Re: [PATCH v2] tracing/uprobe: Add missing PID filter for uretprobe

Oleg Nesterov
2024-08-27T14:26:26Z

On 08/27, Jiri Olsa wrote:
>
> On Tue, Aug 27, 2024 at 12:40:52PM +0200, Oleg Nesterov wrote:
> > static bool
> > uprobe_multi_link_filter(struct uprobe_consumer *con, struct mm_struct *mm)
> > {
> > struct bpf_uprobe *uprobe;
> > + struct task_struct *task, *t;
> > + bool ret = false;
> >
> > uprobe = container_of(con, struct bpf_uprobe, consumer);
> > - return uprobe->link->task->mm == mm;
> > + task = uprobe->link->task;
> > +
> > + rcu_read_lock();
> > + for_each_thread(task, t) {
> > + struct mm_struct *mm = READ_ONCE(t->mm);
> > + if (mm) {
> > + ret = t->mm == mm;
> > + break;
> > + }
> > + }
> > + rcu_read_unlock();
>
> that seems expensive if there's many threads

many threads with ->mm == NULL? In the likely case for_each_thread()
stops after the first t->mm check.

> could we check the leader first and only if it's gone fallback to this?

up to you..

Oleg.