[PATCH v3 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.

Tested-by: syzbot@syzkaller.appspotmail.com
Suggested-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
---
 fs/exfat/exfat_fs.h | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index 7f9d0cfa252b..89ef5368277f 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -552,6 +552,27 @@ 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)
+{
+	unsigned int clu = chain->dir;
+
+	if (unlikely(chain->size < step))
+		return -EIO;
+
+	if (exfat_cluster_walk(sb, &clu, step, chain->flags))
+		return -EIO;
+
+	chain->size -= step;
+
+	if (chain->size == 0 && chain->flags == ALLOC_NO_FAT_CHAIN)
+		chain->dir = EXFAT_EOF_CLUSTER;
+	else
+		chain->dir = clu;
+
+	return 0;
+}
+
 /* inode.c */
 extern const struct inode_operations exfat_file_inode_operations;
 void exfat_sync_inode(struct inode *inode);
-- 
2.43.0