exportfs: add explicit flag to request non-decodeable file handles

So far, all callers of exportfs_encode_inode_fh(), except for fsnotify's
show_mark_fhandle(), check that filesystem can decode file handles, but
we would like to add more callers that do not require a file handle that
can be decoded.

Introduce a flag to explicitly request a file handle that may not to be
decoded later and a wrapper exportfs_encode_fid() that sets this flag
and convert show_mark_fhandle() to use the new wrapper.

This will be used to allow adding fanotify support to filesystems that
do not support NFS export.

Acked-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230502124817.3070545-3-amir73il@gmail.com>

Documentation/filesystems/nfs/exporting.rst

@@ -123,7 +123,7 @@ super_block.  This field must point to a "struct export_operations"
 struct which has the following members:
 
   encode_fh (optional)
-    Takes a dentry and creates a filehandle fragment which can later be used
+    Takes a dentry and creates a filehandle fragment which may later be used
     to find or create a dentry for the same object.  The default
     implementation creates a filehandle fragment that encodes a 32bit inode
     and generation number for the inode encoded, and if necessary the

fs/exportfs/expfs.c

@@ -381,11 +381,27 @@ static int export_encode_fh(struct inode *inode, struct fid *fid,
 	return type;
 }
 
+/**
+ * exportfs_encode_inode_fh - encode a file handle from inode
+ * @inode:   the object to encode
+ * @fid:     where to store the file handle fragment
+ * @max_len: maximum length to store there
+ * @flags:   properties of the requested file handle
+ *
+ * Returns an enum fid_type or a negative errno.
+ */
 int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
-			     int *max_len, struct inode *parent)
+			     int *max_len, struct inode *parent, int flags)
 {
 	const struct export_operations *nop = inode->i_sb->s_export_op;
 
+	/*
+	 * If a decodeable file handle was requested, we need to make sure that
+	 * filesystem can decode file handles.
+	 */
+	if (nop && !(flags & EXPORT_FH_FID) && !nop->fh_to_dentry)
+		return -EOPNOTSUPP;
+
 	if (nop && nop->encode_fh)
 		return nop->encode_fh(inode, fid->raw, max_len, parent);
 
@@ -418,7 +434,7 @@ int exportfs_encode_fh(struct dentry *dentry, struct fid *fid, int *max_len,
 		parent = p->d_inode;
 	}
 
-	error = exportfs_encode_inode_fh(inode, fid, max_len, parent);
+	error = exportfs_encode_inode_fh(inode, fid, max_len, parent, flags);
 	dput(p);
 
 	return error;

fs/notify/fanotify/fanotify.c

@@ -380,7 +380,7 @@ static int fanotify_encode_fh_len(struct inode *inode)
 	if (!inode)
 		return 0;
 
-	exportfs_encode_inode_fh(inode, NULL, &dwords, NULL);
+	exportfs_encode_inode_fh(inode, NULL, &dwords, NULL, 0);
 	fh_len = dwords << 2;
 
 	/*
@@ -443,7 +443,7 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
 	}
 
 	dwords = fh_len >> 2;
-	type = exportfs_encode_inode_fh(inode, buf, &dwords, NULL);
+	type = exportfs_encode_inode_fh(inode, buf, &dwords, NULL, 0);
 	err = -EINVAL;
 	if (!type || type == FILEID_INVALID || fh_len != dwords << 2)
 		goto out_err;

fs/notify/fdinfo.c

@@ -50,7 +50,7 @@ static void show_mark_fhandle(struct seq_file *m, struct inode *inode)
 	f.handle.handle_bytes = sizeof(f.pad);
 	size = f.handle.handle_bytes >> 2;
 
-	ret = exportfs_encode_inode_fh(inode, (struct fid *)f.handle.f_handle, &size, NULL);
+	ret = exportfs_encode_fid(inode, (struct fid *)f.handle.f_handle, &size);
 	if ((ret == FILEID_INVALID) || (ret < 0)) {
 		WARN_ONCE(1, "Can't encode file handler for inotify: %d\n", ret);
 		return;

include/linux/exportfs.h

@@ -136,6 +136,7 @@ struct fid {
 };
 
 #define EXPORT_FH_CONNECTABLE	0x1 /* Encode file handle with parent */
+#define EXPORT_FH_FID		0x2 /* File handle may be non-decodeable */
 
 /**
  * struct export_operations - for nfsd to communicate with file systems
@@ -227,9 +228,18 @@ struct export_operations {
 };
 
 extern int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
-				    int *max_len, struct inode *parent);
+				    int *max_len, struct inode *parent,
+				    int flags);
 extern int exportfs_encode_fh(struct dentry *dentry, struct fid *fid,
 			      int *max_len, int flags);
+
+static inline int exportfs_encode_fid(struct inode *inode, struct fid *fid,
+				      int *max_len)
+{
+	return exportfs_encode_inode_fh(inode, fid, max_len, NULL,
+					EXPORT_FH_FID);
+}
+
 extern struct dentry *exportfs_decode_fh_raw(struct vfsmount *mnt,
 					     struct fid *fid, int fh_len,
 					     int fileid_type,