Deliberately missing the bus

The part I left out earlier is an interesting wrinkle: just before the second timestamp is taken, we insert a bit of code to deliberately miss the sacrosanct deadline! How? It's really very simple:

static void encrypt_decrypt_payload(int work, struct sed_ds *kd, struct sed_ds *kdret)
{
    [ ... ]
    // work done!
    if (make_it_fail == 1)
        msleep(TIMER_EXPIRE_MS + 1);
    t2 = ktime_get_real_ns();

make_it_fail is a module parameter that is set to 0 by default; thus, only if you want to live dangerously (yes, a bit exaggerated!) should you pass it as 1. Let's try it out and see our kernel timer expire. The user mode app will detect this and report the failure as well:

Figure 5.4 – Our sed1 mini-project running with the make_it_fail module parameter set to 1, causing the deadline to be missed

This time, the deadline is exceeded before the timer is canceled, thus causing it to expire and...