[PATCH 4.4 04/31] fuse: fix fuse_write_end() if zero bytes were copied

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Miklos Szeredi <mszeredi@xxxxxxxxxx>

commit 59c3b76cc61d1d676f965c192cc7969aa5cb2744 upstream.

If pos is at the beginning of a page and copied is zero then page is not
zeroed but is marked uptodate.

Fix by skipping everything except unlock/put of page if zero bytes were
copied.

Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Fixes: 6b12c1b37e55 ("fuse: Implement write_begin/write_end callbacks")
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/fuse/file.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1997,6 +1997,10 @@ static int fuse_write_end(struct file *f
 {
 	struct inode *inode = page->mapping->host;
 
+	/* Haven't copied anything?  Skip zeroing, size extending, dirtying. */
+	if (!copied)
+		goto unlock;
+
 	if (!PageUptodate(page)) {
 		/* Zero any unwritten bytes at the end of the page */
 		size_t endoff = (pos + copied) & ~PAGE_CACHE_MASK;
@@ -2007,6 +2011,8 @@ static int fuse_write_end(struct file *f
 
 	fuse_write_update_size(inode, pos + copied);
 	set_page_dirty(page);
+
+unlock:
 	unlock_page(page);
 	page_cache_release(page);