cifs: Add support for follow_link on dfs shares under posix extensions

When using posix extensions, dfs shares in the dfs root show up as
symlinks resulting in userland tools such as 'ls' calling readlink() on
these shares. Since these are dfs shares, we end up returning -EREMOTE.

$ ls -l /mnt
ls: cannot read symbolic link /mnt/test: Object is remote
total 0
lrwxrwxrwx. 1 root root 19 Nov  6 09:47 test

With added follow_link() support for dfs shares, when using unix
extensions, we call GET_DFS_REFERRAL to obtain the DFS referral and
return the first node returned.

The dfs share in the dfs root is now displayed in the following manner.
$ ls -l /mnt
total 0
lrwxrwxrwx. 1 root root 19 Nov  6 09:47 test -> \vm140-31\test
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/smb1ops.c

@@ -907,6 +907,33 @@ cifs_mand_lock(const unsigned int xid, struct cifsFileInfo *cfile, __u64 offset,
 			   (__u8)type, wait, 0);
 }
 
+static int
+cifs_unix_dfs_readlink(const unsigned int xid, struct cifs_tcon *tcon,
+		       const unsigned char *searchName, char **symlinkinfo,
+		       const struct nls_table *nls_codepage)
+{
+#ifdef CONFIG_CIFS_DFS_UPCALL
+	int rc;
+	unsigned int num_referrals = 0;
+	struct dfs_info3_param *referrals = NULL;
+
+	rc = get_dfs_path(xid, tcon->ses, searchName, nls_codepage,
+			  &num_referrals, &referrals, 0);
+
+	if (!rc && num_referrals > 0) {
+		*symlinkinfo = kstrndup(referrals->node_name,
+					strlen(referrals->node_name),
+					GFP_KERNEL);
+		if (!*symlinkinfo)
+			rc = -ENOMEM;
+		free_dfs_info_array(referrals, num_referrals);
+	}
+	return rc;
+#else /* No DFS support */
+	return -EREMOTE;
+#endif
+}
+
 static int
 cifs_query_symlink(const unsigned int xid, struct cifs_tcon *tcon,
 		   const char *full_path, char **target_path,
@@ -922,6 +949,11 @@ cifs_query_symlink(const unsigned int xid, struct cifs_tcon *tcon,
 	if (cap_unix(tcon->ses)) {
 		rc = CIFSSMBUnixQuerySymLink(xid, tcon, full_path, target_path,
 					     cifs_sb->local_nls);
+		if (rc == -EREMOTE)
+			rc = cifs_unix_dfs_readlink(xid, tcon, full_path,
+						    target_path,
+						    cifs_sb->local_nls);
+
 		goto out;
 	}