cifs: avoid dup prefix path in dfs_get_automount_devname()

@server->origin_fullpath already contains the tree name + optional
prefix, so avoid calling __build_path_from_dentry_optional_prefix() as
it might end up duplicating prefix path from @cifs_sb->prepath into
final full path.

Instead, generate DFS full path by simply merging
@server->origin_fullpath with dentry's path.

This fixes the following case

	mount.cifs //root/dfs/dir /mnt/ -o ...
	ls /mnt/link

where cifs_dfs_do_automount() will call smb3_parse_devname() with
@devname set to "//root/dfs/dir/link" instead of
"//root/dfs/dir/dir/link".

Fixes: 7ad54b9 ("cifs: use origin fullpath for automounts")
Cc: <stable@vger.kernel.org> # 6.2+
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Paulo Alcantara

fs/cifs/cifs_dfs_ref.c

@@ -171,8 +171,6 @@ static struct vfsmount *cifs_dfs_do_automount(struct path *path)
 		mnt = ERR_CAST(full_path);
 		goto out;
 	}
-
-	convert_delimiter(full_path, '/');
 	cifs_dbg(FYI, "%s: full_path: %s\n", __func__, full_path);
 
 	tmp = *cur_ctx;

fs/cifs/dfs.h

@@ -34,19 +34,33 @@ static inline int dfs_get_referral(struct cifs_mount_ctx *mnt_ctx, const char *p
 			      cifs_remap(cifs_sb), path, ref, tl);
 }
 
+/* Return DFS full path out of a dentry set for automount */
 static inline char *dfs_get_automount_devname(struct dentry *dentry, void *page)
 {
 	struct cifs_sb_info *cifs_sb = CIFS_SB(dentry->d_sb);
 	struct cifs_tcon *tcon = cifs_sb_master_tcon(cifs_sb);
 	struct TCP_Server_Info *server = tcon->ses->server;
+	size_t len;
+	char *s;
 
 	if (unlikely(!server->origin_fullpath))
 		return ERR_PTR(-EREMOTE);
 
-	return __build_path_from_dentry_optional_prefix(dentry, page,
-							server->origin_fullpath,
-							strlen(server->origin_fullpath),
-							true);
+	s = dentry_path_raw(dentry, page, PATH_MAX);
+	if (IS_ERR(s))
+		return s;
+	/* for root, we want "" */
+	if (!s[1])
+		s++;
+
+	len = strlen(server->origin_fullpath);
+	if (s < (char *)page + len)
+		return ERR_PTR(-ENAMETOOLONG);
+
+	s -= len;
+	memcpy(s, server->origin_fullpath, len);
+	convert_delimiter(s, '/');
+	return s;
 }
 
 static inline void dfs_put_root_smb_sessions(struct list_head *head)