Merge patch series "API for exporting connectable file handles to userspace"

Amir Goldstein <amir73il@gmail.com> says:

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.

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 [1] and a man page draft [2] for the feature.

[1] https://github.com/amir73il/xfstests/commits/connectable-fh/
[2] https://github.com/amir73il/man-pages/commits/connectable-fh/

* patches from https://lore.kernel.org/r/20241011090023.655623-1-amir73il@gmail.com:
  fs: open_by_handle_at() support for decoding "explicit connectable" file handles
  fs: name_to_handle_at() support for "explicit connectable" file handles
  fs: prepare for "explicit connectable" file handles

Link: https://lore.kernel.org/r/20241011090023.655623-1-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 893a1d21dc1c..fcab6ab1d38a 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -160,6 +160,19 @@ struct fid {
 #define EXPORT_FH_FID		0x2 /* File handle may be non-decodeable */
 #define EXPORT_FH_DIR_ONLY	0x4 /* Only decode file handle for a directory */
 
+/*
+ * Filesystems use only lower 8 bits of file_handle type for fid_type.
+ * name_to_handle_at() uses upper 16 bits of type as user flags to be
+ * interpreted by open_by_handle_at().
+ */
+#define FILEID_USER_FLAGS_MASK	0xffff0000
+#define FILEID_USER_FLAGS(type) ((type) & FILEID_USER_FLAGS_MASK)
+
+/* Flags supported in encoded handle_type that is exported to user */
+#define FILEID_IS_CONNECTABLE	0x10000
+#define FILEID_IS_DIR		0x20000
+#define FILEID_VALID_USER_FLAGS	(FILEID_IS_CONNECTABLE | FILEID_IS_DIR)
+
 /**
  * struct export_operations - for nfsd to communicate with file systems
  * @encode_fh:      encode a file handle fragment from a dentry