f2fs: invalidate dentry cache on failed whiteout creation

In the Linux kernel, the following vulnerability has been resolved:

f2fs: invalidate dentry cache on failed whiteout creation

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.

In the error path, VFS does not call d_move() to update the dentry

cache. This results in new_dentry still pointing to the old inode

(new_inode) which has already had its i_nlink decremented to zero.

The stale cache causes subsequent operations to incorrectly reference

the freed inode.

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 does not call d_move() due to error

- VFS cache still has: file1 → inode 7 (stale!)

- inode 7 has i_nlink=0 (already decremented)

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()