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

[Ingo Molnar <mingo@kernel.org>]

* 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!

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?

Thanks,

	Ingo