ovl: Use macros to present ovl_xattr

This patch adds two macros:

OVL_XATTR_PRE_NAME and OVL_XATTR_PRE_LEN

to present ovl_xattr name prefix and its length. Also, a
new macro OVL_XATTR_OPAQUE is introduced to replace old
*ovl_opaque_xattr*.

Fix the length of "trusted.overlay." to *16*.
Signed-off-by: Nhujianyang <hujianyang@huawei.com>
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>

fs/overlayfs/dir.c

@@ -118,14 +118,14 @@ int ovl_create_real(struct inode *dir, struct dentry *newdentry,
 
 static int ovl_set_opaque(struct dentry *upperdentry)
 {
-	return ovl_do_setxattr(upperdentry, ovl_opaque_xattr, "y", 1, 0);
+	return ovl_do_setxattr(upperdentry, OVL_XATTR_OPAQUE, "y", 1, 0);
 }
 
 static void ovl_remove_opaque(struct dentry *upperdentry)
 {
 	int err;
 
-	err = ovl_do_removexattr(upperdentry, ovl_opaque_xattr);
+	err = ovl_do_removexattr(upperdentry, OVL_XATTR_OPAQUE);
 	if (err) {
 		pr_warn("overlayfs: failed to remove opaque from '%s' (%i)\n",
 			upperdentry->d_name.name, err);

fs/overlayfs/inode.c

@@ -205,7 +205,7 @@ static int ovl_readlink(struct dentry *dentry, char __user *buf, int bufsiz)
 
 static bool ovl_is_private_xattr(const char *name)
 {
-	return strncmp(name, "trusted.overlay.", 14) == 0;
+	return strncmp(name, OVL_XATTR_PRE_NAME, OVL_XATTR_PRE_LEN) == 0;
 }
 
 int ovl_setxattr(struct dentry *dentry, const char *name,

fs/overlayfs/overlayfs.h

@@ -23,7 +23,9 @@ enum ovl_path_type {
 #define OVL_TYPE_MERGE_OR_LOWER(type) \
 	(OVL_TYPE_MERGE(type) || !OVL_TYPE_UPPER(type))
 
-extern const char *ovl_opaque_xattr;
+#define OVL_XATTR_PRE_NAME "trusted.overlay."
+#define OVL_XATTR_PRE_LEN  16
+#define OVL_XATTR_OPAQUE   OVL_XATTR_PRE_NAME"opaque"
 
 static inline int ovl_do_rmdir(struct inode *dir, struct dentry *dentry)
 {

fs/overlayfs/super.c

@@ -62,8 +62,6 @@ struct ovl_entry {
 
 #define OVL_MAX_STACK 500
 
-const char *ovl_opaque_xattr = "trusted.overlay.opaque";
-
 static struct dentry *__ovl_dentry_lower(struct ovl_entry *oe)
 {
 	return oe->numlower ? oe->lowerstack[0].dentry : NULL;
@@ -254,7 +252,7 @@ static bool ovl_is_opaquedir(struct dentry *dentry)
 	if (!S_ISDIR(inode->i_mode) || !inode->i_op->getxattr)
 		return false;
 
-	res = inode->i_op->getxattr(dentry, ovl_opaque_xattr, &val, 1);
+	res = inode->i_op->getxattr(dentry, OVL_XATTR_OPAQUE, &val, 1);
 	if (res == 1 && val == 'y')
 		return true;