[PATCH 0/2] Move pidfd to tiny pseudo fs

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This moves pidfds from the anonymous inode infrastructure to a tiny
pseudo filesystem. This has been on my todo for quite a while as it will
unblock further work that we weren't able to do so far simply because of
the very justified limitations of anonymous inodes. So yesterday I sat
down and wrote it down.

Back when I added pidfds the concept was new (on Linux) and the
limitations were acceptable but now it's starting to hurt us. And with
the concept of pidfds having been around quite a while and being widely
used this is worth doing. This makes it so that:

* statx() on pidfds becomes useful for the first time.
* pidfds can be compared simply via statx() for equality.
* pidfds have unique inode numbers for the system lifetime.
* struct pid is now stashed in inode->i_private instead of
  file->private_data. This means it is now possible to introduce
  concepts that operate on a process once all file descriptors have been
  closed. A concrete example is kill-on-last-close.
* file->private_data is freed up for per-file options for pidfds.
* Each struct pid will refer to a different inode but the same struct
  pid will refer to the same inode if it's opened multiple times. In
  contrast to now where each struct pid refers to the same inode. Even
  if we were to move to anon_inode_create_getfile() which creates new
  inodes we'd still be associating the same struct pid with multiple
  different inodes.
* Pidfds now go through the regular dentry_open() path which means that
  all security hooks are called unblocking proper LSM management for
  pidfds. In addition fsnotify hooks are called and allow for listening
  to open events on pidfds.

The tiny pseudo filesystem is not visible anywhere in userspace exactly
like e.g., pipefs and sockfs. There's no lookup, there's no inode
operations in general, so nothing complex. It's hopefully the best kind
of dumb there is. Dentries and inodes are always deleted when the last
pidfd is closed.

I've made the new code optional and placed it under CONFIG_FS_PIDFD but
I'm confident we can remove that very soon. This takes some inspiration
from nsfs which uses a similar stashing mechanism.


Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>

base-commit: 3f643cd2351099e6b859533b6f984463e5315e5f
change-id: 20240212-vfs-pidfd_fs-9a6e49283d80

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