cifs_get_root shouldn't use path with tree name

When a server returns the optional flag SMB_SHARE_IS_IN_DFS in response
to a tree connect, cifs_build_path_to_root() will return a pathname
which includes the hostname. This causes problems with cifs_get_root()
which separates each component and does a lookup for each component of
the path which in this case will incorrectly include looking up the
hostname component as a path component.

We encountered a problem with dfs shares hosted by a Netapp. When
connecting to nodes pointed to by the DFS share. The tree connect for
these nodes return SMB_SHARE_IS_IN_DFS resulting failures in lookup
in cifs_get_root().

RH bz: 1373153
The patch was tested against a Netapp simulator and by a user using an
actual Netapp server.

Signed-off-by: Sachin Prabhu <[e-mail]>
Reported-by: Pierguido Lambri <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/cifsfs.c

···
 		return dget(sb->s_root);
 
 	full_path = cifs_build_path_to_root(vol, cifs_sb,
-					    cifs_sb_master_tcon(cifs_sb));
+				cifs_sb_master_tcon(cifs_sb), 0);
 	if (full_path == NULL)
 		return ERR_PTR(-ENOMEM);
 

fs/cifs/cifsproto.h

···
 extern char *build_path_from_dentry(struct dentry *);
 extern char *cifs_build_path_to_root(struct smb_vol *vol,
 				     struct cifs_sb_info *cifs_sb,
-				     struct cifs_tcon *tcon);
+				     struct cifs_tcon *tcon,
+				     int add_treename);
 extern char *build_wildcard_path_from_dentry(struct dentry *direntry);
 extern char *cifs_compose_mount_options(const char *sb_mountdata,
 		const char *fullpath, const struct dfs_info3_param *ref,

fs/cifs/connect.c

···
 		/*
 		 * cifs_build_path_to_root works only when we have a valid tcon
 		 */
-		full_path = cifs_build_path_to_root(volume_info, cifs_sb, tcon);
+		full_path = cifs_build_path_to_root(volume_info, cifs_sb, tcon,
+					tcon->Flags & SMB_SHARE_IS_IN_DFS);
 		if (full_path == NULL) {
 			rc = -ENOMEM;
 			goto mount_fail_check;

fs/cifs/dir.c

···
 
 char *
 cifs_build_path_to_root(struct smb_vol *vol, struct cifs_sb_info *cifs_sb,
-			struct cifs_tcon *tcon)
+			struct cifs_tcon *tcon, int add_treename)
 {
 	int pplen = vol->prepath ? strlen(vol->prepath) + 1 : 0;
 	int dfsplen;
···
 		return full_path;
 	}
 
-	if (tcon->Flags & SMB_SHARE_IS_IN_DFS)
+	if (add_treename)
 		dfsplen = strnlen(tcon->treeName, MAX_TREE_SIZE + 1);
 	else
 		dfsplen = 0;