On Tue, Feb 06, 2024 at 06:37:22PM +0100, Oleg Nesterov wrote: > > --- a/kernel/pid.c > > +++ b/kernel/pid.c > > @@ -688,7 +688,7 @@ static int pidfd_getfd(struct pid *pid, int fd) > > int ret; > > > > task = get_pid_task(pid, PIDTYPE_PID); > > - if (!task) > > + if (!task || task->flags & PF_EXITING) > > return -ESRCH; > > This looks racy. Suppose that pidfd_getfd() races with the exiting task. > > It is possible that this task sets PF_EXITING and does exit_files() > after the "task->flags & PF_EXITING" check above and before pidfd_getfd() > does __pidfd_fget(), in this case pidfd_getfd() still returns the same > EBADF we want to avoid. > > Perhaps we can change pidfd_getfd() to do > > if (IS_ERR(file)) > return (task->flags & PF_EXITING) ? -ESRCH : PTR_ERR(file); > > instead? > > This needs a comment to explain the PF_EXITING check. And perhaps another > comment to explain that we can't miss PF_EXITING if the target task has > already passed exit_files, both exit_files() and fget_task() take the same > task_lock(task). > > What do you think? Yes, you're absolutely right. Let me resend. Tycho
