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

[Hungyu Lin <dennylin0707@gmail.com>]

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@mail.parknet.co.jp>
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
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@gmail.com

 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
-- 
2.43.0