nilfs2: fix nilfs_empty_dir() misjudgment and long loop on I/O errors

The error handling in nilfs_empty_dir() when a directory folio/page read
fails is incorrect, as in the old ext2 implementation, and if the
folio/page cannot be read or nilfs_check_folio() fails, it will falsely
determine the directory as empty and corrupt the file system.

In addition, since nilfs_empty_dir() does not immediately return on a
failed folio/page read, but continues to loop, this can cause a long loop
with I/O if i_size of the directory's inode is also corrupted, causing the
log writer thread to wait and hang, as reported by syzbot.

Fix these issues by making nilfs_empty_dir() immediately return a false
value (0) if it fails to get a directory folio/page.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ryusuke Konishi <[email protected]>
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=c8166c541d3971bf6c87
Fixes: 2ba466d74ed7 ("nilfs2: directory entry operations")
Tested-by: Ryusuke Konishi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Ryusuke Konishi <[email protected]>

fs/nilfs2/dir.c

@@ -629,7 +629,7 @@ int nilfs_empty_dir(struct inode *inode)
 
 		page = nilfs_get_page(inode, i);
 		if (IS_ERR(page))
-			continue;
+			return 0;
 
 		kaddr = page_address(page);
 		de = (struct nilfs_dir_entry *)kaddr;