Re: [PATCH RESEND2] ptrace: make ptrace() fail if the tracee changed its pid unexpectedly

[Oleg Nesterov <oleg@redhat.com>]

On 05/11, Linus Torvalds wrote:
>
> On Tue, May 11, 2021 at 9:56 AM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > This patch makes ptrace() fail in this case until debugger does wait()
> > and consumes PTHREAD_EVENT_EXEC which reports old_pid.
>
> I'm ok with the patch, just wondering which way it's supposed to come
> to me. Should I just apply it directly?

would be nice!

> That said, why this:
>
> > +       rcu_read_lock();
> > +       pid = task_pid_nr_ns(task, task_active_pid_ns(task->parent));
> > +       rcu_read_unlock();
>
> I don't see why the RCU read lock would be needed? task_pid_nr_ns()
> does any required locking itself, afaik.
>
> And even if it wasn't, this all happens with siglock held, can
> anything actually change.

... and with tasklist_lock held.

Hmm. Linus, I am shy to admit I can't answer immediately, I'll recheck
tomorrow after sleep. But it seems you are right.

Thanks!

Oleg.