Fiasco.OC thread execution time

[Georg Guba]

I'm trying to implement Platform_thread::execution_time() for Fiasco.OC.
However, when using the code below, I'm getting a null pointer error when
trying to read the UTCB's message register:

Possible null pointer READ at 0 IP 704c0d7c

unsigned long long Platform_thread::execution_time() const
{
unsigned long long time = 0;

if (_utcb)
{
l4_thread_stats_time(_thread.local.dst());
time = *(l4_kernel_clock_t*)l4_utcb_mr()->mr[0];
}

return time;
}

The doc for l4_thread_stats_time(...) says

/**
* \brief Get consumed timed of thread in µs.
* \ingroup l4_thread_api
* \param thread Thread to get the consumed time from.
*
* The consumed time is returned as l4_kernel_clock_t at UTCB message
* register 0.
*/
L4_INLINE l4_msgtag_t
l4_thread_stats_time(l4_cap_idx_t thread) L4_NOTHROW;

So my usage should be about right.
What am I missing?
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