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

[Oleg Nesterov]

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().

> +     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.

Oleg.