xattr: Fix error results for non-existent / invisible attributes

Return -ENODATA when trying to read a user.* attribute which cannot
exist: user space otherwise does not have a reasonable way to
distinguish between non-existent and inaccessible attributes.

Likewise, return -ENODATA when an unprivileged process tries to read a
trusted.* attribute: to unprivileged processes, those attributes are
invisible (listxattr() won't include them).

Related to this bug report: https://bugzilla.redhat.com/660613

Signed-off-by: Andreas Gruenbacher <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/xattr.c

···
 		return 0;
 
 	/*
-	 * The trusted.* namespace can only be accessed by a privileged user.
+	 * The trusted.* namespace can only be accessed by privileged users.
 	 */
-	if (!strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN))
-		return (capable(CAP_SYS_ADMIN) ? 0 : -EPERM);
+	if (!strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN)) {
+		if (!capable(CAP_SYS_ADMIN))
+			return (mask & MAY_WRITE) ? -EPERM : -ENODATA;
+		return 0;
+	}
 
-	/* In user.* namespace, only regular files and directories can have
+	/*
+	 * In the user.* namespace, only regular files and directories can have
 	 * extended attributes. For sticky directories, only the owner and
-	 * privileged user can write attributes.
+	 * privileged users can write attributes.
 	 */
 	if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) {
 		if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
-			return -EPERM;
+			return (mask & MAY_WRITE) ? -EPERM : -ENODATA;
 		if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) &&
 		    (mask & MAY_WRITE) && !inode_owner_or_capable(inode))
 			return -EPERM;