exfat: zero invalid tail on partial overwrites

Buffered overwrite that ends before valid_size in the same filesystem
block reads the entire block without IOMAP_F_ZERO_TAIL. If an existing
cluster contains stale bytes beyond valid_size, the synchronous read
loads them into the page cache and marks the block uptodate. Subsequent
buffered reads can return these bytes while i_size exceeds valid_size.

For example, with 512-byte blocks, valid_size of 1000 and i_size of 2048,
a cold-cache overwrite of [900, 950) exposes stale data in [1000, 1024).
Setting ZERO_TAIL for this write would also erase valid data in
[950, 1000).

Use iomap_truncate_page() to zero only the invalid tail at valid_size
before preparing such buffered overwrites. Call it after dropping s_lock
and before iomap locks the write folio. The inode lock held by buffered
writes serializes valid_size against truncation and page_mkwrite.

Skip direct I/O, fault mappings and new allocations. Writes that reach
valid_size continue to use IOMAP_F_ZERO_TAIL.

Fixes: 82a81a7352bc ("exfat: add iomap buffered I/O support")
Cc: stable@vger.kernel.org
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

0 files changed, 0 insertions, 0 deletions