Re: [PATCH v3] fat: fix parent link count when replacing a sibling directory

[OGAWA Hirofumi]

Hungyu Lin <dennylin0707@xxxxxxxxx> writes:

> vfat_rename() and do_msdos_rename() adjust parent link counts only when
> updating the dotdot entry for a move between different parents. Replacing
> an existing empty directory in the same parent also removes a
> subdirectory, but leaves the parent's link count unchanged.
>
> For example, renaming one of two sibling directories over the other
> leaves the parent with four links instead of three. stat and ls report
> the stale count, and find -links 3 does not match the parent.
>
> Drop the parent's link count for this case after the source entry has
> been removed successfully. Leave cross-parent renames, regular files
> and the existing error paths unchanged.
>
> Share the parent link count check with rmdir through fat_dir_drop_nlink().
> Only decrement counts of at least three and report a filesystem error
> otherwise, rather than reducing a corrupted directory's count below two.
> Export the helper for the VFAT and MSDOS modules.
>
> Acked-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>

Looks good. Thank you!

> ---
> Changes in v3:
> - Move the parent link count check into fat_dir_drop_nlink() in the FAT
>   module and export it with EXPORT_SYMBOL_GPL(), as requested by
>   OGAWA Hirofumi.
> - Use the helper in all four places: the VFAT and MSDOS rmdir paths and
>   the two sibling-directory replacement paths.
> - Retain Hirofumi's Acked-by from v1.
>
> Changes in v2:
> - Check the parent link count before decrementing it and report low
>   counts with fat_fs_error(), matching rmdir(), as noted by Sashiko.
>
> v1: https://lore.kernel.org/r/20261028211626.26103-1-dennylin0707@xxxxxxxxx
>
>  fs/fat/dir.c         | 10 ++++++++++
>  fs/fat/fat.h         |  1 +
>  fs/fat/namei_msdos.c |  9 +++------
>  fs/fat/namei_vfat.c  |  9 +++------
>  4 files changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/fs/fat/dir.c b/fs/fat/dir.c
> index 35bdb62944a2..0cebcaf82426 100644
> --- a/fs/fat/dir.c
> +++ b/fs/fat/dir.c
> @@ -963,6 +963,16 @@ int fat_dir_empty(struct inode *dir)
>  }
>  EXPORT_SYMBOL_GPL(fat_dir_empty);
>  
> +void fat_dir_drop_nlink(struct inode *dir)
> +{
> +	if (dir->i_nlink >= 3)
> +		drop_nlink(dir);
> +	else
> +		fat_fs_error(dir->i_sb, "parent dir link count too low (%u)",
> +			     dir->i_nlink);
> +}
> +EXPORT_SYMBOL_GPL(fat_dir_drop_nlink);
> +
>  /*
>   * fat_subdirs counts the number of sub-directories of dir. It can be run
>   * on directories being created.
> diff --git a/fs/fat/fat.h b/fs/fat/fat.h
> index 76978a268410..a8a3ca4f465b 100644
> --- a/fs/fat/fat.h
> +++ b/fs/fat/fat.h
> @@ -332,6 +332,7 @@ extern const struct file_operations fat_dir_operations;
>  extern int fat_search_long(struct inode *inode, const unsigned char *name,
>  			   int name_len, struct fat_slot_info *sinfo);
>  extern int fat_dir_empty(struct inode *dir);
> +void fat_dir_drop_nlink(struct inode *dir);
>  extern int fat_subdirs(struct inode *dir);
>  extern int fat_scan(struct inode *dir, const unsigned char *name,
>  		    struct fat_slot_info *sinfo);
> diff --git a/fs/fat/namei_msdos.c b/fs/fat/namei_msdos.c
> index c7500c2859e0..5a43d042eb01 100644
> --- a/fs/fat/namei_msdos.c
> +++ b/fs/fat/namei_msdos.c
> @@ -333,12 +333,7 @@ static int msdos_rmdir(struct inode *dir, struct dentry *dentry)
>  	err = fat_remove_entries(dir, &sinfo);	/* and releases bh */
>  	if (err)
>  		goto out;
> -	if (dir->i_nlink >= 3)
> -		drop_nlink(dir);
> -	else {
> -		fat_fs_error(sb, "parent dir link count too low (%u)",
> -			dir->i_nlink);
> -	}
> +	fat_dir_drop_nlink(dir);
>  
>  	clear_nlink(inode);
>  	fat_detach(inode);
> @@ -565,6 +560,8 @@ static int do_msdos_rename(struct inode *old_dir, unsigned char *old_name,
>  		(void)sync_inode_metadata(old_dir, 1);
>  
>  	if (new_inode) {
> +		if (is_dir && old_dir == new_dir)
> +			fat_dir_drop_nlink(old_dir);
>  		drop_nlink(new_inode);
>  		if (is_dir)
>  			drop_nlink(new_inode);
> diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
> index ff7652bf0410..791f2d7b8509 100644
> --- a/fs/fat/namei_vfat.c
> +++ b/fs/fat/namei_vfat.c
> @@ -809,12 +809,7 @@ static int vfat_rmdir(struct inode *dir, struct dentry *dentry)
>  	err = fat_remove_entries(dir, &sinfo);	/* and releases bh */
>  	if (err)
>  		goto out;
> -	if (dir->i_nlink >= 3)
> -		drop_nlink(dir);
> -	else {
> -		fat_fs_error(sb, "parent dir link count too low (%u)",
> -			dir->i_nlink);
> -	}
> +	fat_dir_drop_nlink(dir);
>  
>  	clear_nlink(inode);
>  	fat_truncate_time(inode, NULL, FAT_UPDATE_ATIME | FAT_UPDATE_CMTIME);
> @@ -1011,6 +1006,8 @@ static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
>  	vfat_update_dir_metadata(old_dir, &ts);
>  
>  	if (new_inode) {
> +		if (is_dir && old_dir == new_dir)
> +			fat_dir_drop_nlink(old_dir);
>  		drop_nlink(new_inode);
>  		if (is_dir)
>  			drop_nlink(new_inode);
>
> base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8

-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>