exfat: clean stale bdev buffers of newly allocated clusters

Directory entries are read and written through buffer_heads of the
block device. Removing the entries of a directory dirties such a
buffer. If the directory is then removed and its cluster is reused for
file data before writeback, the stale buffer stays dirty in the block
device mapping, and the next flush of the block device (fsync, syncfs
or umount) writes it over the new file data. The file silently reads
back with one sector replaced by the deleted directory entries.

This was harmless while file data went through exfat_get_block(), as
__block_write_begin_int() and do_direct_IO() call clean_bdev_aliases()
for newly allocated blocks. Since data I/O moved to iomap, nothing
drops these buffers any more.

Clean the aliases in exfat_alloc_cluster(), once per contiguous run of
allocated clusters. Doing it at allocation time covers every path that
allocates clusters: buffered and direct writes, and also fallocate()
and an extending truncate(), which allocate clusters before the data is
written, so the later iomap mapping does not see them as new.

Tested on the exfat dev branch in a KVM guest with a reproducer that
removes the entries of a directory, removes the directory and writes a
file over its freed cluster: buffered with and without fsync, O_DIRECT,
after fallocate(), after an extending ftruncate(), and with the freed
cluster in the middle of a fragmented allocation. Without this patch
every run corrupted the file (55/55); with it, none did (0/55). A churn
test that creates, removes, preallocates and truncates directories and
files, then runs fsck.exfat, is clean with and without the patch.

Fixes: 82a81a7352bc ("exfat: add iomap buffered I/O support")
Fixes: 867b9c96dc83 ("exfat: add iomap direct I/O support")
Link: https://lore.kernel.org/all/010001a1222f66ba-0c745f29-d4fe-4862-93ff-1db2055b05fc-000000@email.amazonses.com/
Suggested-by: Namjae Jeon <linkinjeon@kernel.org>
Assisted-by: Claude-Opus-5.5
Signed-off-by: Nicolas Falesy <nicolas@nicolasfalesy.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

0 files changed, 0 insertions, 0 deletions