f2fs: fix possible data corruption in f2fs_write_begin()

f2fs_write_begin() doesn't initialize the 'dn' variable if the inode has
inline data. However it uses its contents to decide whether it should
just zero out the page or load data to it. Thus if we are unlucky we can
zero out page contents instead of loading inline data into a page.

CC: stable@vger.kernel.org
CC: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1017,21 +1017,19 @@ static int f2fs_write_begin(struct file *file, struct address_space *mapping,
 		goto out;
 	}
 
-	if (dn.data_blkaddr == NEW_ADDR) {
+	if (f2fs_has_inline_data(inode)) {
+		err = f2fs_read_inline_data(inode, page);
+		if (err) {
+			page_cache_release(page);
+			goto fail;
+		}
+	} else if (dn.data_blkaddr == NEW_ADDR) {
 		zero_user_segment(page, 0, PAGE_CACHE_SIZE);
 	} else {
-		if (f2fs_has_inline_data(inode)) {
-			err = f2fs_read_inline_data(inode, page);
-			if (err) {
-				page_cache_release(page);
-				goto fail;
-			}
-		} else {
-			err = f2fs_submit_page_bio(sbi, page, dn.data_blkaddr,
-							READ_SYNC);
-			if (err)
-				goto fail;
-		}
+		err = f2fs_submit_page_bio(sbi, page, dn.data_blkaddr,
+					   READ_SYNC);
+		if (err)
+			goto fail;
 
 		lock_page(page);
 		if (unlikely(!PageUptodate(page))) {