Re: [PATCH] procfs: fix missing RCU protection when reading real_parent in do_task_stat()

[Jinliang Zheng]

On Tue, 27 Jan 2026 19:49:11 +0100, mjguzik@xxxxxxxxx wrote:
> On Tue, Jan 27, 2026 at 06:25:25PM +0100, Oleg Nesterov wrote:
> > On 01/27, alexjlzheng@xxxxxxxxx wrote:
> > > --- a/fs/proc/array.c
> > > +++ b/fs/proc/array.c
> > > @@ -528,7 +528,9 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
> > >  		}
> > >
> > >  		sid = task_session_nr_ns(task, ns);
> > > -		ppid = task_tgid_nr_ns(task->real_parent, ns);
> > > +		rcu_read_lock();
> > > +		ppid = task_tgid_nr_ns(rcu_dereference(task->real_parent), ns);
> > > +		rcu_read_unlock();
> > 
> > But this can't really help. If task->real_parent has already exited and
> > it was reaped, then it is actually "Too late!" for rcu_read_lock().
> > 
> > Please use task_ppid_nr_ns() which does the necessary pid_alive() check.
> > 
> 
> That routine looks bogus in its own right though.
> 
> Suppose it fits the time window between the current parent exiting and
> the task being reassigned to init. Then you transiently see 0 as the pid,
> instead of 1 (or whatever). This reads like a bug to me.
> 
> But suppose task_ppid_nr_ns() managed to get the right value at the
> time. As per usual, such an exit + reaping could have happened before
> the caller even looks at the returned pid.
> 
> Or to put it differently, imo the check in the routine not only does not
> help, but introduces a corner case with a bogus result.
> 
> It probably should do precisely the same thing proposed in this patch,
> as in:
> 	rcu_read_lock();
> 	ppid = task_tgid_nr_ns(rcu_dereference(task->real_parent), ns);
> 	rcu_read_unlock();

I agree.

Thanks,
Jinliang Zheng. :)