cifs: store a pointer to the root dentry in cifs_sb_info once we have completed mounting the share

And use this to only allow to take out a shared handle once the mount has completed and the
sb becomes available.
This will become important in follow up patches where we will start holding a reference to the
directory dentry for the shared handle during the lifetime of the handle.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifs_fs_sb.h

@@ -81,5 +81,9 @@ struct cifs_sb_info {
 	 * (cifs_autodisable_serverino) in order to match new mounts.
 	 */
 	bool mnt_cifs_serverino_autodisabled;
+	/*
+	 * Available once the mount has completed.
+	 */
+	struct dentry *root;
 };
 #endif				/* _CIFS_FS_SB_H */

fs/cifs/cifsfs.c

@@ -257,6 +257,12 @@ cifs_read_super(struct super_block *sb)
 static void cifs_kill_sb(struct super_block *sb)
 {
 	struct cifs_sb_info *cifs_sb = CIFS_SB(sb);
+
+	if (cifs_sb->root) {
+		dput(cifs_sb->root);
+		cifs_sb->root = NULL;
+	}
+
 	kill_anon_super(sb);
 	cifs_umount(cifs_sb);
 }
@@ -886,6 +892,9 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
 	if (IS_ERR(root))
 		goto out_super;
 
+	if (cifs_sb)
+		cifs_sb->root = dget(root);
+
 	cifs_dbg(FYI, "dentry root is: %p\n", root);
 	return root;
 

fs/cifs/smb2ops.c

@@ -751,8 +751,11 @@ int open_cached_dir(unsigned int xid, struct cifs_tcon *tcon,
 	if (tcon->nohandlecache)
 		return -ENOTSUPP;
 
+	if (cifs_sb->root == NULL)
+		return -ENOENT;
+
 	if (strlen(path))
-		return -ENOTSUPP;
+		return -ENOENT;
 
 	mutex_lock(&tcon->crfid.fid_mutex);
 	if (tcon->crfid.is_valid) {