Kernel threads (and processes) are supposed to call kthread_exit() (or
kproc_exit()) to terminate. However, the kernel includes a fallback in
fork_exit() to force a kthread exit if a kernel thread's "main" routine
returns. This fallback was added back when the kernel only had processes
and was not updated to call kthread_exit() instead of kproc_exit() when
threads were added to the kernel.
This mistake was particular exciting when the errant thread belonged to
proc0. Due to the missing P_KTHREAD flag the fallback did not kick in
and instead tried to return to userland via whatever garbage was in the
trapframe. With P_KTHREAD set it tried to terminate proc0 resulting in
other amusements.
PR: 204999
MFC after: 1 week
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