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

[Yuezhang.Mo@xxxxxxxx]

> On 4/3/26 10:34 AM, Yuezhang.Mo@xxxxxxxx wrote:
> >> 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.
> 
> Okay,
> 
> v3:
> 
> static inline int exfat_chain_advance(struct super_block *sb,
>                  struct exfat_chain *chain, unsigned int step)
> {
>          if (chain->size < step)
>                  return -EIO;
> 
>          chain->size -= step;
> 
>          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;
> }
> 

chain->dir should also not be changed.

static inline int exfat_chain_advance(struct super_block *sb,
                 struct exfat_chain *chain, unsigned int step)
{
         unsigned int clu = chain->dir;

         if (chain->size < step)
                 return -EIO;

         if (exfat_fat_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;
}

> 
> Thanks,
> >
> >> +       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
> >
> >