[PATCH 1/2] pid: add pidfd_open()

[Christian Brauner]

On Wed, May 15, 2019 at 05:35:15PM +0200, Oleg Nesterov wrote:
> On 05/15, Oleg Nesterov wrote:
> >
> > On 05/15, Christian Brauner wrote:
> > >
> > > +SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
> > > +{
> > > +	int fd, ret;
> > > +	struct pid *p;
> > > +	struct task_struct *tsk;
> > > +
> > > +	if (flags)
> > > +		return -EINVAL;
> > > +
> > > +	if (pid <= 0)
> > > +		return -EINVAL;
> > > +
> > > +	p = find_get_pid(pid);
> > > +	if (!p)
> > > +		return -ESRCH;
> > > +
> > > +	rcu_read_lock();
> > > +	tsk = pid_task(p, PIDTYPE_PID);
> >
> > You do not need find_get_pid() before rcu_lock and put_pid() at the end.
> > You can just do find_vpid() under rcu_read_lock().
> 
> Ah, sorry. Somehow I forgot you need to call pidfd_create(pid), you can't
> do this under rcu_read_lock().
> 
> So I was wrong, you can't avoid get/put_pid.

Yeah, I haven't made any changes yet.

Christian