[PATCH v4 0/3] API for exporting connectable file handles to userspace

[Amir Goldstein <amir73il@gmail.com>]

Christian,

These patches bring the NFS connectable file handles feature to
userspace servers.

They rely on your and Aleksa's changes recently merged to v6.12.

This v4 incorporates the review comments on Jeff and Jan (thanks!)
and there does not seem to be any objection for this new API, so
I think it is ready for staging.

The API I chose for encoding conenctable file handles is pretty
conventional (AT_HANDLE_CONNECTABLE).

open_by_handle_at(2) does not have AT_ flags argument, but also, I find
it more useful API that encoding a connectable file handle can mandate
the resolving of a connected fd, without having to opt-in for a
connected fd independently.

I chose to implemnent this by using upper bits in the handle type field
It may be that out-of-tree filesystems return a handle type with upper
bits set, but AFAIK, no in-tree filesystem does that.
I added some warnings just in case we encouter that.

I have written an fstest [4] and a man page draft [5] for the feature.

Thanks,
Amir.

Changes since v3 [3]:
- Relax WARN_ON in decode and replace with pr_warn in encode (Jeff)
- Loose the macro FILEID_USER_TYPE_IS_VALID() (Jan)
- Add explicit check for negative type values (Jan)
- Added fstest and man-page draft

Changes since v2 [2]:
- Use bit arithmetics instead of bitfileds (Jeff)
- Add assertions about use of high type bits

Changes since v1 [1]:
- Assert on encode for disconnected path (Jeff)
- Don't allow AT_HANDLE_CONNECTABLE with AT_EMPTY_PATH
- Drop the O_PATH mount_fd API hack (Jeff)
- Encode an explicit "connectable" flag in handle type

[1] https://lore.kernel.org/linux-fsdevel/20240919140611.1771651-1-amir73il@gmail.com/
[2] https://lore.kernel.org/linux-fsdevel/20240923082829.1910210-1-amir73il@gmail.com/
[3] https://lore.kernel.org/linux-fsdevel/20241008152118.453724-1-amir73il@gmail.com/
[4] https://github.com/amir73il/xfstests/commits/connectable-fh/
[5] https://github.com/amir73il/man-pages/commits/connectable-fh/

Amir Goldstein (3):
  fs: prepare for "explicit connectable" file handles
  fs: name_to_handle_at() support for "explicit connectable" file
    handles
  fs: open_by_handle_at() support for decoding "explicit connectable"
    file handles

 fs/exportfs/expfs.c        | 17 ++++++++-
 fs/fhandle.c               | 75 +++++++++++++++++++++++++++++++++++---
 include/linux/exportfs.h   | 13 +++++++
 include/uapi/linux/fcntl.h |  1 +
 4 files changed, 98 insertions(+), 8 deletions(-)

-- 
2.34.1