Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe

Joel Fernandes
2019-12-04T04:10:03Z

On Tue, Dec 03, 2019 at 08:13:29AM +0100, Ingo Molnar wrote:
>
> * Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > On Mon, Dec 02, 2019 at 04:32:13PM +0900, Masami Hiramatsu wrote:
> > > Anders reported that the lockdep warns that suspicious
> > > RCU list usage in register_kprobe() (detected by
> > > CONFIG_PROVE_RCU_LIST.) This is because get_kprobe()
> > > access kprobe_table[] by hlist_for_each_entry_rcu()
> > > without rcu_read_lock.
> > >
> > > If we call get_kprobe() from the breakpoint handler context,
> > > it is run with preempt disabled, so this is not a problem.
> > > But in other cases, instead of rcu_read_lock(), we locks
> > > kprobe_mutex so that the kprobe_table[] is not updated.
> > > So, current code is safe, but still not good from the view
> > > point of RCU.
> > >
> > > Let's lock the rcu_read_lock() around get_kprobe() and
> > > ensure kprobe_mutex is locked at those points.
> > >
> > > Note that we can safely unlock rcu_read_lock() soon after
> > > accessing the list, because we are sure the found kprobe has
> > > never gone before unlocking kprobe_mutex. Unless locking
> > > kprobe_mutex, caller must hold rcu_read_lock() until it
> > > finished operations on that kprobe.
> > >
> > > Reported-by: Anders Roxell <anders.roxell@linaro.org>
> > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> >
> > Instead of this, can you not just pass the lockdep_is_held() expression as
> > the last argument to list_for_each_entry_rcu() to silence the warning? Then
> > it will be a simpler patch.
>
> Come on, we do not silence warnings!

By silence, I mean remove a false-positive warning. In this case since lock
is held, it is not a valid warning.

> If it's safely inside the lock then why not change it from
> hlist_for_each_entry_rcu() to hlist_for_each_entry()?
>
> I do think that 'lockdep flag' inside hlist_for_each_entry_rcu():
>
> /**
> * hlist_for_each_entry_rcu - iterate over rcu list of given type
> * @pos: the type * to use as a loop cursor.
> * @head: the head for your list.
> * @member: the name of the hlist_node within the struct.
> * @cond: optional lockdep expression if called from non-RCU protection.
> *
> * This list-traversal primitive may safely run concurrently with
> * the _rcu list-mutation primitives such as hlist_add_head_rcu()
> * as long as the traversal is guarded by rcu_read_lock().
> */
> #define hlist_for_each_entry_rcu(pos, head, member, cond...) \
>
> is actively harmful. Why is it there?

Because as Paul also said, the code can be common between regular lock
holders and RCU lock holders. I am not sure if this is the case with the
kprobe code though.

thanks,

- Joel