vfs: default to generic_readlink()

If i_op->readlink is NULL, but i_op->get_link is set then vfs_readlink()
defaults to calling generic_readlink().

The IOP_DEFAULT_READLINK flag indicates that the above conditions are met
and the default action can be taken.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/namei.c

@@ -4682,10 +4682,19 @@ int vfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
 {
 	struct inode *inode = d_inode(dentry);
 
-	if (!inode->i_op->readlink)
-		return -EINVAL;
+	if (unlikely(!(inode->i_opflags & IOP_DEFAULT_READLINK))) {
+		if (unlikely(inode->i_op->readlink))
+			return inode->i_op->readlink(dentry, buffer, buflen);
 
-	return inode->i_op->readlink(dentry, buffer, buflen);
+		if (!d_is_symlink(dentry))
+			return -EINVAL;
+
+		spin_lock(&inode->i_lock);
+		inode->i_opflags |= IOP_DEFAULT_READLINK;
+		spin_unlock(&inode->i_lock);
+	}
+
+	return generic_readlink(dentry, buffer, buflen);
 }
 EXPORT_SYMBOL(vfs_readlink);