Re: [PATCH 2/5] kprobes: Use guard() for external locks

[Peter Zijlstra]

On Tue, Dec 10, 2024 at 11:15:28AM +0900, Masami Hiramatsu wrote:
> On Tue, 10 Dec 2024 11:04:28 +0900
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote:
> 
> > On Mon, 9 Dec 2024 12:04:11 +0100
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > 
> > > On Mon, Dec 09, 2024 at 11:41:26AM +0900, Masami Hiramatsu (Google) wrote:
> > > > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > > > 
> > > > Use guard() for text_mutex, cpu_read_lock, and jump_label_lock in
> > > > the kprobes.
> > > 
> > > > @@ -853,29 +850,24 @@ static void try_to_optimize_kprobe(struct kprobe *p)
> > > >  		return;
> > > >  
> > > >  	/* For preparing optimization, jump_label_text_reserved() is called. */
> > > > -	cpus_read_lock();
> > > > -	jump_label_lock();
> > > > -	mutex_lock(&text_mutex);
> > > > +	guard(cpus_read_lock)();
> > > > +	guard(jump_label_lock)();
> > > > +	guard(mutex)(&text_mutex);
> > > >  
> > > 
> > > > @@ -1294,62 +1280,55 @@ static int register_aggr_kprobe(struct kprobe *orig_p, struct kprobe *p)
> > > >  	int ret = 0;
> > > >  	struct kprobe *ap = orig_p;
> > > >  
> > > > -	cpus_read_lock();
> > > > -
> > > > -	/* For preparing optimization, jump_label_text_reserved() is called */
> > > > -	jump_label_lock();
> > > > -	mutex_lock(&text_mutex);
> > > 
> > > Why does kprobe need jump_label_lock and how does it then not also need
> > > static_call_lock ?
> > 
> > Good catch! It has not been updated for static_call_text_reserved().
> > We need static_call_lock() here too.
> 
> Wait, this is for checking the jump_label_text_reserved(), but as far as
> I know, the text reserved area of jump_label will be updated when the
> module is loaded or removed. And the static call too, right?

Correct.

> In that case, what we need is to lock the modules (for the short term,
> can we use rcu_read_lock?) for using both jump_label_text_reserved()
> and static_call_text_reserved()?

Yes, rcu_read_lock() is sufficient to observe fully loaded modules. I
don't think you care about placing kprobes on modules that are still
loading (that doesn't really make sense).

Also see:

  https://lkml.kernel.org/r/20241205215102.hRywUW2A@xxxxxxxxxxxxx