> If these concerns are correct So, ok. I misremebered this. The scenario I had been thinking of is basically the following. We have a thread-group with thread-group leader 1234 and a thread with 4567 in that thread-group. Assume current thread-group leader is tsk1 and the non-thread-group leader is tsk2. tsk1 uses struct pid *tg_pid and tsk2 uses struct pid *t_pid. The struct pids look like this after creation of both thread-group leader tsk1 and thread tsk2: TGID 1234 TID 4567 tg_pid[PIDTYPE_PID] = tsk1 t_pid[PIDTYPE_PID] = tsk2 tg_pid[PIDTYPE_TGID] = tsk1 t_pid[PIDTYPE_TGID] = NULL IOW, tsk2's struct pid has never been used as a thread-group leader and thus PIDTYPE_TGID is NULL. Now assume someone does create pidfds for tsk1 and for tsk2: tg_pidfd = pidfd_open(tsk1) t_pidfd = pidfd_open(tsk2) -> tg_pidfd->private_data = tg_pid -> t_pidfd->private_data = t_pid So we stash away struct pid *tg_pid for a pidfd_open() on tsk1 and we stash away struct pid *t_pid for a pidfd_open() on tsk2. If we wait on that task via P_PIDFD we get: /* waiting through pidfd */ waitid(P_PIDFD, tg_pidfd) waitid(P_PIDFD, t_pidfd) tg_pid[PIDTYPE_TGID] == tsk1 t_pid[PIDTYPE_TGID] == NULL => succeeds => fails Because struct pid *tg_pid is used a thread-group leader struct pid we can wait on that tsk1. But we can't via the non-thread-group leader pidfd because the struct pid *t_pid has never been used as a thread-group leader. Now assume, t_pid exec's and the struct pids are transfered. IIRC, we get: tg_pid[PIDTYPE_PID] = tsk2 t_pid[PIDTYPE_PID] = tsk1 tg_pid[PIDTYPE_TGID] = tsk2 t_pid[PIDTYPE_TGID] = NULL If we wait on that task via P_PIDFD we get: /* waiting through pidfd */ waitid(P_PIDFD, tg_pidfd) waitid(P_PIDFD, t_pid) tg_pid[PIDTYPE_TGID] == tsk2 t_pid[PIDTYPE_TGID] == NULL => succeeds => fails Which is what we want. So effectively this should all work and I misremembered the struct pid linkage. So afaict we don't even have a problem here which is great.
