// https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/xattr.c?h=v4.17-rc5#n122 :
// In the user.* namespace, only regular files and directories can have
// extended attributes.

       The file permission bits of regular files and directories are
       interpreted differently from the file permission bits of special
       files and symbolic links.  For regular files and directories the file
       permission bits define access to the file's contents, while for
       device special files they define access to the device described by
       the special file.  The file permissions of symbolic links are not
       used in access checks.  These differences would allow users to
       consume filesystem resources in a way not controllable by disk quotas
       for group or world writable special files and directories.

       For this reason, extended user attributes are allowed only for
       regular files and directories, and access to extended user attributes
       is restricted to the owner and to users with appropriate capabilities
       for directories with the sticky bit set (see the chmod(1) manual page
       for an explanation of the sticky bit).