exportfs: support encoding non-decodeable file handles by default

AT_HANDLE_FID was added as an API for name_to_handle_at() that request
the encoding of a file id, which is not intended to be decoded.

This file id is used by fanotify to describe objects in events.

So far, overlayfs is the only filesystem that supports encoding
non-decodeable file ids, by providing export_operations with an
->encode_fh() method and without a ->decode_fh() method.

Add support for encoding non-decodeable file ids to all the filesystems
that do not provide export_operations, by encoding a file id of type
FILEID_INO64_GEN from { i_ino, i_generation }.

A filesystem may that does not support NFS export, can opt-out of
encoding non-decodeable file ids for fanotify by defining an empty
export_operations struct (i.e. with a NULL ->encode_fh() method).

This allows the use of fanotify events with file ids on filesystems
like 9p which do not support NFS export to bring fanotify in feature
parity with inotify on those filesystems.

Note that fanotify also requires that the filesystems report a non-null
fsid.  Currently, many simple filesystems that have support for inotify
(e.g. debugfs, tracefs, sysfs) report a null fsid, so can still not be
used with fanotify in file id reporting mode.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231023180801.2953446-5-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 29 insertions, 3 deletions

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 8f883c4758f5..7d9fdcc187b7 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -383,6 +383,32 @@ int generic_encode_ino32_fh(struct inode *inode, __u32 *fh, int *max_len,
 }
 EXPORT_SYMBOL_GPL(generic_encode_ino32_fh);
 
+#define FILEID_INO64_GEN_LEN 3
+
+/**
+ * exportfs_encode_ino64_fid - encode non-decodeable 64bit ino file id
+ * @inode:   the object to encode
+ * @fid:     where to store the file handle fragment
+ * @max_len: maximum length to store there (in 4 byte units)
+ *
+ * This generic function is used to encode a non-decodeable file id for
+ * fanotify for filesystems that do not support NFS export.
+ */
+static int exportfs_encode_ino64_fid(struct inode *inode, struct fid *fid,
+				     int *max_len)
+{
+	if (*max_len < FILEID_INO64_GEN_LEN) {
+		*max_len = FILEID_INO64_GEN_LEN;
+		return FILEID_INVALID;
+	}
+
+	fid->i64.ino = inode->i_ino;
+	fid->i64.gen = inode->i_generation;
+	*max_len = FILEID_INO64_GEN_LEN;
+
+	return FILEID_INO64_GEN;
+}
+
 /**
  * exportfs_encode_inode_fh - encode a file handle from inode
  * @inode:   the object to encode
@@ -401,10 +427,10 @@ int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
 	if (!exportfs_can_encode_fh(nop, flags))
 		return -EOPNOTSUPP;
 
-	if (nop && nop->encode_fh)
-		return nop->encode_fh(inode, fid->raw, max_len, parent);
+	if (!nop && (flags & EXPORT_FH_FID))
+		return exportfs_encode_ino64_fid(inode, fid, max_len);
 
-	return -EOPNOTSUPP;
+	return nop->encode_fh(inode, fid->raw, max_len, parent);
 }
 EXPORT_SYMBOL_GPL(exportfs_encode_inode_fh);