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

[Yuezhang.Mo@xxxxxxxx]

> From: Chi Zhiling <chizhiling@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Tested-by: syzbot@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ---
> 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;

It would be better to not change *chain if return an error.

> +       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