Merge tag 'vfs-6.15-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Merge tag 'vfs-6.15-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull vfs pidfs updates from Christian Brauner:

 - Allow retrieving exit information after a process has been reaped
   through pidfds via the new PIDFD_INTO_EXIT extension for the
   PIDFD_GET_INFO ioctl. Various tools need access to information about
   a process/task even after it has already been reaped.

   Pidfd polling allows waiting on either task exit or for a task to
   have been reaped. The contract for PIDFD_INFO_EXIT is simply that
   EPOLLHUP must be observed before exit information can be retrieved,
   i.e., exit information is only provided once the task has been reaped
   and then can be retrieved as long as the pidfd is open.

 - Add PIDFD_SELF_{THREAD,THREAD_GROUP} sentinels allowing userspace to
   forgo allocating a file descriptor for their own process. This is
   useful in scenarios where users want to act on their own process
   through pidfds and is akin to AT_FDCWD.

 - Improve premature thread-group leader and subthread exec behavior
   when polling on pidfds:

   (1) During a multi-threaded exec by a subthread, i.e.,
       non-thread-group leader thread, all other threads in the
       thread-group including the thread-group leader are killed and the
       struct pid of the thread-group leader will be taken over by the
       subthread that called exec. IOW, two tasks change their TIDs.

   (2) A premature thread-group leader exit means that the thread-group
       leader exited before all of the other subthreads in the
       thread-group have exited.

   Both cases lead to inconsistencies for pidfd polling with
   PIDFD_THREAD. Any caller that holds a PIDFD_THREAD pidfd to the
   current thread-group leader may or may not see an exit notification
   on the file descriptor depending on when poll is performed. If the
   poll is performed before the exec of the subthread has concluded an
   exit notification is generated for the old thread-group leader. If
   the poll is performed after the exec of the subthread has concluded
   no exit notification is generated for the old thread-group leader.

   The correct behavior is to simply not generate an exit notification
   on the struct pid of a subhthread exec because the struct pid is
   taken over by the subthread and thus remains alive.

   But this is difficult to handle because a thread-group may exit
   premature as mentioned in (2). In that case an exit notification is
   reliably generated but the subthreads may continue to run for an
   indeterminate amount of time and thus also may exec at some point.

   After this pull no exit notifications will be generated for a
   PIDFD_THREAD pidfd for a thread-group leader until all subthreads
   have been reaped. If a subthread should exec before no exit
   notification will be generated until that task exits or it creates
   subthreads and repeates the cycle.

   This means an exit notification indicates the ability for the father
   to reap the child.

* tag 'vfs-6.15-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: (25 commits)
  selftests/pidfd: third test for multi-threaded exec polling
  selftests/pidfd: second test for multi-threaded exec polling
  selftests/pidfd: first test for multi-threaded exec polling
  pidfs: improve multi-threaded exec and premature thread-group leader exit polling
  pidfs: ensure that PIDFS_INFO_EXIT is available
  selftests/pidfd: add seventh PIDFD_INFO_EXIT selftest
  selftests/pidfd: add sixth PIDFD_INFO_EXIT selftest
  selftests/pidfd: add fifth PIDFD_INFO_EXIT selftest
  selftests/pidfd: add fourth PIDFD_INFO_EXIT selftest
  selftests/pidfd: add third PIDFD_INFO_EXIT selftest
  selftests/pidfd: add second PIDFD_INFO_EXIT selftest
  selftests/pidfd: add first PIDFD_INFO_EXIT selftest
  selftests/pidfd: expand common pidfd header
  pidfs/selftests: ensure correct headers for ioctl handling
  selftests/pidfd: fix header inclusion
  pidfs: allow to retrieve exit information
  pidfs: record exit code and cgroupid at exit
  pidfs: use private inode slab cache
  pidfs: move setting flags into pidfs_alloc_file()
  pidfd: rely on automatic cleanup in __pidfd_prepare()
  ...