Re: [PATCH] uprobes: get RCU trace lock before list iteration

[Breno Leitao]

On Fri, Nov 08, 2024 at 09:28:17AM -0800, Andrii Nakryiko wrote:
> On Fri, Nov 8, 2024 at 1:00 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Nov 07, 2024 at 09:14:45AM -0800, Breno Leitao wrote:
> > > Acquire RCU trace lock in filter_chain() to protect
> > > list_for_each_entry_rcu() iteration, protecting the list iteration in a
> > > RCU read section.
> > >
> > > Prior to this fix, list_for_each_entry_srcu() was called without holding
> > > the required lock, triggering warnings when RCU_PROVING is enabled:
> > >
> > >       kernel/events/uprobes.c:937 RCU-list traversed without holding the required lock!!
> > >
> > > Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> > > Fixes: cc01bd044e6a ("uprobes: travers uprobe's consumer list locklessly under SRCU protection")
> > > ---
> > >  kernel/events/uprobes.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > > index fa04b14a7d72353adc440742016b813da6c812d2..afdaa45a43ac3948f7983175eda808c989e8738a 100644
> > > --- a/kernel/events/uprobes.c
> > > +++ b/kernel/events/uprobes.c
> > > @@ -1103,11 +1103,13 @@ static bool filter_chain(struct uprobe *uprobe, struct mm_struct *mm)
> > >       bool ret = false;
> > >
> > >       down_read(&uprobe->consumer_rwsem);
> > > +     rcu_read_lock_trace();
> > >       list_for_each_entry_rcu(uc, &uprobe->consumers, cons_node, rcu_read_lock_trace_held()) {
> >
> > Maybe I'm confused, but isn't uprobe->consumer list protected by
> > uprobe->consumer_rwsem, which we hold for reading?
> >
> > That is, AFAICT this is a false positive and we should be doing this
> > instead, no?
> 
> Yep, you are absolutely right. RCU-protected traversal is important
> only for handler_chain() and handle_uretprobe_chain(). Here it's all
> under lock, so no need for RCU protection.

Thanks. I will update