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

[Joel Fernandes]

On Tue, Dec 03, 2019 at 08:13:29AM +0100, Ingo Molnar wrote:
> 
> * Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > > Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > 
> > 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