[PATCH v2 5/6] exfat: introduce exfat_chain_advance helper

[Chi Zhiling <chizhiling@163.com>]

From: Chi Zhiling <chizhiling@kylinos.cn>

Introduce exfat_chain_advance() to walk a exfat_chain structure by a
given step, updating both ->dir and ->size fields atomically. This
helper handles both ALLOC_NO_FAT_CHAIN and ALLOC_FAT_CHAIN modes with
proper boundary checking.

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Tested-by: syzbot@syzkaller.appspotmail.com

---
 fs/exfat/exfat_fs.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index 530459ab9acc..1035d20ba563 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -552,6 +552,23 @@ int exfat_read_volume_label(struct super_block *sb,
 int exfat_write_volume_label(struct super_block *sb,
 			     struct exfat_uni_name *label);
 
+static inline int exfat_chain_advance(struct super_block *sb,
+		struct exfat_chain *chain, unsigned int step)
+{
+	if (chain->size >= step)
+		chain->size -= step;
+	else
+		return -EIO;
+
+	if (exfat_fat_walk(sb, &chain->dir, step, chain->flags))
+		return -EIO;
+
+	if (chain->size == 0 && chain->flags == ALLOC_NO_FAT_CHAIN)
+		chain->dir = EXFAT_EOF_CLUSTER;
+
+	return 0;
+}
+
 /* inode.c */
 extern const struct inode_operations exfat_file_inode_operations;
 void exfat_sync_inode(struct inode *inode);
-- 
2.43.0