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

[Masami Hiramatsu]

Hi Joel,

On Mon, 2 Dec 2019 16:08:54 -0500
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.

Ah, I see. That is more natural to silence the warning.

Thank you!

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>