[PATCH v2 0/2] API for exporting connectable file handles to userspace

[Amir Goldstein <amir73il@gmail.com>]

Jeff,

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

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

I am aware of the usability implications with connectable file handles,
which are not consistent throughout the inode lifetime (i.e. when moved
to another parent), but the nfsd feature does exists and some users (me)
are interested in exporting this feature to userspace.

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.

Therefore, the whacky API from RFC was replaced with an explicit
connectable flag in the unused (*) upper bits of the handle_type.

(*) It may be valid for filesystems to return a handle type with upper
bits set, but AFAIK, no filesystem does that.

I chose to implemnent this by re-farmatting struct file_handle using bit
feilds.  While using bit fields in UAPI is a questionable practice,
file_handle is not actually in the UAPI and the legacy struct
file_handle which is described in the man page, is binary compatible
with the modified kernel definition with bit fields.
If this is a problem, I can add (and strip) the connectable bit using
plain arithmetics.

Thought and flames are welcome.

Thanks,
Amir.

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/

Amir Goldstein (2):
  fs: name_to_handle_at() support for "explicit connectable" file
    handles
  fs: open_by_handle_at() support for decoding "explicit connectable"
    file handles

 fs/fhandle.c               | 70 ++++++++++++++++++++++++++++++++++----
 include/linux/exportfs.h   |  2 ++
 include/linux/fs.h         |  3 +-
 include/uapi/linux/fcntl.h |  1 +
 4 files changed, 69 insertions(+), 7 deletions(-)

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2.34.1