exfat: support handle zero-size directory

After repairing a corrupted file system with exfatprogs' fsck.exfat,
zero-size directories may result. It is also possible to create
zero-size directories in other exFAT implementation, such as Paragon
ufsd dirver.

As described in the specification, the lower directory size limits
is 0 bytes.

Without this commit, sub-directories and files cannot be created
under a zero-size directory, and it cannot be removed.
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

fs/exfat/namei.c

@@ -351,14 +351,20 @@ static int exfat_find_empty_entry(struct inode *inode,
 		if (exfat_check_max_dentries(inode))
 			return -ENOSPC;
 
-		/* we trust p_dir->size regardless of FAT type */
-		if (exfat_find_last_cluster(sb, p_dir, &last_clu))
-			return -EIO;
-
 		/*
 		 * Allocate new cluster to this directory
 		 */
-		exfat_chain_set(&clu, last_clu + 1, 0, p_dir->flags);
+		if (ei->start_clu != EXFAT_EOF_CLUSTER) {
+			/* we trust p_dir->size regardless of FAT type */
+			if (exfat_find_last_cluster(sb, p_dir, &last_clu))
+				return -EIO;
+
+			exfat_chain_set(&clu, last_clu + 1, 0, p_dir->flags);
+		} else {
+			/* This directory is empty */
+			exfat_chain_set(&clu, EXFAT_EOF_CLUSTER, 0,
+					ALLOC_NO_FAT_CHAIN);
+		}
 
 		/* allocate a cluster */
 		ret = exfat_alloc_cluster(inode, 1, &clu, IS_DIRSYNC(inode));
@@ -368,6 +374,11 @@ static int exfat_find_empty_entry(struct inode *inode,
 		if (exfat_zeroed_cluster(inode, clu.dir))
 			return -EIO;
 
+		if (ei->start_clu == EXFAT_EOF_CLUSTER) {
+			ei->start_clu = clu.dir;
+			p_dir->dir = clu.dir;
+		}
+
 		/* append to the FAT chain */
 		if (clu.flags != p_dir->flags) {
 			/* no-fat-chain bit is disabled,
@@ -646,7 +657,7 @@ static int exfat_find(struct inode *dir, struct qstr *qname,
 	info->type = exfat_get_entry_type(ep);
 	info->attr = le16_to_cpu(ep->dentry.file.attr);
 	info->size = le64_to_cpu(ep2->dentry.stream.valid_size);
-	if ((info->type == TYPE_FILE) && (info->size == 0)) {
+	if (info->size == 0) {
 		info->flags = ALLOC_NO_FAT_CHAIN;
 		info->start_clu = EXFAT_EOF_CLUSTER;
 	} else {
@@ -889,6 +900,9 @@ static int exfat_check_dir_empty(struct super_block *sb,
 
 	dentries_per_clu = sbi->dentries_per_clu;
 
+	if (p_dir->dir == EXFAT_EOF_CLUSTER)
+		return 0;
+
 	exfat_chain_dup(&clu, p_dir);
 
 	while (clu.dir != EXFAT_EOF_CLUSTER) {
@@ -1256,7 +1270,8 @@ static int __exfat_rename(struct inode *old_parent_inode,
 		}
 
 		/* Free the clusters if new_inode is a dir(as if exfat_rmdir) */
-		if (new_entry_type == TYPE_DIR) {
+		if (new_entry_type == TYPE_DIR &&
+		    new_ei->start_clu != EXFAT_EOF_CLUSTER) {
 			/* new_ei, new_clu_to_free */
 			struct exfat_chain new_clu_to_free;