F2FS can mount filesystems with corrupted directory depth values that
get runtime-clamped to MAX_DIR_HASH_DEPTH. When RENAME_WHITEOUT
operations are performed on such directories, f2fs_rename performs
directory modifications (updating target entry and deleting source
entry) before attempting to add the whiteout entry via f2fs_add_link.

If f2fs_add_link fails due to the corrupted directory structure, the
function returns an error to VFS, but the partial directory
modifications have already been committed to disk. VFS assumes the
entire rename operation failed and does not update the dentry cache,
leaving stale mappings.

This causes subsequent operations to use cached dentry information that
no longer matches the on-disk state. When a second rename targets the
same entry, VFS attempts to decrement i_nlink on the stale inode, which
may already have i_nlink=0, triggering a WARNING in drop_nlink().

Example sequence:
1. First rename (RENAME_WHITEOUT): file2 → file1
   - f2fs updates file1 entry on disk (points to inode 8)
   - f2fs deletes file2 entry on disk
   - f2fs_add_link(whiteout) fails (corrupted directory)
   - Returns error to VFS
   - VFS cache still has: file1 → inode 7 (stale!)

2. Second rename: file3 → file1
   - VFS uses stale cache: file1 → inode 7
   - Tries to drop_nlink on inode 7 (i_nlink already 0)
   - WARNING in drop_nlink()

Fix this by explicitly invalidating old_dentry and new_dentry when
f2fs_add_link fails during whiteout creation. This forces VFS to
refresh from disk on subsequent operations, ensuring cache consistency
even when the rename partially succeeds.

Reproducer:
1. Mount F2FS image with corrupted i_current_depth
2. renameat2(file2, file1, RENAME_WHITEOUT)
3. renameat2(file3, file1, 0)
4. System triggers WARNING in drop_nlink()

Reported-by: syzbot+632cf32276a9a564188d@syzkaller.appspotmail.com
Tested-by: syzbot+632cf32276a9a564188d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=632cf32276a9a564188d
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 fs/f2fs/namei.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c
index b882771e4699..712479b7b93d 100644
--- a/fs/f2fs/namei.c
+++ b/fs/f2fs/namei.c
@@ -1053,9 +1053,11 @@ static int f2fs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 	if (whiteout) {
 		set_inode_flag(whiteout, FI_INC_LINK);
 		err = f2fs_add_link(old_dentry, whiteout);
-		if (err)
+		if (err) {
+			d_invalidate(old_dentry);
+			d_invalidate(new_dentry);
 			goto put_out_dir;
-
+		}
 		spin_lock(&whiteout->i_lock);
 		whiteout->i_state &= ~I_LINKABLE;
 		spin_unlock(&whiteout->i_lock);
-- 
2.43.0

On 10/23/25 07:33, Deepanshu Kartikey wrote:
> F2FS can mount filesystems with corrupted directory depth values that
> get runtime-clamped to MAX_DIR_HASH_DEPTH. When RENAME_WHITEOUT
> operations are performed on such directories, f2fs_rename performs
> directory modifications (updating target entry and deleting source
> entry) before attempting to add the whiteout entry via f2fs_add_link.
> 
> If f2fs_add_link fails due to the corrupted directory structure, the
> function returns an error to VFS, but the partial directory
> modifications have already been committed to disk. VFS assumes the
> entire rename operation failed and does not update the dentry cache,
> leaving stale mappings.

Nice catch!

More detail is in error path, vfs won't call d_exchange to exchange old_dentry
and new_dentry, result in new_dentry still link to new_inode which has zeroed
nlink, once we try to remove this new_inode, it will trigger the bug.

Can you please add above info into your description?

> 
> This causes subsequent operations to use cached dentry information that
> no longer matches the on-disk state. When a second rename targets the
> same entry, VFS attempts to decrement i_nlink on the stale inode, which
> may already have i_nlink=0, triggering a WARNING in drop_nlink().
> 
> Example sequence:
> 1. First rename (RENAME_WHITEOUT): file2 → file1
>    - f2fs updates file1 entry on disk (points to inode 8)
>    - f2fs deletes file2 entry on disk
>    - f2fs_add_link(whiteout) fails (corrupted directory)
>    - Returns error to VFS
>    - VFS cache still has: file1 → inode 7 (stale!)
> 
> 2. Second rename: file3 → file1
>    - VFS uses stale cache: file1 → inode 7
>    - Tries to drop_nlink on inode 7 (i_nlink already 0)
>    - WARNING in drop_nlink()
> 
> Fix this by explicitly invalidating old_dentry and new_dentry when
> f2fs_add_link fails during whiteout creation. This forces VFS to
> refresh from disk on subsequent operations, ensuring cache consistency
> even when the rename partially succeeds.
> 
> Reproducer:
> 1. Mount F2FS image with corrupted i_current_depth
> 2. renameat2(file2, file1, RENAME_WHITEOUT)
> 3. renameat2(file3, file1, 0)
> 4. System triggers WARNING in drop_nlink()
> 

Can you please add Fixes flag and Cc stable@kernel.org?

Otherwise it looks good to me.

Thanks,

> Reported-by: syzbot+632cf32276a9a564188d@syzkaller.appspotmail.com
> Tested-by: syzbot+632cf32276a9a564188d@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=632cf32276a9a564188d
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
> ---
>  fs/f2fs/namei.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c
> index b882771e4699..712479b7b93d 100644
> --- a/fs/f2fs/namei.c
> +++ b/fs/f2fs/namei.c
> @@ -1053,9 +1053,11 @@ static int f2fs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
>  	if (whiteout) {
>  		set_inode_flag(whiteout, FI_INC_LINK);
>  		err = f2fs_add_link(old_dentry, whiteout);
> -		if (err)
> +		if (err) {
> +			d_invalidate(old_dentry);
> +			d_invalidate(new_dentry);
>  			goto put_out_dir;
> -
> +		}
>  		spin_lock(&whiteout->i_lock);
>  		whiteout->i_state &= ~I_LINKABLE;
>  		spin_unlock(&whiteout->i_lock);

Hi Chao,

Thank you for reviewing the patch . I will send patch v2 shortly.

Thanks

Deepanshu