exportfs: require ->fh_to_parent() to encode connectable file handles

When user requests a connectable file handle explicitly with the
AT_HANDLE_CONNECTABLE flag, fail the request if filesystem (e.g. nfs)
does not know how to decode a connected non-dir dentry.

Fixes: c374196 ("fs: name_to_handle_at() support for "explicit connectable" file handles")
Signed-off-by: Amir Goldstein <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: amir73il

include/linux/exportfs.h

@@ -314,13 +314,23 @@ static inline bool exportfs_can_decode_fh(const struct export_operations *nop)
 static inline bool exportfs_can_encode_fh(const struct export_operations *nop,
 					  int fh_flags)
 {
+	if (!nop)
+		return false;
+
 	/*
 	 * If a non-decodeable file handle was requested, we only need to make
 	 * sure that filesystem did not opt-out of encoding fid.
 	 */
 	if (fh_flags & EXPORT_FH_FID)
 		return exportfs_can_encode_fid(nop);
 
+	/*
+	 * If a connectable file handle was requested, we need to make sure that
+	 * filesystem can also decode connected file handles.
+	 */
+	if ((fh_flags & EXPORT_FH_CONNECTABLE) && !nop->fh_to_parent)
+		return false;
+
 	/*
 	 * If a decodeable file handle was requested, we need to make sure that
 	 * filesystem can also decode file handles.