exportfs: add helpers to check if filesystem can encode/decode file handles

The logic of whether filesystem can encode/decode file handles is open
coded in many places.

In preparation to changing the logic, move the open coded logic into
inline helpers.

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/20231023180801.2953446-2-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/exportfs/expfs.c

@@ -396,11 +396,7 @@ int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
 {
 	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)
+	if (!exportfs_can_encode_fh(nop, flags))
 		return -EOPNOTSUPP;
 
 	if (nop && nop->encode_fh)
@@ -456,7 +452,7 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
 	/*
 	 * Try to get any dentry for the given file handle from the filesystem.
 	 */
-	if (!nop || !nop->fh_to_dentry)
+	if (!exportfs_can_decode_fh(nop))
 		return ERR_PTR(-ESTALE);
 	result = nop->fh_to_dentry(mnt->mnt_sb, fid, fh_len, fileid_type);
 	if (IS_ERR_OR_NULL(result))

fs/fhandle.c

@@ -26,12 +26,8 @@ static long do_sys_name_to_handle(const struct path *path,
 	/*
 	 * We need to make sure whether the file system support decoding of
 	 * the file handle if decodeable file handle was requested.
-	 * Otherwise, even empty export_operations are sufficient to opt-in
-	 * to encoding FIDs.
 	 */
-	if (!path->dentry->d_sb->s_export_op ||
-	    (!(fh_flags & EXPORT_FH_FID) &&
-	     !path->dentry->d_sb->s_export_op->fh_to_dentry))
+	if (!exportfs_can_encode_fh(path->dentry->d_sb->s_export_op, fh_flags))
 		return -EOPNOTSUPP;
 
 	if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle)))

fs/nfsd/export.c

@@ -421,8 +421,7 @@ static int check_export(struct path *path, int *flags, unsigned char *uuid)
 		return -EINVAL;
 	}
 
-	if (!inode->i_sb->s_export_op ||
-	    !inode->i_sb->s_export_op->fh_to_dentry) {
+	if (!exportfs_can_decode_fh(inode->i_sb->s_export_op)) {
 		dprintk("exp_export: export of invalid fs type.\n");
 		return -EINVAL;
 	}

fs/notify/fanotify/fanotify_user.c

@@ -1595,7 +1595,7 @@ static int fanotify_test_fid(struct dentry *dentry, unsigned int flags)
 	 * file handles so user can use name_to_handle_at() to compare fids
 	 * reported with events to the file handle of watched objects.
 	 */
-	if (!nop)
+	if (!exportfs_can_encode_fid(nop))
 		return -EOPNOTSUPP;
 
 	/*
@@ -1603,7 +1603,7 @@ static int fanotify_test_fid(struct dentry *dentry, unsigned int flags)
 	 * supports decoding file handles, so user has a way to map back the
 	 * reported fids to filesystem objects.
 	 */
-	if (mark_type != FAN_MARK_INODE && !nop->fh_to_dentry)
+	if (mark_type != FAN_MARK_INODE && !exportfs_can_decode_fh(nop))
 		return -EOPNOTSUPP;
 
 	return 0;

fs/overlayfs/util.c

@@ -55,7 +55,7 @@ int ovl_can_decode_fh(struct super_block *sb)
 	if (!capable(CAP_DAC_READ_SEARCH))
 		return 0;
 
-	if (!sb->s_export_op || !sb->s_export_op->fh_to_dentry)
+	if (!exportfs_can_decode_fh(sb->s_export_op))
 		return 0;
 
 	return sb->s_export_op->encode_fh ? -1 : FILEID_INO32_GEN;

include/linux/exportfs.h

@@ -233,6 +233,33 @@ extern int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
 extern int exportfs_encode_fh(struct dentry *dentry, struct fid *fid,
 			      int *max_len, int flags);
 
+static inline bool exportfs_can_encode_fid(const struct export_operations *nop)
+{
+	return nop;
+}
+
+static inline bool exportfs_can_decode_fh(const struct export_operations *nop)
+{
+	return nop && nop->fh_to_dentry;
+}
+
+static inline bool exportfs_can_encode_fh(const struct export_operations *nop,
+					  int fh_flags)
+{
+	/*
+	 * If a non-decodeable file handle was requested, we only need to make
+	 * sure that filesystem can encode file handles.
+	 */
+	if (fh_flags & EXPORT_FH_FID)
+		return exportfs_can_encode_fid(nop);
+
+	/*
+	 * If a decodeable file handle was requested, we need to make sure that
+	 * filesystem can also decode file handles.
+	 */
+	return exportfs_can_decode_fh(nop);
+}
+
 static inline int exportfs_encode_fid(struct inode *inode, struct fid *fid,
 				      int *max_len)
 {