should mark this process is
> going to die and never try to kretprobe on it.
> 
> > 
> > The kretprobe_trampoline handler is then executed with already
> > locked kretprobe_table_locks, and first thing it does is to
> > lock kretprobe_table_locks ;-) the whole lockup path like:
> > 
> >   kprobe_flush_task
> >     kretprobe_table_lock
> >       raw_spin_lock_irqsave
> >         _raw_spin_lock_irqsave ---> probe triggered, kretprobe_trampoline installed
> > 
> >         ---> kretprobe_table_locks locked
> > 
> >         kretprobe_trampoline
> >           trampoline_handler
> >             kretprobe_hash_lock(current, &head, &flags);  <--- deadlock
> > 
> > The change below sets current_kprobe in kprobe_flush_task, so the probe
> > recursion protection check is hit and the probe is never set. It seems
> > to fix the deadlock.
> > 
> > I'm not sure this is the best fix, any ideas are welcome ;-)
> 
> Hmm, this is a bit tricky to fix this issue. Of course, temporary disable
> kprobes (and kretprobe) on an area by filling current_kprobe might
> be a good idea, but it also involves other kprobes.
> 
> How about let kretprobe skip the task which state == TASK_DEAD ?

hum, isn't that considerable amount of paths (with state == TASK_DEAD)
that we would kill kprobes for? someone might want to trace it

thanks,
jirka