On Thu, May 28, 2020 at 12:45:02AM +0200, Christian Brauner wrote: > On Wed, May 27, 2020 at 03:37:58PM -0700, Kees Cook wrote: > > On Thu, May 28, 2020 at 12:05:32AM +0200, Christian Brauner wrote: > > > The main question also is, is there precedence where the kernel just > > > closes the file descriptor for userspace behind it's back? I'm not sure > > > I've heard of this before. That's not how that works afaict; it's also > > > not how we do pidfds. We don't just close the fd when the task > > > associated with it goes away, we notify and then userspace can close. > > > > But there's a mapping between pidfd and task struct that is separate > > from task struct itself, yes? I.e. keeping a pidfd open doesn't pin > > struct task in memory forever, right? > > No, but that's an implementation detail and we discussed that. It pins > struct pid instead of task_struct. Once the process is fully gone you > just get ESRCH. > For example, fds to /proc/<pid>/<tid>/ fds aren't just closed once the > task has gone away, userspace will just get ESRCH when it tries to open > files under there but the fd remains valid until close() is called. > > In addition, of all the anon inode fds, none of them have the "close the > file behind userspace back" behavior: io_uring, signalfd, timerfd, btf, > perf_event, bpf-prog, bpf-link, bpf-map, pidfd, userfaultfd, fanotify, > inotify, eventpoll, fscontext, eventfd. These are just core kernel ones. > I'm pretty sure that it'd be very odd behavior if we did that. I'd > rather just notify userspace and leave the close to them. But maybe I'm > missing something. I'm also starting to think this isn't even possible or currently doable safely. The fdtable in the kernel would end up with a dangling pointer, I would think. Unless you backtrack all fds that still have a reference into the fdtable and refer to that file and close them all in the kernel which I don't think is possible and also sounds very dodgy. This also really seems like we would be breaking a major contract, namely that fds stay valid until userspace calls close, execve(), or exits. Christian _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
