[PATCH] fuse: clear the old EOF tail before writeback-cache writes

[Hungyu Lin <dennylin0707@gmail.com>]

Buffered writeback-cache writes go through iomap_file_buffered_write(),
but do not clear cached data between the old EOF and the write position.
If a shared mapping dirtied the tail of the old EOF page, an extending
write can expose those bytes as file contents instead of zeros. A later
writeback can also send them to the server.

Clear the gap under the existing inode lock before the buffered write.
Use the same page-cache cleanup as fuse_perform_write() does for
write-through writes, leaving the write range itself untouched.

Fixes: 4d99ff8f12eb ("fuse: Turn writeback cache on")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 fs/fuse/file.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index d5e1cb53d86a6b35e63ca31b1065068ea5bdb6be..1c43c2f58e0a127004d74f3a28548b20d3d8a013 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1565,6 +1565,11 @@ static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		written = direct_write_fallback(iocb, from, written,
 				fuse_perform_write(iocb, from));
 	} else if (writeback) {
+		loff_t old_size = i_size_read(inode);
+
+		if (iocb->ki_pos > old_size)
+			truncate_pagecache_range(inode, old_size, iocb->ki_pos - 1);
+
 		/*
 		 * Use iomap so that we can do granular uptodate reads
 		 * and granular dirty tracking for large folios.

base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
-- 
2.43.0