On 05/16, Christian Brauner wrote:
With the introduction of pidfds through CLONE_PIDFD it is possible to
created pidfds at process creation time.
Now I am wondering why do we need CLONE_PIDFD, you can just do
pid = fork();
pidfd_open(pid);
+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;
+
+ ret = 0;
+ rcu_read_lock();
+ /*
+ * If this returns non-NULL the pid was used as a thread-group
+ * leader. Note, we race with exec here: If it changes the
+ * thread-group leader we might return the old leader.
+ */
+ tsk = pid_task(p, PIDTYPE_TGID);
+ if (!tsk)
+ ret = -ESRCH;
+ rcu_read_unlock();
+
+ fd = ret ?: pidfd_create(p);
+ put_pid(p);
+ return fd;
+}
Looks correct, feel free to add Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
But why do we need task_struct *tsk?
rcu_read_lock();
if (!pid_task(PIDTYPE_TGID))
ret = -ESRCH;
rcu_read_unlock();
and in fact we do not even need rcu_read_lock(), we could do
// shut up rcu_dereference_check()
rcu_lock_acquire(&rcu_lock_map);
if (!pid_task(PIDTYPE_TGID))
ret = -ESRCH;
rcu_lock_release(&rcu_lock_map);
Well... I won't insist, but the comment about the race with exec looks a bit
confusing to me. It is true, but we do not care at all, we are not going to
use the task_struct returned by pid_task().
Oleg.
