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

[Christian Brauner]

On Wed, May 15, 2019 at 04:38:58PM +0200, 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().

Will do.

> 
> > +	if (!tsk)
> > +		ret = -ESRCH;
> > +	else if (unlikely(!thread_group_leader(tsk)))
> > +		ret = -EINVAL;
> 
> it seems that you can do a single check
> 
> 	tsk = pid_task(p, PIDTYPE_TGID);
> 	if (!tsk)
> 		ret = -ESRCH;
> 
> this even looks more correct if we race with exec changing the leader.

The logic here being that you can only reach the thread_group leader
from struct pid if PIDTYPE_PID == PIDTYPE_TGID for this struct pid?

Thanks, Oleg.
Christian