With glibc 2.27, cupsd crashed with SIGSYS in function
raise

glibc has change implementation of raise as below. We need to
whiltelist gettid and tgkill

int
raise (int sig)
{
  /* rt_sigprocmask may fail if:

     1. sigsetsize != sizeof (sigset_t) (EINVAL)
     2. a failure in copy from/to user space (EFAULT)
     3. an invalid 'how' operation (EINVAL)

     The first case is already handle in glibc syscall call by using the arch
     defined _NSIG.  Second case is handled by using a stack allocated mask.
     The last one should be handled by the block/unblock functions.  */

  sigset_t set;
  __libc_signal_block_app (&set);

  INTERNAL_SYSCALL_DECL (err);
  pid_t pid = INTERNAL_SYSCALL (getpid, err, 0);
  pid_t tid = INTERNAL_SYSCALL (gettid, err, 0);

  int ret = INLINE_SYSCALL (tgkill, 3, pid, tid, sig);

  __libc_signal_restore_set (&set);

  return ret;
}