virtiofsd: Don't allow empty paths in lookup_name()

When passed an empty filename, lookup_name() returns the inode of
the parent directory, unless the parent is the root in which case
the st_dev doesn't match and lo_find() returns NULL. This is
because lookup_name() passes AT_EMPTY_PATH down to fstatat() or
statx().

This behavior doesn't quite make sense because users of lookup_name()
then pass the name to unlinkat(), renameat() or renameat2(), all of
which will always fail on empty names.

Drop AT_EMPTY_PATH from the flags in lookup_name() so that it has
the consistent behavior of "returning an existing child inode or
NULL" for all directories.

Signed-off-by: Greg Kurz <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Connor Kuehl <[email protected]>
Reviewed-by: Vivek Goyal <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>

tools/virtiofsd/passthrough_ll.c

@@ -1330,8 +1330,7 @@ static struct lo_inode *lookup_name(fuse_req_t req, fuse_ino_t parent,
         return NULL;
     }
 
-    res = do_statx(lo, dir->fd, name, &attr,
-                   AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW, &mnt_id);
+    res = do_statx(lo, dir->fd, name, &attr, AT_SYMLINK_NOFOLLOW, &mnt_id);
     lo_inode_put(lo, &dir);
     if (res == -1) {
         return NULL;